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_Is Mars Habitable?_

A CRITICAL EXAMINATION OF PROFESSOR PERCIVAL LOWELL'S BOOK
"MARS AND ITS CANALS," WITH AN ALTERNATIVE EXPLANATION

BY ALFRED RUSSEL WALLACE F.R.S., ETC.



PREFACE.

This small volume was commenced as a review article on Professor
Percival Lowell's book, _Mars and its Canals_, with the object of
showing that the large amount of new and interesting facts contained in
this work did not invalidate the conclusion I had reached in 1902, and
stated in my book on _Man's Place in the Universe_, that Mars was not
habitable.

But the more complete presentation of the opposite view in the volume
now under discussion required a more detailed examination of the various
physical problems involved, and as the subject is one of great, popular,
as well as scientific interest, I determined to undertake the task.

This was rendered the more necessary by the fact that in July last
Professor Lowell published in the _Philosophical Magazine_ an elaborate
mathematical article claiming to demonstrate that, notwithstanding its
much greater distance from the sun and its excessively thin atmosphere,
Mars possessed a climate on the average equal to that of the south of
England, and in its polar and sub-polar regions even less severe than
that of the earth. Such a contention of course required to be dealt
with, and led me to collect information bearing upon temperature in all
its aspects, and so enlarging my criticism that I saw it would be
necessary to issue it in book form.

Two of my mathematical friends have pointed out the chief omission which
vitiates Professor Lowell's mathematical conclusions--that of a failure
to recognise the very large conservative and _cumulative_ effect of a
dense atmosphere. This very point however I had already myself discussed
in Chapter VI., and by means of some remarkable researches on the heat
of the moon and an investigation of the causes of its very low
temperature, I have, I think, demonstrated the incorrectness of Mr.
Lowell's results. In my last chapter, in which I briefly summarise the
whole argument, I have further strengthened the case for very severe
cold in Mars, by adducing the rapid lowering of temperature universally
caused by diminution of atmospheric pressure, as manifested in the
well-known phenomenon of temperate climates at moderate heights even
close to the equator, cold climates at greater heights even on extensive
plateaux, culminating in arctic climates and perpetual snow at heights
where the air is still far denser than it is on the surface of Mars.
This argument itself is, in my opinion, conclusive; but it is enforced
by two others equally complete, neither of which is adequately met by
Mr. Lowell.

The careful examination which I have been led to give to the whole of
the phenomena which Mars presents, and especially to the discoveries of
Mr. Lowell, has led me to what I hope will be considered a satisfactory
physical explanation of them. This explanation, which occupies the whole
of my seventh chapter, is founded upon a special mode of origin for
Mars, derived from the Meteoritic Hypothesis, now very widely adopted by
astronomers and physicists. Then, by a comparison with certain
well-known and widely spread geological phenomena, I show how the great
features of Mars--the 'canals' and 'oases'--may have been caused. This
chapter will perhaps be the most interesting to the general reader, as
furnishing a quite natural explanation of features of the planet which
have been termed 'non-natural' by Mr. Lowell.

Incidentally, also, I have been led to an explanation of the highly
volcanic nature of the moon's surface. This seems to me absolutely to
require some such origin as Sir George Darwin has given it, and thus
furnishes corroborative proof of the accuracy of the hypothesis that our
moon has had an unique origin among the known satellites, in having been
thrown off from the earth itself.

I am indebted to Professor J. H. Poynting, of the University of
Birmingham, for valuable suggestions on some of the more difficult
points of mathematical physics here discussed, and also for the critical
note (at the end of Chapter V.) on Professor Lowell's estimate of the
temperature of Mars.

BROADSTONE, DORSET, _October_ 1907.



TABLE OF CONTENTS.


CHAPTER I.

EARLY OBSERVERS OF MARS,
--Mars the only planet the surface of which is
  distinctly visible
--Early observation of the snow-caps and seas
--The 'canals' seen by Schiaparelli in 1877
--Double canals first seen in 1881
--Round spots at intersection of canals seen
  by Pickering in 1892
--Confirmed by Lowell in 1894
--Changes of colour seen in 1892 and 1894
--Existence of seas doubted by Pickering and
  Barnard in 1894.


CHAPTER II.

MR. LOWELL'S DISCOVERIES AND THEORIES,
--Observatory at Flagstaff, Arizona
--Illustrated book on his observations of
  Mars
--Volume on Mars and its canals, 1906
--Non-natural features
--The canals as irrigation works of an intelligent
  race
--A challenge to the thinking world
--The canals as described and mapped by Mr. Lowell
--The double canals
--Dimensions of the canals
--They cross the supposed seas
--Circular black spots termed oases
--An interesting volume.


CHAPTER III.

THE CLIMATE AND PHYSIOGRAPHY OF MARS,
--No permanent water on Mars
--Rarely any clouds and no rain
--Snow-caps the only source of water
--No mountains, hills, or valleys on Mars
--Two-thirds of the surface a desert
--Water from the snow-caps too scanty to supply
  the canals
--Miss Clerke's views as to the water-supply
--Description of some of the chief canals
--Mr. Lowell on the purpose of the canals
--Remarks on the same
--Mr. Lowell on relation of canals to oases and
  snow-caps
--Critical remarks on the same.


CHAPTER IV.

IS ANIMAL LIFE POSSIBLE ON MARS?
--Water and air essential for animal life
--Atmosphere of Mars assumed to be like ours
--Blue tint near melting snow the only evidence
  of water
--Fallacy of this argument
--Dr. Johnstone Stoney's proof that water-vapour
  cannot exist on Mars
--Spectroscope gives no evidence of water.


CHAPTER V.

TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE,
--Problem of terrestrial temperature
--Ice under recent lava
--Tropical oceans ice-cold at bottom
--Earth's surface-heat all from the sun
--Absolute zero of temperature
--Complex problem of planetary temperatures
--Mr. Lowell's investigation of the problem
--Abstract of Mr. Lowell's paper
--Critical remarks on Mr. Lowell's paper.


CHAPTER VI.

A NEW ESTIMATE OF THE TEMPERATURE OF MARS,
--Langley's determination of lunar heat
--Rapid loss of heat by radiation on the
  earth
--Rapid loss of heat on moon during eclipse
--Sir George Darwin's theory of the moon's origin
--Very's researches on the moon's temperature
--Application of these results to the case of Mars
--Cause of great difference of temperatures of earth
  and moon
--Special features of Mars influencing its
  temperature
--Further criticism of Mr. Lowell's article
--Very low temperature of arctic regions on Mars.


CHAPTER VII.

A SUGGESTION AS TO THE 'CANALS' OF MARS,
--Special features of the canals
--Mr. Pickering's suggested explanation
--The meteoritic hypotheses of origin of planets
--Probable mode of origin of Mars
--Structural straight lines on the earth
--Probable origin of the surface-features of Mars
--Symmetry of basaltic columns
--How this applies to Mars
--Suggested explanation of the oases
--Probable function of the great fissures
--Suggested origin of blue patches adjacent to snow-caps
--The double canals
--Concluding remarks on the canals.


CHAPTER VIII.

PAGE SUMMARY AND CONCLUSION,
--The canals the origin of Mr. Lowell's theory
--Best explained as natural features
--Evaporation difficulty not met by Mr. Lowell
--How did Martians live without the canals
--Radiation due to scanty atmosphere not taken account
  of
--Three independent proofs of low temperature and
  uninhabitability of Mars
--Conclusion.


CHAPTER I.

EARLY OBSERVERS OF MARS.

Few persons except astronomers fully realise that of all the planets of
the Solar system the only one whose solid surface has been seen with
certainty is Mars; and, very fortunately, that is also the only one
which is sufficiently near to us for the physical features of the
surface to be determined with any accuracy, even if we could see it in
the other planets. Of Venus we probably see only the upper surface of
its cloudy atmosphere.[1] As regards Jupiter and Saturn this is still
more certain, since their low density will only permit of a
comparatively small proportion of their huge bulk being solid. Their
belts are but the cloud-strata of their upper atmosphere, perhaps
thousands of miles above their solid surfaces, and a somewhat similar
condition seems to prevail in the far more remote planets Uranus and
Neptune. It has thus happened, that, although as telescopic objects of
interest and beauty, the marvellous rings of Saturn, the belts and
ever-changing aspects of the satellites of Jupiter, and the moon-like
phases of Venus, together with its extreme brilliancy, still remain
unsurpassed, yet the greater amount of details of these features when
examined with the powerful instruments of the nineteenth century have
neither added much to our knowledge of the planets themselves or led to
any sensational theories calculated to attract the popular imagination.

[Footnote 1: Mercury also seems to have a scanty atmosphere, but as its
mass is only one-thirtieth that of the earth it can retain only the
heavier gases, and its atmosphere may be dust-laden, as is that of Mars,
according to Mr. Lowell. Its dusky markings, as seen by Schiaparelli,
seem to be permanent, and they are also for considerable periods
unchangeable in position, indicating that the planet keeps the same face
towards the sun as does Venus. This was confirmed by Mr. Lowell in 1896.
Its distance from us and unfavourable position for observation must
prevent us from obtaining any detailed knowledge of its actual surface,
though its low reflective power indicates that the surface may be really
visible.]

But in the case of Mars the progress of discovery has had a very
different result. The most obvious peculiarity of this planet--its polar
snow-caps--were seen about 250 years ago, but they were first proved to
increase and decrease alternately, in the summer and winter of each
hemisphere, by Sir William Herschell in the latter part of the
eighteenth century. This fact gave the impulse to that idea of
similarity in the conditions of Mars and the earth, which the
recognition of many large dusky patches and streaks as water, and the
more ruddy and brighter portions as land, further increased. Added to
this, a day only about half an hour longer than our own, and a
succession of seasons of the same character as ours but of nearly double
the length owing to its much longer year, seemed to leave little wanting
to render this planet a true earth on a smaller scale. It was therefore
very natural to suppose that it must be inhabited, and that we should
some day obtain evidence of the fact.

_The Canals discovered by Schiaparelli._

Hence the great interest excited when Schiaparelli, at the Milan
Observatory, during the very favourable opposition of 1877 and 1879,
observed that the whole of the tropical and temperate regions from 60 deg.
N. to 60 deg. S. Lat. were covered with a remarkable network of broader
curved and narrower straight lines of a dark colour. At each successive
favourable opposition, these strange objects called _canali_ (channels)
by their discoverer, but rather misleadingly 'canals' in England and
America, were observed by means of all the great telescopes in the
world, and their reality and general features became well established.
In Schiaparelli's first map they were represented as being much broader
and less sharply defined than he himself and other observers found by
later and equally favourable observations that they really were.

_Discovery of the Double Canals._

In 1881 another strange feature was discovered by Schiaparelli, who
found that about twenty canals which had previously been seen single
were now distinctly double, that is, that they consisted of two parallel
lines, equally distinct and either very close together or a considerable
distance apart. This curious appearance was at first thought to be due
to some instrumental defect or optical illusion; but as it was soon
confirmed by other observers with the best instruments and in widely
different localities it became in time accepted as a real phenomenon of
the planet's surface.

_Round Spots discovered in_ 1892.

At the favourable opposition of 1892, Mr. W. H. Pickering noticed that
besides the 'seas' of various sizes there were numerous very small black
spots apparently quite circular and occurring at every intersection or
starting-point of the 'canals.' Many of these had been seen by
Schiaparelli as larger and ill-defined dark patches, and were termed
seas or lakes; but Mr. Pickering's observatory was at Arequipa in Peru,
about 8000 feet above the sea, and with such perfect atmospheric
conditions as were, in his opinion, equal to a doubling of telescopic
aperture. They were soon detected by other observers, especially by Mr.
Lowell in 1894, who thus wrote of them:

"Scattered over the orange-ochre groundwork of the continental regions
of the planet, are any number of dark round spots. How many there may be
it is not possible to state, as the better the seeing, the more of them
there seem to be. In spite, however, of their great number, there is no
instance of one unconnected with a canal. What is more, there is
apparently none that does not lie at the junction of several canals.
Reversely, all the junctions appear to be provided with spots. Plotted
upon a globe they and their connecting canals make a most curious
network over all the orange-ochre equatorial parts of the planet, a mass
of lines and knots, the one marking being as omnipresent as the other."

_Changes of Colour recognised._

During the oppositions of 1892 and 1894 it was fully recognised that a
regular course of change occurred dependent upon the succession of the
seasons, as had been first suggested by Schiaparelli. As the polar snows
melt the adjacent seas appear to overflow and spread out as far as the
tropics, and are often seen to assume a distinctly green colour. These
remarkable changes and the extraordinary phenomena of perfect straight
lines crossing each other over a large portion of the planet's surface,
with the circular spots at their intersections, had such an appearance
of artificiality that the idea that they were really 'canals' made by
intelligent beings for purposes of irrigation, was first hinted at, and
then adopted as the only intelligible explanation, by Mr. Lowell and a
few other persons. This at once seized upon the public imagination and
was spread by the newspapers and magazines over the whole civilised
world.

_Existence of Seas doubted._

At this time (1894) it began to be doubted whether there were any seas
at all on Mars. Professor Pickering thought they were far more limited
in size than had been supposed, and even might not exist as true seas.
Professor Barnard, with the Lick thirty-six inch telescope, discerned an
astonishing wealth of detail on the surface of Mars, so intricate,
minute, and abundant, that it baffled all attempts to delineate it; and
these peculiarities were seen upon the supposed seas as well as on the
land-surfaces. In fact, under the best conditions these 'seas' lost all
trace of uniformity, their appearance being that of a mountainous
country, broken by ridges, rifts, and canyons, seen from a great
elevation. As we shall see later on these doubts soon became
certainties, and it is now almost universally admitted that Mars
possesses no permanent bodies of water.



CHAPTER II.


MR. PERCIVAL LOWELL'S DISCOVERIES AND THEORIES.

_The Observatory in Arizona._

In 1894, after a careful search for the best atmospheric conditions, Mr.
Lowell established his observatory near the town of Flagstaff in
Arizona, in a very dry and uniform climate, and at an elevation of 7300
feet above the sea. He then possessed a fine equatorial telescope of 18
inches aperture and 26 feet focal length, besides two smaller ones, all
of the best quality. To these he added in 1896 a telescope with 24 inch
object glass, the last work of the celebrated firm of Alvan Clark &
Sons, with which he has made his later discoveries. He thus became
perhaps more favourably situated than any astronomer in the northern
hemisphere, and during the last twelve years has made a specialty of the
study of Mars, besides doing much valuable astronomical work on other
planets.

_Mr, Lowell's recent Books upon Mars._

In 1905 Mr. Lowell published an illustrated volume giving a full account
of his observations of Mars from 1894 to 1903, chiefly for the use of
astronomers; and he has now given us a popular volume summarising the
whole of his work on the planet, and published both in America and
England by the Macmillan Company. This very interesting volume is fully
illustrated with twenty plates, four of them , and more than
forty figures in the text, showing the great variety of details from
which the larger general maps have been constructed.

_Non-natural Features of Mars._

But what renders this work especially interesting to all intelligent
readers is, that the author has here, for the first time, fully set
forth his views both as to the habitability of Mars and as to its being
actually inhabited by beings comparable with ourselves in intellect. The
larger part of the work is in fact devoted to a detailed description of
what he terms the 'Non-natural Features' of the planet's surface,
including especially a full account of the 'Canals,' single and double;
the 'Oases,' as he terms the dark spots at their intersections; and the
varying visibility of both, depending partly on the Martian seasons;
while the five concluding chapters deal with the possibility of animal
life and the evidence in favour of it. He also upholds the theory of the
canals having been constructed for the purpose of 'husbanding' the
scanty water-supply that exists; and throughout the whole of this
argument he clearly shows that he considers the evidence to be
satisfactory, and that the only intelligible explanation of the whole of
the phenomena he so clearly sets forth is, that the inhabitants of Mars
have carried out on their small and naturally inhospitable planet a vast
system of irrigation-works, far greater both in its extent, in its
utility, and its effect upon their world as a habitation for civilised
beings, than anything we have yet done upon our earth, where our
destructive agencies are perhaps more prominent than those of an
improving and recuperative character.

_A Challenge to the Thinking World._

This volume is therefore in the nature of a challenge, not so much to
astronomers as to the educated world at large, to investigate the
evidence for so portentous a conclusion. To do this requires only a
general acquaintance with modern science, more especially with mechanics
and physics, while the main contention (with which I shall chiefly deal)
that the features termed 'canals' are really works of art and
necessitate the presence of intelligent organic beings, requires only
care and judgment in drawing conclusions from admitted facts. As I have
already paid some attention to this problem and have expressed the
opinion that Mars is not habitable,[2] judging from the evidence then
available, and as few men of science have the leisure required for a
careful examination of so speculative a subject, I propose here to point
out what the facts, as stated by Mr. Lowell himself, do _not_ render
even probable much less prove. Incidentally, I may be able to adduce
evidence of a more or less weighty character, which seems to negative
the possibility of any high form of animal life on Mars, and, _a
fortiori_, the development of such life as might culminate in a being
equal or superior to ourselves. As most popular works on Astronomy for
the last ten years at least, as well as many scientific periodicals and
popular magazines, have reproduced some of the maps of Mars by
Schiaparelli, Lowell, and others, the general appearance of its surface
will be familiar to most readers, who will thus be fully able to
appreciate Mr. Lowell's account of his own further discoveries which I
may have to quote. One of the _best_ of these maps I am able to give as
a frontispiece to this volume, and to this I shall mainly refer.

[Footnote 2: _Man's Place in the Universe_ p. 267 (1903).]

_The Canals as described by Mr. Lowell._

In the clear atmosphere of Arizona, Mr. Lowell has been able on various
favourable occasions to detect a network of straight lines, meeting or
crossing each other at various angles, and often extending to a thousand
or even over two thousand miles in length. They are seen to cross both
the light and the dark regions of the planet's surface, often extending
up to or starting from the polar snow-caps. Most of these lines are so
fine as only to be visible on special occasions of atmospheric clearness
and steadiness, which hardly ever occur at lowland stations, even with
the best instruments, and almost all are seen to be as perfectly
straight as if drawn with a ruler.

_The Double Canals._

Under exceptionally favourable conditions, many of the lines that have
been already seen single appear double--a pair of equally fine lines
exactly parallel throughout their whole length, and appearing, as Mr.
Lowell says, "clear cut upon the disc, its twin lines like the rails of
a railway track." Both Schiaparelli and Lowell were at first so
surprised at this phenomenon that they thought it must be an optical
illusion, and it was only after many observations in different years,
and by the application of every conceivable test, that they both became
convinced that they witnessed a real feature of the planet's surface.
Mr. Lowell says he has now seen them hundreds of times, and that his
first view of one was 'the most startlingly impressive' sight he has
ever witnessed.

_Dimensions of the Canals._

A few dimensions of these strange objects must be given in order that
readers may appreciate their full strangeness and inexplicability. Out
of more than four hundred canals seen and recorded by Mr. Lowell,
fifty-one, or about one eighth, are either constantly or occasionally
seen to be double, the appearance of duplicity being more or less
periodical. Of 'canals' generally, Mr. Lowell states that they vary in
length from a few hundred to a few thousand miles long, one of the
largest being the Phison, which he terms 'a typical double canal,' and
which is said to be 2250 miles long, while the distance between its two
constituents is about 130 miles.[3] The actual width of each canal is
from a minimum of about a mile up to several miles, in one case over
twenty. A great feature of the doubles is, that they are strictly
parallel throughout their whole course, and that in almost all cases
they are so truly straight as to form parts of a great circle of the
planet's sphere. A few however follow a gradual but very distinct curve,
and such of these as are double present the same strict parallelism as
those which are straight.

[Footnote 3: This is on the opposite side of Mars from that shown in the
frontispiece.]

_Canals extend across the Seas._

It was only after seventeen years of observation of the canals that it
was found that they extended also into and across the dark spots and
surfaces which by the earlier observers were termed seas, and which then
formed the only clearly distinguishable and permanent marks on the
planet's surface. At the present time, Professor Lowell states that this
"curious triangulation has been traced over almost every portion of the
planet's surface, whether dark or light, whether greenish, ochre, or
brown in colour." In some parts they are much closer together than in
others, "forming a perfect network of lines and spots, so that to
identify them all was a matter of extreme difficulty." Two such portions
are figured at pages 247 and 256 of Mr. Lowell's volume.

_The Oases._

The curious circular black spots which are seen at the intersections of
many of the canals, and which in some parts of the surface are very
numerous, are said to be more difficult of detection than even the
lines, being often blurred or rendered completely invisible by slight
irregularities in our own atmosphere, while the canals themselves
continue visible. About 180 of these have now been found, and the more
prominent of them are estimated to vary from 75 to 100 miles in
diameter. There are however many much smaller, down to minute and barely
visible black points. Yet they all seem a little larger than the canals
which enter them. Where the canals are double, the spots (or 'oases' as
Mr. Lowell terms them) lie between the two parallel canals.

No one can read this book without admiration for the extreme
perseverance in long continued and successful observation, the results
of which are here recorded; and I myself accept unreservedly the
substantial accuracy of the whole series. It must however always be
remembered that the growth of knowledge of the detailed markings has
been very gradual, and that much of it has only been seen under very
rare and exceptional conditions. It is therefore quite possible that, if
at some future time a further considerable advance in instrumental power
should be made, or a still more favourable locality be found, the new
discoveries might so modify present appearances as to render a
satisfactory explanation of them more easy than it is at present.

But though I wish to do the fullest justice to Mr. Lowell's technical
skill and long years of persevering work, which have brought to light
the most complex and remarkable appearances that any of the heavenly
bodies present to us, I am obliged absolutely to part company with him
as regards the startling theory of artificial production which he thinks
alone adequate to explain them. So much is this the case, that the very
phenomena, which to him seem to demonstrate the intervention of
intelligent beings working for the improvement of their own environment,
are those which seem to me to bear the unmistakable impress of being due
to natural forces, while they are wholly unintelligible as being useful
works of art. I refer of course to the great system of what are termed
'canals,' whether single or double. Of these I shall give my own
interpretation later on.



CHAPTER III.


THE CLIMATE AND PHYSIOGRAPHY OF MARS.

Mr. Lowell admits, and indeed urges strongly, that there are no
permanent bodies of water on Mars; that the dark spaces and spots,
thought by the early observers to be seas, are certainly not so now,
though they may have been at an earlier period; that true clouds are
rare, even if they exist, the appearances that have been taken for them
being either dust-storms or a surface haze; that there is consequently
no rain, and that large portions (about two-thirds) of the planet's
surface have all the characteristics of desert regions.

_Snow-caps the only Source of Water._

This state of things is supposed to be ameliorated by the fact of the
polar snows, which in the winter cover the arctic and about half the
temperate regions of each hemisphere alternately. The maximum of the
northern snow-caps is reached at a period of the Martian winter
corresponding to the end of February with us. About the end of March the
cap begins to shrink in size (in the Northern Hemisphere), and this goes
on so rapidly that early in the June of Mars it is reduced to its
minimum. About the same time changes of colour take place in the
adjacent darker portions of the surface, which become at first bluish,
and later a decided blue-green; but by far the larger portion, including
almost all the equatorial regions of the planet, remain always of a
reddish-ochre tint.[4]

[Footnote 4: In 1890 at Mount Wilson, California, Mr. W.H. Pickering's
photographs of Mars on April 9th showed the southern polar cap of
moderate dimensions, but with a large dim adjacent area. Twenty-four
hours later a corresponding plate showed this same area brilliantly
white; the result apparently of a great Martian snowfall. In 1882 the
same observer witnessed the steady disappearance of 1,600,000 square
miles of the southern snow-cap, an area nearly one-third of that
hemisphere of the planet.]

The rapid and comparatively early disappearance of the white covering
is, very reasonably, supposed to prove that it is of small thickness,
corresponding perhaps to about a foot or two of snow in north-temperate
America and Europe, and that by the increasing amount of sun-heat it is
converted, partly into liquid and partly into vapour. Coincident with
this disappearance and as a presumed result of the water (or other
liquid) producing inundations, the bluish-green tinge which appears on
the previously dark portion of the surface is supposed to be due to a
rapid growth of vegetation.

But the evidence on this point does not seem to be clear or harmonious,
for in the four  plates showing the planet's surface at
successive Martian dates from December 30th to February 21st, not only
is a considerable extent of the south temperate zone shown to change
rapidly from bluish-green to chocolate-brown and then again to
bluish-green, but the portions furthest from the supposed fertilising
overflow are permanently green, as are also considerable portions in the
opposite or northern hemisphere, which one would think would then be
completely dried up.

_No Hills upon Mars._

The special point to which I here wish to call attention is this. Mr.
Lowell's main contention is, that the surface of Mars is wonderfully
smooth and level. Not only are there no mountains, but there are no
hills or valleys or plateaux. This assumption is absolutely essential to
support the other great assumption, that the wonderful network of
perfectly straight lines over nearly the whole surface of the planet are
irrigation canals. It is not alleged that irregularities or undulations
of a few hundreds or even one or two thousands of feet could possibly be
detected, while certainly all we know of planetary formation or
structure point strongly towards _some_ inequalities of surface. Mr.
Lowell admits that the dark portions of the surface, when examined on
the terminator (the margin of the illuminated portion), do _look_ like
hollows and _may be_ the beds of dried-up seas; yet the supposed canals
run across these old sea-beds in perfect straight lines just as they do
across the many thousand miles of what are admitted to be deserts--which
he describes in these forcible terms: "Pitiless as our deserts are, they
are but faint forecasts of the state of things existent on Mars at the
present time."

It appears, then, that Mr. Lowell has to face this dilemma--_Only if the
whole surface of Mars is an almost perfect level could the enormous
network of straight canals, each from hundreds to thousands of miles
long, have been possibly constructed by intelligent beings for purposes
of irrigation; but, if a complete and universal level surface exists no
such system would be necessary._ For on a level surface--or on a
surface slightly inclined from the poles towards the equator, which
would be advantageous in either case--the melting water would of itself
spread over the ground and naturally irrigate as much of the surface as
it was possible for it to reach. If the surface were not level, but
consisted of slight elevations and expressions to the extent of a few
scores or a few hundreds of feet, then there would be no possible
advantage in cutting straight troughs through these elevations in
various directions with water flowing at the bottom of them. In neither
case, and in hardly any conceivable case, could these perfectly straight
canals, cutting across each other in every direction and at very varying
angles, be of any use, or be the work of an intelligent race, if any
such race could possibly have been developed under the adverse
conditions which exist in Mars.

_The Scanty Water-supply._

But further, if there were any superfluity of water derived from the
melting snow beyond what was sufficient to moisten the hollows indicated
by the darker portions of the surface, which at the time the water
reaches them acquire a green tint (a superfluity under the circumstances
highly improbable), that superfluity could be best utilised by widening,
however little, the borders to which natural overflow had carried it.
Any attempt to make that scanty surplus, by means of overflowing canals,
travel across the equator into the opposite hemisphere, through such a
terrible desert region and exposed to such a cloudless sky as Mr. Lowell
describes, would be the work of a body of madmen rather than of
intelligent beings. It may be safely asserted that not one drop of water
would escape evaporation or insoak at even a hundred miles from its
source. [5]

[Footnote 5: What the evaporation is likely to be in Mars may be
estimated by the fact, stated by Professor J.W. Gregory in his recent
volume on 'Australia' in _Stanford's Compendium_, that in North-West
Victoria evaporation is at the rate of ten feet per annum, while in
Central Australia it is very much more. The greatly diminished
atmospheric pressure in Mars will probably more than balance the loss of
sun-heat in producing rapid evaporation.]

_Miss Clerke on the Scanty Water-supply._

On this point I am supported by no less an authority than the historian
of modern astronomy, the late Miss Agnes Clerke. In the _Edinburgh
Review_ (of October 1896) there is an article entitled 'New Views about
Mars,' exhibiting the writer's characteristic fulness of knowledge and
charm of style. Speaking of Mr. Lowell's idea of the 'canals' carrying
the surplus water across the equator, far into the opposite hemisphere,
for purposes of irrigation there (which we see he again states in the
present volume), Miss Clerke writes: "We can hardly imagine so shrewd a
people as the irrigators of Thule and Hellas[6] wasting labour, and the
life-giving fluid, after so unprofitable a fashion. There is every
reason to believe that the Martian snow-caps are quite flimsy
structures. Their material might be called snow _souffle_, since, owing
to the small power of gravity on Mars, snow is almost three times
lighter there than here. Consequently, its own weight can have very
little effect in rendering it compact. Nor, indeed, is there time for
much settling down. The calotte does not form until several months after
the winter solstice, and it begins to melt, as a rule, shortly after the
vernal equinox. (The interval between these two epochs in the southern
hemisphere of Mars is 176 days.) The snow lies on the ground, at the
outside, a couple of months. At times it melts while it is still fresh
fallen. Thus, at the opposition of 1881-82 the spreading of the northern
snows was delayed until seven weeks after the equinox: and they had,
accordingly, no sooner reached their maximum than they began to decline.
And Professor Pickering's photographs of April 9th and 10th, 1890,
proved that the southern calotte may assume its definitive proportions
in a single night.

[Footnote 6: Areas on Mars so named.]

"No attempt has yet been made to estimate the quantity of water
derivable from the melting of one of these formations; yet the
experiment is worth trying as a help towards defining ideas. Let us
grant that the average depth of snow in them, of the delicate Martian
kind, is twenty feet, equivalent at the most to one foot of water. The
maximum area covered, of 2,400,000 square miles, is nearly equal to that
of the United States, while the whole globe of Mars measures 55,500,000
square miles, of which one-third, on the present hypothesis, is under
cultivation, and in need of water. Nearly the whole of the dark areas,
as we know, are situated in the southern hemisphere, of which they
extend over, at the very least, 17,000,000 square miles; that is to say,
they cover an area, in round numbers, seven times that of the snow-cap.
Only one-seventh of a foot of water, accordingly, could possibly be
made available for their fertilisation, supposing them to get the entire
advantage of the spring freshet. Upon a stint of less than two inches of
water these fertile lands are expected to flourish and bear abundant
crops; and since they completely enclose the polar area they are
necessarily served first. The great emissaries for carrying off the
surplus of their aqueous riches, would then appear to be superfluous
constructions, nor is it likely that the share in those riches due to
the canals and oases, intricately dividing up the wide, dry, continental
plains, can ever be realised.

"We have assumed, in our little calculation, that the entire contents of
a polar hood turn to water; but in actual fact a considerable proportion
of them must pass directly into vapour, omitting the intermediate stage.
Even with us a large quantity of snow is removed aerially; and in the
rare atmosphere of Mars this cause of waste must be especially
effective. Thus the polar reservoirs are despoiled in the act of being
opened. Further objections might be taken to Mr. Lowell's irrigation
scheme, but enough has been said to show that it is hopelessly
unworkable."

It will be seen that the writer of this article accepted the existence
of water on Mars, on the testimony of Sir W. Huggins, which, in view of
later observations, he has himself acknowledged to be valueless. Dr.
Johnstone Stoney's proof of its absence, derived from the molecular
theory of gases, had not then been made public.

_Description of some of the Canals._

At the end of his volume Mr. Lowell gives a large chart of Mars on
Mercator's projection, showing the canals and other features seen during
the opposition of 1905. This contains many canals not shown on the map
here reproduced (see frontispiece), and some of the differences between
the two are very puzzling. Looking at our map, which shows the
north-polar snow below, so that the south pole is out of the view at the
top of the map, the central feature is the large spot Ascraeeus Lucus,
from which ten canals diverge centrally, and four from the sides,
forming wide double canals, fourteen in all. There is also a canal named
Ulysses, which here passes far to the right of the spot, but in the
large chart enters it centrally. Looking at our map we see, going
downwards a little to the left, the canal Udon, which runs through a
dark area quite to the outer margin. In the dark area, however, there is
shown on the chart a spot Aspledon Lucus, where five canals meet, and if
this is taken as a terminus the Udon canal is almost exactly 2000 miles
long, and another on its right, Lapadon, is the same length, while Ich,
running in a slightly curved line to a large spot (Lucus Castorius on
the chart) is still longer. The Ulysses canal, which (on the chart) runs
straight from the point of the Mare Sirenum to the Astraeeus Lucus is
about 2200 miles long. Others however are even longer, and Mr. Lowell
says: "With them 2000 miles is common; while many exceed 2500; and the
Eumenides-Orcus is 3540 miles from the point where it leaves Lucus
Phoeniceus to where it enters the Trivium Charontis." This last canal is
barely visible on our map, its commencement being indicated by the word
Eumenides.

The Trivium Charontis is situated just beyond the right-hand margin of
our map. It is a triangular dark area, the sides about 200 miles long,
and it is shown on the chart as being the centre from which radiate
thirteen canals. Another centre is Aquae Calidae situated at the point
of a dark area running obliquely from 55 deg. to 35 deg. N. latitude, and, as
shown on a map of the opposite hemisphere to our map, has nearly twenty
canals radiating from it in almost every direction. Here at all events
there seems to be no special connection with the polar snow-caps, and
the radiating lines seem to have no intelligent purpose whatever, but
are such as might result from fractures in a glass globe produced by
firing at it with very small shots one at a time. Taking the whole
series of them, Mr. Lowell very justly compares them to "a network which
triangulates the surface of the planet like a geodetic survey, into
polygons of all shapes and sizes."

At the very lowest estimate the total length of the canals observed and
mapped by Mr. Lowell must be over a hundred thousand miles, while he
assures us that numbers of others have been seen over the whole surface,
but so faintly or on such rare occasions as to elude all attempts to fix
their position with certainty. But these, being of the same character
and evidently forming part of the same system, must also be artificial,
and thus we are led to a system of irrigation of almost unimaginable
magnitude on a planet which has no mountains, no rivers, and no rain to
support it; whose whole water-supply is derived from polar snows, the
amount of which is ludicrously inadequate to need any such world-wide
system; while the low atmospheric pressure would lead to rapid
evaporation, thus greatly diminishing the small amount of moisture that
is available. Everyone must, I think, agree with Miss Clerke, that, even
admitting the assumption that the polar snows consist of frozen water,
the excessively scanty amount of water thus obtained would render any
scheme of world-wide distribution of it hopelessly unworkable.

The very remarkable phenomena of the duplication of many of the lines,
together with the darkspots--the so-called oases--at their
intersections, are doubtless all connected in some unknown way with the
constitution and past history of the planet; but, on the theory of the
whole being works of art, they certainly do _not_ help to remove any of
the difficulties which have been shown to attend the theory that the
single lines represent artificial canals of irrigation with a strip of
verdure on each side of them produced by their overflow.

_Lowell on the Purpose of the Canals._

Before leaving this subject it will be well to quote Mr. Lowell's own
words as to the supposed perfectly level surface of Mars, and his
interpretation of the origin and purpose of the 'canals':

"A body of planetary size, if unrotating, becomes a sphere, except for
solar tidal deformation; if rotating, it takes on a spheroidal form
exactly expressive, so far as observation goes, of the so-called
centrifugal force at work. Mars presents such a figure, being flattened
out to correspond to its axial rotation. Its surface therefore is in
fluid equilibrium, or, in other words, a particle of liquid at any point
of its surface at the present time would stay where it was devoid of
inclination to move elsewhere. Now the water which quickens the verdure
of the canals moves from the pole down to the equator as the season
advances. This it does then irrespective of gravity. No natural force
propels it, and the inference is forthright and inevitable that it is
artificially helped to its end. There seems to be no escape from this
deduction. Water only flows downhill, and there is no such thing as
downhill on a surface already in fluid equilibrium. A few canals might
presumably be so situated that their flow could, by inequality of
terrane, lie equatorward, but not all....Now it is not in particular but
by general consent that the canal-system of Mars develops from pole to
equator. From the respective times at which the minima take place, it
appears that the canal quickening occupies fifty-two days, as evidenced
by the successive vegetal darkenings, to descend from latitude 72 deg. north
to latitude 0 deg., a journey of 2650 miles. This gives for the water a
speed of fifty-one miles a day, or 2.1 miles an hour. The rate of
progression is remarkably uniform, and this abets the deduction as to
assisted transference. But the fact is more unnatural yet. The growth
pays no regard to the equator, but proceeds across it as if it did not
exist into the planet's other hemisphere. Here is something still more
telling than travel to this point. For even if we suppose, for the sake
of argument, that natural forces took the water down to the equator,
their action must there be certainly reversed, and the equator prove a
dead-line, to pass which were impossible" (pp. 374-5).

I think my readers will agree with me that this whole argument is one of
the most curious ever put forth seriously by an eminent man of science.
Because the polar compression of Mars is about what calculation shows it
ought to be in accordance with its rate of rotation, its surface is in a
state of 'fluid equilibrium,' and must therefore be absolutely level
throughout. But the polar compression of the earth equally agrees with
calculation; therefore its surface is also in 'fluid equilibrium';
therefore it also ought to be as perfectly level on land as it is on the
ocean surface! But as we know this is very far from being the case, why
must it be so in Mars? Are we to suppose Mars to have been formed in
some totally different way from other planets, and that there neither is
nor ever has been any reaction between its interior and exterior forces?
Again, the assumption of perfect flatness is directly opposed to all
observation and all analogy with what we see on the earth and moon. It
gives no account whatever of the numerous and large dark patches, once
termed seas, but now found to be not so, and to be full of detailed
markings and varied depths of shadow. To suppose that these are all the
same dead-level as the light- portions are assumed to be,
implies that the darkness is one of material and colour only, not of
diversified contour, which again is contrary to experience, since
difference of material with us always leads to differences in rate of
degradation, and hence of diversified contour, as these dark spaces
actually show themselves under favourable conditions to independent
observers.

_Lowell on the System of Canals as a whole._

We will now see what Mr. Lowell claims to be the plain teaching of the
'canals' as a whole:

"But last and all-embracing in its import is the system which the canals
form. Instead of running at hap-hazard, the canals are interconnected in
a most remarkable manner. They seek centres instead of avoiding them.
The centres are linked thus perfectly one with another, an arrangement
which could not result from centres, whether of explosion or otherwise,
which were themselves discrete. Furthermore, the system covers the whole
surface of the planet, dark areas and light ones alike, a world-wide
distribution which exceeds the bounds of natural possibility. Any force
which could act longitudinally on such a scale must be limited
latitudinally in its action, as witness the belts of Jupiter and the
spots upon the sun. Rotational, climatic, or other physical cause could
not fail of zonal expression. Yet these lines are grandly indifferent to
such competing influences. Finally, the system, after meshing the
surface in its entirety, runs straight into the polar caps.

"It is, then, a system whose end and aim is the tapping of the snow-cap
for the water there semi-annually let loose; then to distribute it over
the planet's face" (p. 373).

Here, again, we have curiously weak arguments adduced to support the
view that these numerous straight lines imply works of art rather than
of nature, especially in the comparison made with the belts of Jupiter
and the spots on the sun, both purely atmospheric phenomena, whereas the
lines on Mars are on the solid surface of the planet. Why should there
be any resemblance between them? Every fact stated in the above
quotation, always keeping in mind the physical conditions of the
planet--its very tenuous atmosphere and rainless desert-surface--seem
wholly in favour of a purely natural as opposed to an artificial origin;
and at the close of this discussion I shall suggest one which seems to
me to be at least possible, and to explain the whole series of the
phenomena set forth and largely discovered by Mr. Lowell, in a simpler
and more probable manner than does his tremendous assumption of their
being works of art. Readers who may not possess Mr. Lowell's volume will
find three of his most recent maps of the 'canals' reproduced in
_Nature_ of October 11th, 1906.



CHAPTER IV.


IS ANIMAL LIFE POSSIBLE ON MARS?

Having now shown, that, even admitting the accuracy of all Mr. Lowell's
observations, and provisionally accepting all his chief conclusions as
to the climate, the nature of the snow-caps, the vegetation, and the
animal life of Mars, yet his interpretation of the lines on its surface
as being veritably 'canals,' constructed by intelligent beings for the
special purpose of carrying water to the more arid regions, is wholly
erroneous and rationally inconceivable. I now proceed to discuss his
more fundamental position as to the actual habitability of Mars by a
highly organised and intellectual race of material organic beings.

_Water and Air essential to Life._

Here, fortunately, the issue is rendered very simple, because Mr. Lowell
fully recognises the identity of the constitution of matter and of
physical laws throughout the solar-system, and that for any high form of
organic life certain conditions which are absolutely essential on our
earth must also exist in Mars. He admits, for example, that water is
essential, that an atmosphere containing oxygen, nitrogen, aqueous
vapour, and carbonic acid gas is essential, and that an abundant
vegetation is essential; and these of course involve a
surface-temperature through a considerable portion of the year that
renders the existence of these--especially of water--possible and
available for the purposes of a high and abundant animal life.

_Blue Colour the only Evidence of Water._

In attempting to show that these essentials actually exist on Mars he is
not very successful. He adduces evidence of an atmosphere, but of an
exceedingly scanty one, since the greatest amount he can give to it is--
"not more than about four inches of barometric pressure as we reckon
it";[7] and he assumes, as he has a fair right to do till disproved,
that it consists of oxygen and nitrogen, with carbon-dioxide and
water-vapour, in approximately the same proportions as with us. With
regard to the last item--the water-vapour--there are however many
serious difficulties. The water-vapour of our atmosphere is derived from
the enormous area of our seas, oceans, lakes, and rivers, as well as
from the evaporation from heated lands and tropical forests of much of
the moisture produced by frequent and abundant rains. All these sources
of supply are admittedly absent from Mars, which has no permanent bodies
of water, no rain, and tropical regions which are almost entirely
desert. Many writers have therefore doubted the existence of water in
any form upon this planet, supposing that the snow-caps are not formed
of frozen water but of carbon-dioxide, or some other heavy gas, in a
frozen state; and Mr. Lowell evidently feels this to be a difficulty,
since the only fact he is able to adduce in favour of the melting snows
of the polar caps producing water is, that at the time they are melting
a marginal blue band appears which accompanies them in their retreat,
and this blue colour is said to prove conclusively that the liquid is
not carbonic acid but water. This point he dwells upon repeatedly,
stating, of these blue borders: "This excludes the possibility of their
being formed by carbon-dioxide, and shows that of all the substances we
know the material composing them must be water."

[Footnote 7: In a paper written since the book appeared the density of
air at the surface of Mars is said to be 1/12 of the earth's.]

This is the only proof of the existence of _water_ he adduces, and it is
certainly a most extraordinary and futile one. For it is perfectly well
known that although water, in large masses and by transmitted light, is
of a blue colour, yet shallow water by reflected light is not so; and in
the case of the liquid produced by the snow-caps of Mars, which the
whole conditions of the planet show must be shallow, and also be more or
less turbid, it cannot possibly be the cause of the 'deep blue' tint
said to result from the melting of the snow.

But there is a very weighty argument depending on the molecular theory
of gases against the polar caps of Mars being composed of frozen water
at all. The mass and elastic force of the several gases is due to the
greater or less rapidity of the vibratory motion of their molecules
under identical conditions. The speed of these molecular motions has
been ascertained for all the chief gases, and it is found to be so great
as in certain cases to enable them to overcome the force of gravity and
escape from a planet's surface into space. Dr. G. Johnstone Stoney has
specially investigated this subject, and he finds that the force of
gravity on the earth is sufficient to retain all the gases composing its
atmosphere, but not sufficient to retain hydrogen; and as a consequence,
although this gas is produced in small quantities by volcanoes and by
decomposing vegetation, yet no trace of it is found in our atmosphere.
The moon however, having only one-eightieth the mass of the earth,
cannot retain any gas, hence its airless and waterless condition.

_Water Vapour cannot exist on Mars._

Now, Dr. Stoney finds that in order to retain water vapour permanently a
planet must have a mass at least a quarter that of the earth. But the
mass of Mars is only one-ninth that of the earth; therefore, unless
there are some special conditions that prevent its loss, this gas cannot
be present in the atmosphere. Mr. Lowell does not refer to this argument
against his view, neither does he claim the evidence of spectroscopy in
his favour. This was alleged more than thirty years ago to show the
existence of water-vapour in the atmosphere of Mars, but of late years
it has been doubted, and Mr. Lowell's complete silence on the subject,
while laying stress on such a very weak and inconclusive argument as
that from the tinge of colour that is observed a little distance from
the edge of the diminishing snow-caps, shows that he himself does not
think the fact to be thus proved. If he did he would hardly adduce such
an argument for its presence as the following: "The melting of the caps
on the one hand and their re-forming on the other affirm the presence of
water-vapour in the Martian atmosphere, of whatever else that air
consists" (p. 162). Yet absolutely the only proof he gives that the caps
are frozen water is the almost frivolous colour-argument above referred
to!

_No Spectroscopic Evidence of Water Vapour._

As Sir William Huggins is the chief authority quoted for this fact, and
is referred to as being almost conclusive in the third edition of Miss
Clerke's _History of Astronomy_ in 1893, I have ascertained that his
opinion at the present time is that "there is no conclusive proof of the
presence of aqueous vapour in the atmosphere of Mars, and that
observations at the Lick Observatory (in 1895), where the conditions and
instruments are of the highest order, were negative." He also informs me
that Marchand at the Pic du Midi Observatory was unable to obtain lines
of aqueous vapour in the spectrum of Mars; and that in 1905, Slipher, at
Mr. Lowell's observatory, was unable to detect any indications of
aqueous vapour in the spectrum of Mars.

It thus appears that spectroscopic observations are quite accordant with
the calculations founded on the molecular theory of gases as to the
absence of aqueous vapour, and therefore presumably of liquid water,
from Mars. It is true that the spectroscopic argument is purely
negative, and this may be due to the extreme delicacy of the
observations required; but that dependent on the inability of the force
of gravity to retain it is positive scientific evidence against its
presence, and, till shown to be erroneous, must be held to be
conclusive.

This absence of water is of itself conclusive against the existence of
animal life, unless we enter the regions of pure conjecture as to the
possibility of some other liquid being able to take its place, and that
liquid being as omnipresent there as water is here. Mr. Lowell however
never takes this ground, but bases his whole theory on the fundamental
identity of the substance of the bodies of living organisms wherever
they may exist in the solar system. In the next two chapters I shall
discuss an equally essential condition, that of temperature, which
affords a still more conclusive and even crushing argument against the
suitability of Mars for the existence of organic life.



CHAPTER V.


THE TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE.

We have now to consider a still more important and fundamental question,
and one which Mr. Lowell does not grapple with in this volume, the
actual temperatures on Mars due to its distance from the sun and the
atmospheric conditions on which he himself lays so much stress. If I am
not greatly mistaken we shall arrive at conclusions on this subject
which are absolutely fatal to the conception of any high form of organic
life being possible on this planet.

_The Problem of Terrestrial Temperatures._

In order that the problem may be understood and its importance
appreciated, it is necessary to explain the now generally accepted
principles as to the causes which determine the temperatures on our
earth, and, presumably, on all other planets whose conditions are not
wholly unlike ours. The fact of the internal heat of the earth which
becomes very perceptible even at the moderate depths reached in mines
and deep borings, and in the deepest mines becomes a positive
inconvenience, leads many people to suppose that the surface-
temperatures of the earth are partly due to this cause. But it is now
generally admitted that this is not the case, the reason being that all
rocks and soils, in their natural compacted state, are exceedingly bad
conductors of heat.

A striking illustration of this is the fact, that a stream of lava often
continues to be red hot at a few feet depth for years after the surface
is consolidated, and is hardly any warmer than that of the surrounding
land. A still more remarkable case is that of a glacier on the
south-east side of the highest cone of Etna underneath a lava stream
with an intervening bed of volcanic sand only ten feet thick. This was
visited by Sir Charles Lyell in 1828, and a second time thirty years
later, when he made a very careful examination of the strata, and was
quite satisfied that the sand and the lava stream together had actually
preserved this mass of ice, which neither the heat of the lava above it
at its first outflow, nor the continued heat rising from the great
volcano below it, had been able to melt or perceptibly to diminish in
thirty years. Another fact that points in the same direction is the
existence over the whole floor of the deepest oceans of ice-cold water,
which, originating in the polar seas, owing to its greater density sinks
and creeps slowly along the ocean bottom to the depths of the Atlantic
and Pacific, and is not perceptibly warmed by the internal heat of the
earth.

Now the solid crust of the earth is estimated as at least about twenty
miles in average thickness; and, keeping in mind the other facts just
referred to, we can understand that the heat from the interior passes
through it with such extreme slowness as not to be detected at the
surface, the varying temperatures of which are due entirely to solar
heat. A large portion of this heat is stored up in the surface soil, and
especially in the surface layer of the oceans and seas, thus partly
equalising the temperatures of day and night, of winter and summer, so
as greatly to ameliorate the rapid changes of temperature that would
otherwise occur. Our dense atmosphere is also of immense advantage to us
as an equaliser of temperature, charged as it almost always is with a
large quantity of water-vapour. This latter gas, when not condensed into
cloud, allows the solar heat to pass freely to the earth; but it has the
singular and highly beneficial property of absorbing and retaining the
dark or lower-grade heat given off by the earth which would otherwise
radiate into space much more rapidly. We must therefore always remember
that, very nearly if not quite, the _whole_ of _the warmth we experience
on the earth is derived from the sun._[8]

[Footnote 8: Professor J.H. Poynting, in his lecture to the British
Association at Cambridge in 1904, says: "The surface of the earth
receives, we know, an amount of heat from the inside almost
infinitesimal compared with that which it receives from the sun, and on
the sun, therefore, we depend for our temperature."]

In order to understand the immense significance of this conclusion we
must know what is meant by the _whole_ heat or warmth; as unless we know
this we cannot define what half or any other proportion of sun-heat
really means. Now I feel pretty sure that nine out of ten of the average
educated public would answer the following question incorrectly: The
mean temperature of the southern half of England is about 48 deg. F.
Supposing the earth received only half the sun-heat it now receives,
what would then be the probable mean temperature of the South of
England? The majority would, I think, answer at once--About 24 deg. F.
Nearly as many would perhaps say--48 deg. F. is 16 deg. above the freezing
point; therefore half the heat received would bring us down to 8 deg. above
the freezing point, or 40 deg. F. Very few, I think, would realise that our
share of half the amount of sun-heat received by the earth would
probably result in reducing our mean temperature to about 100 deg. F. below
the freezing point, and perhaps even lower. This is about the very
lowest temperature yet experienced on the earth's surface. To understand
how such results are obtained a few words must be said about the
absolute zero of temperature.

_The Zero of Temperature._

Heat is now believed to be entirely due to ether-vibration, which
produces a correspondingly rapid vibration of the molecules of matter,
causing it to expand and producing all the phenomena we term 'heat.' We
can conceive this vibration to increase indefinitely, and thus there
would appear to be no necessary limit to the amount of heat possible,
but we cannot conceive it to decrease indefinitely at the same uniform
rate, as it must soon inevitably come to nothing. Now it has been found
by experiment that gases under uniform pressure expand 1/273 of their
volume for each degree Centigrade of increased temperature, so that in
passing from 0 deg. C. to 273 deg. C. they are doubled in volume. They also
decrease in volume at the same rate for each degree below 0 deg. C. (the
freezing point of water). Hence if this goes on to-273 deg. C. a gas will
have no volume, or it will undergo some change of nature. Hence this is
called the zero of temperature, or the temperature to which any matter
falls which receives no heat from any other matter. It is also sometimes
called the temperature of space, or of the ether in a state of rest, if
that is possible. All the gases have now been proved to become, first
liquid and then (most of them) solid, at temperatures considerably above
this zero.

The only way to compare the proportional temperatures of bodies, whether
on the earth or in space, is therefore by means of a scale beginning at
this natural zero, instead of those scales founded on the artificial
zero of the freezing point of water, or, as in Fahrenheit's, 32 deg. below
it. Only by using the natural zero and measuring continuously from it
can we estimate temperatures in relative proportion to the amount of
heat received. This is termed the absolute zero, and so that we start
reckoning from that point it does not matter whether the scale adopted
is the Centigrade or that of Fahrenheit.

_The Complex Problem of Planetary Temperatures._

Now if, as is the case with Mars, a planet receives only half the amount
of solar heat that we receive, owing to its greater distance from the
sun, and if the mean temperature of our earth is 60 deg. F., this is equal
to 551 deg. F. on the absolute scale. It would therefore appear very simple
to halve this amount and obtain 275.5 deg. F. as the mean temperature of
that planet. But this result is erroneous, because the actual amount of
sun heat intercepted by a planet is only one condition out of many that
determine its resulting temperature. Radiation, that is loss of heat, is
going on concurrently with gain, and the rate of loss varies with the
temperature according to a law recently discovered, the loss being much
greater at high temperatures in proportion to the 4th power of the
absolute temperature. Then, again, the whole heat intercepted by a
planet does not reach its surface unless it has no atmosphere. When it
has one, much is reflected or absorbed according to complex laws
dependent on the density and composition of the atmosphere. Then, again,
the heat that reaches the actual surface is partly reflected and partly
absorbed, according to the nature of that surface--land or water, desert
or forest or snow-clad--that part which is absorbed being the chief
agent in raising the temperature of the surface and of the air in
contact with it. Very important too is the loss of heat by radiation
from these various heated surfaces at different rates; while the
atmosphere itself sends back to the surface an ever varying portion of
both this radiant and reflected heat according to distinct laws. Further
difficulties arise from the fact that much of the sun's heat consists of
dark or invisible rays, and it cannot therefore be measured by the
quantity of light only.

From this rough statement it will be seen that the problem is an
exceedingly complex one, not to be decided off-hand, or by any simple
method. It has in fact been usually considered as (strictly speaking)
insoluble, and only to be estimated by a more or less rough
approximation, or by the method of general analogy from certain known
facts. It will be seen, from what has been said in previous chapters,
that Mr. Lowell, in his book, has used the latter method, and, by taking
the presence of water and water-vapour in Mars as proved by the
behaviour of the snow-caps and the bluish colour that results from their
melting, has deduced a temperature above the freezing point of water, as
prevalent in the equatorial regions permanently, and in the temperate
and arctic zones during a portion of each year.

_Mr. Lowell's Mathematical Investigation of the Problem._

But as this result has been held to be both improbable in itself and
founded on no valid evidence, he has now, in the _London, Edinburgh, and
Dublin Philosophical Magazine_ of July 1907, published an elaborate
paper of 15 pages, entitled _A General Method for Evaluating the
Surface-Temperatures of the Planets; with special reference to the
Temperature of Mars_, by Professor Percival Lowell; and in this paper,
by what purports to be strict mathematical reasoning based on the most
recent discoveries as to the laws of heat, as well as on measurements or
estimates of the various elements and constants used in the
calculations, he arrives at a conclusion strikingly accordant with that
put forward in the recently published volume. Having myself neither
mathematical nor physical knowledge sufficient to enable me to criticise
this elaborate paper, except on a few points, I will here limit myself
to giving a short account of it, so as to explain its method of
procedure; after which I may add a few notes on what seem to me doubtful
points; while I also hope to be able to give the opinions of some more
competent critics than myself.

_Mr. Lowell's Mode of Estimating the Surface-temperature of Mars._

The author first states, that Professor Young, in his _General
Astronomy_ (1898), makes the mean temperature of Mars 223.6 deg. absolute,
by using Newton's law of heat being radiated in proportion to
temperature, and 363 deg. abs. (=-96 deg. F.) by Dulong and Petit's law; but
adds, that a closer determination has been made by Professor Moulton,
using Stefan's law, that radiation is as the _/4th_ power of the
temperature, whence results a mean temperature of-31 deg. F. These estimates
assume identity of atmospheric conditions of Mars and the Earth.

But as none of these estimates take account of the many complex factors
which interfere with such direct and simple calculations, Mr. Lowell
then proceeds to enunciate them, and work out mathematically the effects
they produce:

(1) The whole radiant energy of the sun on striking a planet becomes
divided as follows: Part is reflected back into space, part absorbed by
the atmosphere, part transmitted to the surface of the planet. This
surface again reflects a portion and only the balance left goes to warm
the planet.

(2) To solve this complex problem we are helped by the _albedoes_ or
intrinsic brilliancy of the planets, which depend on the proportion of
the visible rays which are reflected and which determines the
comparative brightness of their respective surfaces. We also have to
find the ratio of the invisible to the visible rays and the heating
power of each.

(3) He then refers to the actinometer and pyroheliometer, instruments
for measuring the actual heat derived from the sun, and also to the
Bolometer, an instrument invented by Professor Langley for measuring the
invisible heat rays, which he has proved to extend to more than three
times the length of the whole heat-spectrum as previously known, and
has also shown that the invisible rays contribute 68 per cent, of the
sun's total energy.[9]

[Footnote 9: For a short account of this remarkable instrument, see my
_Wonderful Century_, new ed., pp. 143-145.]

(4) Then follows an elaborate estimate of the loss of heat during the
passage of the sun's rays through our atmosphere from experiments made
at different altitudes and from the estimated reflective power of the
various parts of the earth's surface--rocks and soil, ocean, forest and
snow--the final result being that three-fourths of the whole sun-heat
is reflected back into space, forming our _albedo_, while only
one-fourth is absorbed by the soil and goes to warm the air and
determine our mean temperature.

(5) We now have another elaborate estimate of the comparative amounts of
heat actually received by Mars and the Earth, dependent on their very
different amounts of atmosphere, and this estimate depends almost wholly
on the comparative _albedoes_, that of Mars, as observed by astronomers
being 0.27, while ours has been estimated in a totally different way as
being 0.75, whence he concludes that nearly three-fourths of the
sun-heat that Mars receives reaches the surface and determines its
temperature, while we get only one-fourth of our total amount. Then by
applying Stefan's law, that the radiation is as the 4th power of the
surface temperature, he reaches the final result that the actual heating
power at the surface of Mars is considerably _more_ than on the Earth,
and would produce a mean temperature of 72 deg. F., if it were not for the
greater conservative or blanketing power of our denser and more
water-laden atmosphere. The difference produced by this latter fact he
minimises by dwelling on the probability of a greater proportion of
carbonic-acid gas and water-vapour in the Martian atmosphere, and thus
brings down the mean temperature of Mars to 48 deg. F., which is almost
exactly the same as that of the southern half of England. He has also,
as the result of observations, reduced the probable density of the
atmosphere of Mars to 2-1/2 inches of mercury, or only one-twelfth of
that of the Earth.

_Critical Remarks on Mr. Lowell's Paper._

The last part of this paper, indicated under pars. 4 and 5, is the most
elaborate, occupying eight pages, and it contains much that seems
uncertain, if not erroneous. In particular, it seems very unlikely that
under a clear sky over the whole earth we should only receive at the
sea-level 0.23 of the solar rays which the earth intercepts (p. 167).
These data largely depend on observations made in California and other
parts of the southern United States, where the lower atmosphere is
exceptionally dust-laden. Till we have similar observations made in the
tropical forest-regions, which cover so large an area, and where the
atmosphere is purified by frequent rains, and also on the prairies and
the great oceans, we cannot trust these very local observations for
general conclusions affecting the whole earth. Later, in the same
article (p. 170), Mr. Lowell says: "Clouds transmit approximately 20 per
cent. of the heat reaching them: a clear sky at sea-level 60 per cent.
As the sky is half the time cloudy the mean transmission is 35 per
cent." These statements seem incompatible with that quoted above.

The figure he uses in his calculations for the actual albedo of the
earth, 0.75, is also not only improbable, but almost self-contradictory,
because the albedo of cloud is 0.72, and that of the great cloud-covered
planet, Jupiter, is given by Lowell as 0.75, while Zollner made it only
0.62. Again, Lowell gives Venus an albedo of 0.92, while Zollner made it
only 0.50 and Mr. Gore 0.65. This shows the extreme uncertainty of these
estimates, while the fact that both Venus and Jupiter are wholly
cloud-covered, while we are only half-covered, renders it almost
certain that our albedo is far less than Mr. Lowell makes it. It is
evident that mathematical calculations founded upon such uncertain data
cannot yield trustworthy results. But this is by no means the only case
in which the data employed in this paper are of uncertain value.
Everywhere we meet with figures of somewhat doubtful accuracy. Here we
have somebody's 'estimate' quoted, there another person's 'observation,'
and these are adopted without further remark and used in the various
calculations leading to the result above quoted. It requires a practised
mathematician, and one fully acquainted with the extensive literature of
this subject, to examine these various data, and track them through the
maze of formulae and figures so as to determine to what extent they
affect the final result.

There is however one curious oversight which I must refer to, as it is a
point to which I have given much attention. Not only does Mr. Lowell
assume, as in his book, that the 'snows' of Mars consist of frozen
water, and that therefore there _is_ water on its surface and
water-vapour in its atmosphere, not only does he ignore altogether Dr.
Johnstone Stoney's calculations with regard to it, which I have already
referred to, but he uses terms that imply that water-vapour is one of
the heavier components of our atmosphere. The passage is at p. 168 of
the _Philosophical Magazine._ After stating that, owing to the very
small barometric pressure in Mars, water would boil at 110 deg. F., he adds:
"The sublimation at lower temperatures would be correspondingly
increased. Consequently the amount of water-vapour in the Martian air
must on that score be relatively greater than our own." Then follows
this remarkable passage: "Carbon-dioxide, because of its greater
specific gravity, would also be in relatively greater amount so far as
this cause is considered. For the planet would part, _caeteris paribus_,
with its lighter gases the quickest. Whence as regards both water-vapour
and carbon-dioxide we have reason to think them in relatively greater
quantity than in our own air at corresponding barometric pressure." I
cannot understand this passage except as implying that 'water-vapour and
carbon-dioxide' are among the heavier and not among the lighter gases of
the atmosphere--those which the planet 'parts with quickest.' But this
is just what water-vapour _is_, being a little less than two-thirds the
weight of air (0.6225), and one of those which the planet _would_ part
with the quickest, and which, according to Dr. Johnstone Stoney, it
loses altogether.
       *       *       *       *       *

Note on Professor Lowell's article in the _Philosophical Magazine_; by
J.H. Poynting, F.R.S., Professor of Physics in the University of
Birmingham.

"I think Professor Lowell's results are erroneous through his neglect of
the heat stored in the air by its absorption of radiation both from the
sun and from the surface. The air thus heated radiates to the surface
and keeps up the temperature. I have sent to the _Philosophical
Magazine_ a paper in which I think it is shown that when the radiation
by the atmosphere is taken into account the results are entirely
changed. The temperature of Mars, with Professor Lowell's data, still
comes out far below the freezing-point--still further below than the
increased distance alone would make it. Indeed, the lower temperature on
elevated regions of the earth's surface would lead us to expect this. I
think it is impossible to raise the temperature of Mars to anything like
the value obtained by Professor Lowell, unless we assume some quality in
his atmosphere entirely different from any found in our own atmosphere."
J.H. POYNTING. October 19, 1907.



CHAPTER VI.


A NEW ESTIMATE OF THE TEMPERATURE OF MARS.

When we are presented with a complex problem depending on a great number
of imperfectly ascertained data, we may often check the results thus
obtained by the comparison of cases in which some of the more important
of these data are identical, while others are at a maximum or a minimum.
In the present case we can do this by a consideration of the Moon as
compared with the Earth and with Mars.

_Langley's Determination of the Moon's Temperature._

In the moon we see the conditions that prevail in Mars both exaggerated
and simplified. Mars has a very scanty atmosphere, the moon none at all,
or if there is one it is so excessively scanty that the most refined
observations have not detected it. All the complications arising from
the possible nature of the atmosphere, and its complex effects upon
reflection, absorption, and radiation are thus eliminated. The mean
distance of the moon from the sun being identical with that of the
earth, the total amount of heat intercepted must also be identical; only
in this case the whole of it reaches the surface instead of one-fourth
only, according to Mr. Lowell's estimate for the earth.

Now, by the most refined observations with his Bolometer, Mr. Langley
was able to determine the temperature of the moon's surface exposed to
undimmed sunshine for fourteen days together; and he found that, even in
that portion of it on which the sun was shining almost vertically, the
temperature rarely rose above the freezing point of water. However
extraordinary this result may seem, it is really a striking confirmation
of the accuracy of the general laws determining temperature which I have
endeavoured to explain in the preceding chapter. For the same surface
which has had fourteen days of sunshine has also had a preceding
fourteen days of darkness, during which the heat which it had
accumulated in its surface layers would have been lost by free radiation
into stellar space. It thus acquires during its day a maximum
temperature of only 491 deg. F. absolute, while its minimum, after 14 days'
continuous radiation, must be very low, and is, with much reason,
supposed to approach the absolute zero.

_Rapid Loss of Heat by Radiation on the Earth._

In order better to comprehend what this minimum may be under extreme
conditions, it will be useful to take note of the effects it actually
produces on the earth in places where the conditions are nearest to
those existing on the moon or on Mars, though never quite equalling, or
even approaching very near them. It is in our great desert regions, and
especially on high plateaux, that extreme aridity prevails, and it is in
such districts that the differences between day and night temperatures
reach their maximum. It is stated by geographers that in parts of the
Great Sahara the surface temperature is sometimes 150 deg. F., while during
the night it falls nearly or quite to the freezing point--a difference
of 118 degrees in little more than 12 hours.[10] In the high desert
plains of Central Asia the extremes are said to be even greater.[11]
Again, in his _Universal Geography_, Reclus states that in the Armenian
Highlands the thermometer oscillates between 13 deg. F. and 112 deg.F. We may
therefore, without any fear of exaggeration, take it as proved that a
fall of 100 deg. F. in twelve or fifteen hours not infrequently occurs where
there is a very dry and clear atmosphere permitting continuous
insolation by day and rapid radiation by night.

[Footnote 10: Keith Johnston's 'Africa' in _Stanford's Compendium._]

[Footnote 11: _Chambers's Encyclopaedia_, Art. 'Deserts.']

Now, as it is admitted that our dense atmosphere, however dry and clear,
absorbs and reflects some considerable portion of the solar heat, we
shall certainly underestimate the radiation from the moon's surface
during its long night if we take as the basis of our calculation a
lowering of temperature amounting to 100 deg. F. during twelve hours, as not
unfrequently occurs with us. Using these data--with Stefan's law of
decrease of radiation as the 4th power of the temperature--a
mathematical friend finds that the temperature of the moon's surface
would be reduced during the lunar night to nearly 200 deg. F. absolute
(equal to-258 deg. F.).

_More Rapid Loss of Heat by the Moon._

Although such a calculation as the above may afford us a good
approximation to the rate of loss of heat by Mars with its very scanty
atmosphere, we have now good evidence that in the case of the moon the
loss is much more rapid. Two independent workers have investigated this
subject with very accordant results--Dr. Boeddicker, with Lord Rosse's
3-foot reflector and a Thermopile to measure the heat, and Mr. Frank
Very, with a glass reflector of 12 inches diameter and the Bolometer
invented by Mr. Langley. The very striking and unexpected fact in which
these observers agree is the sudden disappearance of much of the
stored-up heat during the comparatively short duration of a total
eclipse of the moon--less than two hours of complete darkness, and about
twice that period of partial obscuration.

Dr. Boeddicker was unable to detect any appreciable heat at the period
of greatest obscuration; but, owing to the extreme sensitiveness of the
Bolometer, Mr. Very ascertained that those parts of the surface which
had been longest in the shadow still emitted heat "to the amount of one
per cent. of the heat to be expected from the full moon." This however
is the amount of radiation measured by the Bolometer, and to get the
temperature of the radiating surface we must apply Stefan's law of the
4th power. Hence the temperature of the moon's dark surface will be the
[fourth root of (1 over 100)] = 1 over 3.2 [A] of the highest temperature
 (which we may take at the freezing-point, 491 deg. F. abs.), or 154 deg. F. abs.,
 just below the liquefaction point of air. This is about 50 deg. lower than the
amount found by calculation from our most rapid radiation; and as this
amount is produced in a few hours, it is not too much to expect that,
when continued for more than two weeks (the lunar night), it might reach
a temperature sufficient to liquefy hydrogen (60 deg. F. abs.), or perhaps
even below it.

[Note A: LaTex markup $\root 4  \of {1 \over 100}  = {1 \over 3.2}$  ]

_Theory of the Moon's Origin._

This extremely rapid loss of heat by radiation, at first sight so
improbable as to be almost incredible, may perhaps be to some extent
explained by the physical constitution of the moon's surface, which,
from a theoretical point of view, does not appear to have received the
attention it deserves. It is clear that our satellite has been long
subjected to volcanic eruptions over its whole visible face, and these
have evidently been of an explosive nature, so as to build up the very
lofty cones and craters, as well as thousands of smaller ones, which,
owing to the absence of any degrading or denuding agencies, have
remained piled up as they were first formed.

This highly volcanic structure can, I think, be well explained by an
origin such as that attributed to it by Sir George Darwin, and which has
been so well described by Sir Robert Ball in his small volume, _Time and
Tide._ These astronomers adduce strong evidence that the earth once
rotated so rapidly that the equatorial protuberance was almost at the
point of separation from the planet as a ring. Before this occurred,
however, the tension was so great that one large portion of the
protuberance where it was weakest broke away, and began to move around
the earth at some considerable distance from it. As about 1/50 of the
bulk of the earth thus escaped, it must have consisted of a considerable
portion of the solid crust and a much larger quantity of the liquid or
semi-liquid interior, together with a proportionate amount of the gases
which we know formed, and still form, an important part of the earth's
substance.

As the surface layers of the earth must have been the lightest, they
would necessarily, when broken up by this gigantic convulsion, have come
together to form the exterior of the new satellite, and be soon adjusted
by the forces of gravity and tidal disturbance into a more or less
irregular spheroidal form, all whose interstices and cavities would be
filled up and connected together by the liquid or semi-liquid mass
forced up between them. Thence-forward, as the moon increased its
distance and reduced its time of rotation, in the way explained by Sir
Robert Ball, there would necessarily commence a process of escape of the
imprisoned gases at every fissure and at all points and lines of
weakness, giving rise to numerous volcanic outlets, which, being
subjected only to the small force of lunar gravity (only one-sixth that
of the earth), would, in the course of ages, pile up those gigantic
cones and ridges which form its great characteristic.

But this small gravitative power of the moon would prevent its retaining
on its surface any of the gases forming our atmosphere, which would all
escape from it and probably be recaptured by the earth. By no process of
external aggregation of solid matter to such a relatively small amount
as that forming the moon, even if the aggregation was so violent as to
produce heat enough to cause liquefaction, could any such
long-continued volcanic action arise by gradual cooling, in the absence
of internal gases. There might be fissures, and even some outflows of
molten rock; but without imprisoned gases, and especially without water
and water-vapour producing explosive outbursts, could any such amount of
scoriae and ashes be produced as were necessary for the building up of
the vast volcanic cones, craters, and craterlets we see upon the moon's
surface.

I am not aware that either Sir Robert Ball or Sir George Darwin have
adduced this highly volcanic condition of the moon's surface as a
phenomenon which can _only_ be explained by our satellite having been
thrown off a very much larger body, whose gravitative force was
sufficient to acquire and retain the enormous quantity of gases and of
water which we possess, and which are _absolutely essential_ for that
_special form of cone-building volcanic action_ which the moon exhibits
in so pre-eminent a degree. Yet it seems to me clear, that some such
hypothetical origin for our satellite would have had to be assumed if
Sir George Darwin had not deduced it by means of purely mathematical
argument based upon astronomical facts.

Returning now to the problem of the moon's temperature, I think the
phenomena this presents may be in part due to the mode of formation here
described. For, its entire surface being the result of long-continued
gaseous explosions, all the volcanic products--scoriae, pumice, and
ashes--would necessarily be highly porous throughout; and, never having
been compacted by water-action, as on the earth, and there having been
no winds to carry the finer dust so as to fill up their pores and
fissures, the whole of the surface material to a very considerable depth
must be loose and porous to a high degree. This condition has been
further increased owing to the small power of gravity and the extreme
irregularity of the surface, consisting very largely of lofty cones and
ridges very loosely piled up to enormous heights.

Now this condition of the substance of the moon's surface is such as
would produce a high specific heat, so that it would absorb a large
amount of heat in proportion to the rise of temperature produced, the
heat being conducted downwards to a considerable depth. Owing, however,
to the total absence of atmosphere radiation would very rapidly cool the
surface, but afterwards more slowly, both on account of the action of
Stefan's law and because the heat stored up in the deeper portions could
be carried to the surface by conduction only, and with extreme slowness.

_Very's Researches on the Moon's Heat._

The results of the eclipse observations are supported by the detailed
examination of the surface-temperature of the moon by Mr. Very in his
_Prize Essay on the Distribution of the Moon's Heat_ (published by the
Utrecht Society of Arts and Sciences in 1891). He shows, by a diagram of
the 'Phase-curve,' that at the commencement of the Lunar day the surface
just within the illuminated limb has acquired about 1/7 of its maximum
temperature, or about 70 deg. F. abs. As the surface exposed to the
Bolometer at each observation is about 1/30 of the moon's surface, and
in order to ensure accuracy the instrument has to be directed to a spot
lying wholly within the edge of the moon, it is evident that the surface
measured has already been for several hours exposed to oblique sunshine.
The curve of temperature then rises gradually and afterwards more
rapidly, till it attains its maximum (of about +30 to 40 deg. F.) a few
hours _before_ noon. This, Mr. Very thinks, is due to the fact that the
half of the moon's face first illuminated for us has, on the average, a
darker surface than that of the afternoon, or second quarter, during
which the curve descends not quite so rapidly, the temperature near
sunset being only a little higher than that near sunrise. This rapid
fall while exposed to oblique sunshine is quite in harmony with the
rapid loss of heat during the few hours of darkness during an eclipse,
both showing the prepotency of radiation over insolation on the moon.

Two other diagrams show the distribution of heat at the time of
full-moon, one half of the curve showing the temperatures along the
equator from the edge of the disc to the centre, the other along a
meridian from this centre to the pole. This diagram (here reproduced)
exhibits the quick rise of temperature of the oblique rim of the moon
and the nearly uniform heat of the central half of its surface; the
diminution of heat towards the pole, however, is slower for the first
half and more rapid for the latter portion.

It is an interesting fact that the temperature near the margin of the
full-moon increases towards the centre more rapidly than it does when
the same parts are observed during the early phases of the first
quarter. Mr. Very explains this difference as being due to the fact that
the full-moon to its very edges is fully illuminated, all the shadows of
the ridges and mountains being thrown vertically or obliquely _behind
them._ We thus measure the heat reflected from the _whole_ visible
surface. But at new moon, and somewhat beyond the first quarter, the
deep shadows thrown by the smallest cones and ridges, as well as by the
loftiest mountains, cover a considerable portion of the visible surface,
thus largely reducing the quantity of light and heat reflected or
radiated in our direction. It is only at the full, therefore, that the
maximum temperature of the whole lunar surface can be measured. It must
be considered a proof of the delicacy of the heat-measuring instruments
that this difference in the curves of temperature of the different parts
of the moon's surface and under different conditions is so clearly
shown.

_The Application of the Preceding Results to the Case of Mars._

This somewhat lengthy account of the actual state of the moon's surface
and temperature is of very great importance in our present enquiry,
because it shows us the extraordinary difference in mean and extreme
temperatures of two bodies situated at the same distance from the sun,
and therefore receiving exactly the same amount of solar heat per unit
of surface. We have learned also what are the main causes of this almost
incredible difference, namely: (1) a remarkably rugged surface with
porous and probably cavernous rock-texture, leading to extremely rapid
radiation of heat in the one; as compared with a comparatively even and
well-compacted surface largely clad with vegetation, leading to
comparatively slow and gradual loss by radiation in the other: and (2),
these results being greatly intensified by the total absence of a
protecting atmosphere in the former, while a dense and cloudy atmosphere
with an ever-present supply of water-vapour, accumulates and equalises
the heat received by the latter.

The only other essential difference in the two bodies which may possibly
aid in the production of this marvellous result, is the fact of our day
and night having a mean length of 12 hours, while those of the moon are
about 14-1/2 of our days. But the altogether unexpected fact, in which
two independent enquirers agree, that during the few hours' duration of
a total eclipse of the moon so large a proportion of the heat is lost by
radiation renders it almost certain that the resulting low temperature
would be not very much less if the moon had a day and night the same
length as our own.

The great lesson we learn by this extreme contrast of conditions
supplied to us by nature, as if to enable us to solve some of her
problems, is, the overwhelming importance, first, of a dense and
well-compacted surface, due to water-action and strong gravitative
force; secondly, of a more or less general coat of vegetation; and,
thirdly, of a dense vapour-laden atmosphere. These three favourable
conditions result in a mean temperature of about +60 deg. F. with a range
seldom exceeding 40 deg. above or below it, while over more than half the
land-surface of the earth the temperature rarely falls below the
freezing point. On the other hand, we have a globe of the same materials
and at the same distance from the sun, with a maximum temperature of
freezing water, and a minimum not very far from the absolute zero, the
monthly mean being probably much below the freezing point of
carbonic-acid gas--a difference entirely due to the absence of these
three favourable conditions.

_The Special Features of Mars as influencing Temperature._

Coming now to the special feature of Mars and its probable temperature,
we find that most writers have arrived at a very different conclusion
from that of Mr. Lowell, who himself quotes Mr. Moulton as an authority
who 'recently, by the application of Stefan's law,' has found the mean
temperature of this planet to be-35 deg. F. Again, Professor J.H. Poynting,
in his lecture on 'Radiation in the Solar System,' delivered before the
British Association at Cambridge in 1904, gave an estimate of the mean
temperature of the planets, arrived at from measurements of the sun's
emissive power and the application of Stefan's law to the distances of
the several planets, and he thus finds the earth to have a mean
temperature of 17 deg. C. (=62-1/2 deg. F.) and Mars one of-38 deg. C. (=-36-1/2 deg.
F.), a wonderfully close approximation to the mean temperature of the
earth as determined by direct measurement, and therefore, presumably, an
equally near approximation to that of Mars as dependent on distance from
the sun, and '_on the supposition that it is earth-like in all its
conditions._'

But we know that it is far from being earth-like in the very conditions
which we have found to be those which determine the extremely different
temperatures of the earth, and moon; and, as regards each of these, we
shall find that, so far as it differs from the earth, it approximates to
the less favourable conditions that prevail in the moon. The first of
these conditions which we have found to be essential in regulating the
absorption and radiation of heat, and thus raising the mean temperature
of a planet, is a compact surface well covered with vegetation, two
conditions arising from, and absolutely dependent on, an ample amount of
water. But Mr. Lowell himself assures us, as a fact of which he has no
doubt, that there are no permanent bodies of water, great or small, upon
Mars; that rain, and consequently rivers, are totally wanting; that its
sky is almost constantly clear, and that what appear to be clouds are
not formed of water-vapour but of dust. He dwells, emphatically, on the
terrible desert conditions of the greater part of the surface of the
planet.

That being the case now, we have no right to assume that it has ever
been otherwise; and, taking full account of the fact, neither denied nor
disputed by Mr. Lowell, that the force of gravity on Mars is not
sufficient to retain water-vapour in its atmosphere, we must conclude
that the surface of that planet, like that of the moon, has been moulded
by some form of volcanic action modified probably by wind, but not by
water. Adding to this, that the force of gravity on Mars is nearer that
of the moon than to that of the earth, and we may r reasonably conclude
that its surface is formed of volcanic matter in a light and porous
condition, and therefore highly favourable for the rapid loss of surface
heat by radiation. The surface-conditions of Mars are therefore,
presumably, much more like those of the moon than like those of the
earth.

The next condition favourable to the storing up of heat--a covering of
vegetation--is almost certainly absent from Mars except, possibly, over
limited areas and for short periods. In this feature also the surface of
Mars approximates much nearer to lunar than to earth-conditions. The
third condition--a dense, vapour-laden atmosphere--is also wanting in
Mars. For although it possesses an atmosphere it is estimated by Mr.
Lowell (in his latest article) to have a pressure equivalent to only
2-1/2 inches of mercury with us, giving it a density of only one-twelfth
part that of ours; while aqueous vapour, the chief accumulator of heat,
cannot permanently exist in it, and, notwithstanding repeated
spectroscopic observations for the purpose of detecting it, has never
been proved to exist.

I submit that I have now shown from the statements--and largely as the
result of the long-continued observations--of Mr. Lowell himself, that,
so far as the physical conditions of Mars are known to differ from those
of the earth, the differences are all _unfavourable_ to the conservation
and _favourable_ to the dissipation of the scanty heat it receives from
the sun--that they point unmistakeably towards the temperature
conditions of the moon rather than to those of the earth, and that the
cumulative effect of these adverse conditions, acting upon a
heat-supply, reduced by solar distance to less than one-half of ours,
_must_ result in a mean temperature (as well as in the extremes) nearer
to that of our satellite than to that of our own earth.

_Further Criticism of Mr. Lowell's Article._

We are now in a position to test some further conclusions of Mr.
Lowell's _Phil. Mag._ article by comparison with actual phenomena. We
have seen, in the outline I have given of this article, that he
endeavours to show how the small amount of solar heat received by Mars
is counterbalanced, largely by the greater transparency to light and
heat of its thin and cloudless atmosphere, and partially also by a
greater conservative or 'blanketing' power of its atmosphere due to the
presence in it of a large proportion of carbonic acid gas and aqueous
vapour. The first of these statements may be admitted as a fact which he
is entitled to dwell upon, but the second--the presence of large
quantities of carbon-dioxide and aqueous vapour is a pure hypothesis
unsupported by any item of scientific evidence, while in the case of
aqueous vapour it is directly opposed to admitted results founded upon
the molecular theory of gaseous elasticity. But, although Mr. Lowell
refers to the conservative or 'blanketing' effect of the earth's
atmosphere, he does not consider or allow for its very great cumulative
effect, as is strikingly shown by the comparison with the actual
temperature conditions of the moon. This cumulative effect is due to the
_continuous_ reflection and radiation of heat from the clouds as well as
from the vapour-laden strata of air in our lower atmosphere, which
latter, though very transparent to the luminous and accompanying heat
rays of the sun, are opaque to the dark heat-rays whether radiated or
reflected from the earth's surface. We are therefore in a position
strictly comparable with that of the interior of some huge glass house,
which not only becomes intensely heated by the direct rays of the sun,
but also to a less degree by reflected rays from the sky and those
radiated from the clouds, so that even on a cloudy or misty day its
temperature rises many degrees above that of the outer air. Such a
building, if of large size, of suitable form, and well protected at
night by blinds or other covering, might be so arranged as to accumulate
heat in its soil and walls so as to maintain a tolerably uniform
temperature though exposed to a considerable range of external heat and
cold. It is to such a power of accumulation of heat in our soil and
lower atmosphere that we must impute the overwhelming contrast between
our climate and that of the moon. With us, the solar heat that
penetrates our vapour-laden and cloudy atmosphere is shut in by that
same atmosphere, accumulates there for weeks and months together, and
can only slowly escape. It is this great cumulative power which Mr.
Lowell has not taken account of, while he certainly has not estimated
the enormous loss of heat by free radiation, which entirely neutralises
the effects of increase of sun-heat, however great, when these
cumulative agencies are not present.[12]

[Footnote 12: The effects of this 'cumulative' power of a dense
atmosphere are further discussed and illustrated in the last chapter of
this book, where I show that the universal fact of steadily diminishing
temperatures at high altitudes is due solely to the diminution of this
cumulative power of our atmosphere, and that from this cause alone the
temperature of Mars must be that which would be found on a lofty plateau
about 18,000 feet higher than the average of the peaks of the Andes!]

_Temperature on Polar Regions of Mars._

There is also a further consideration which I think Mr. Lowell has
altogether omitted to discuss. Whatever may be the _mean_ temperature
of Mars, we must take account of the long nights in its polar and
high-temperate latitudes, lasting nearly twice as long as ours, with the
resulting lowering of temperature by radiation into a constantly clear
sky. Even in Siberia, in Lat. 67-1/2 deg.N. a cold of-88 deg.F. has been
attained; while over a large portion of N. Asia and America above 60 deg.
Lat. the _mean_ January temperature is from-30 deg.F. to-60 deg.F., and the
whole subsoil is permanently frozen from a depth of 6 or 7 feet to
several hundreds. But the winter temperatures, _over the same latitudes_
in Mars, must be very much lower; and it must require a proportionally
larger amount of its feeble sun-heat to raise the surface even to the
freezing-point, and an additional very large amount to melt any
considerable depth of snow. But this identical area, from a little below
60 deg. to the pole, is that occupied by the snow-caps of Mars, and over the
whole of it the winter temperature must be far lower than the
earth-minimum of-88 deg.F. Then, as the Martian summer comes on, there is
less than half the sun-heat available to raise this low temperature
after a winter nearly double the length of ours. And when the summer
does come with its scanty sun-heat, that heat is not accumulated as it
is by our dense and moisture-laden atmosphere, the marvellous effects of
which we have already shown. Yet with all these adverse conditions, each
assisting the other to produce a climate approximating to that which the
earth would have if it had no atmosphere (but retaining our superiority
over Mars in receiving double the amount of sun-heat), we are asked to
accept a mean temperature for the more distant planet almost exactly the
same as that of mild and equable southern England, and a disappearance
of the vast snowfields of its polar regions as rapid and complete as
what occurs with us! If the moon, even at its equator, has not its
temperature raised above the freezing-point of water, how can the more
_distant_ Mars, with its _oblique_ noon-day sun falling upon the
snow-caps, receive heat enough, first to raise their temperature to 32 deg.
F., and then to melt with marked rapidity the vast frozen plains of its
polar regions?

Mr. Lowell is however so regardless of the ordinary teachings of
meteorological science that he actually accounts for the supposed mild
climate of the polar regions of Mars by the absence of water on its
surface and in its atmosphere. He concludes his fifth chapter with the
following words: "Could our earth but get rid of its oceans, we too
might have temperate regions stretching to the poles." Here he runs
counter to two of the best-established laws of terrestrial climatology--
the wonderful equalising effects of warm ocean-currents which are the
chief agents in diminishing polar cold; the equally striking effects of
warm moist winds derived from these oceans, and the great storehouse of
heat we possess in our vapour-laden atmosphere, its vapour being
primarily derived from these same oceans! But, in Mr. Lowell's opinion,
all our meteorologists are quite mistaken. Our oceans are our great
drawbacks. Only get rid of them and we should enjoy the exquisite
climate of Mars--with its absence of clouds and fog, of rain or rivers,
and its delightful expanses of perennial deserts, varied towards the
poles by a scanty snow-fall in winter, the melting of which might, with
great care, supply us with the necessary moisture to grow wheat and
cabbages for about one-tenth, or more likely one-hundredth, of our
present population. I hope I may be excused for not treating such an
argument seriously. The various considerations now advanced, especially
those which show the enormous cumulative and conservative effect of our
dense and water-laden atmosphere, and the disastrous effect--judging by
the actual condition of the moon--which the loss of it would have upon
our temperature, seem to me quite sufficient to demonstrate important
errors in the data or fallacies in the complex mathematical argument by
which Mr. Lowell has attempted to uphold his views as to the temperature
and consequent climatic conditions of Mars. In concluding this portion
of my discussion of the problem of Mars, I wish to call attention to the
fact that my argument, founded upon a comparison of the physical
conditions of the earth and moon with those of Mars, is dependent upon a
small number of generally admitted scientific facts; while the
conclusions drawn from those facts are simple and direct, requiring no
mathematical knowledge to follow them, or to appreciate their weight and
cogency. I claim for them, therefore, that they are in no degree
speculative, but in their data and methods exclusively scientific. In
the next chapter I will put forward a suggestion as to how the very
curious markings upon the surface of Mars may possibly be interpreted,
so as to be in harmony with the planet's actual physical condition and
its not improbable origin and past history.



CHAPTER VII.


A SUGGESTION AS TO THE 'CANALS' OF MARS.

The special characteristics of the numerous lines which intersect the
whole of the equatorial and temperate regions of Mars are, their
straightness combined with their enormous length. It is this which has
led Mr. Lowell to term them 'non-natural features.' Schiaparelli, in his
earlier drawings, showed them curved and of comparatively great width.
Later, he found them to be straight fine lines when seen under the best
conditions, just as Mr. Lowell has always seen them in the pure
atmosphere of his observatory. Both of these observers were at first
doubtful of their reality, but persistent observation continued at many
successive oppositions compelled acceptance of them as actual features
of the planet's disc. So many other observers have now seen them that
the objection of unreality seems no longer valid.

Mr. Lowell urges, however, that their perfect straightness, their
extreme tenuity, their uniformity throughout their whole length, the
dual character of many of them, their relation to the 'oases' and the
form and position of these round black spots, are all proofs of
artificiality and are suggestive of design. And considering that some of
them are actually as long as from Boston to San Francisco, and
relatively to their globe as long as from London to Bombay, his
objection that "no natural phenomena within our knowledge show such
regularity on such a scale" seems, at first, a mighty one.

It is certainly true that we can point to nothing exactly like them
either on the earth or on the moon, and these are the only two planetary
bodies we are in a position to compare with Mars. Yet even these do, I
think, afford us some hints towards an interpretation of the mysterious
lines. But as our knowledge of the internal structure and past history
even of our earth is still imperfect, that of the moon only conjectural,
and that of Mars a perfect blank, it is not perhaps surprising that the
surface-features of the latter do not correspond with those of either of
the others.

_Mr. Pickering's Suggestion._

The best clue to a natural interpretation of the strange features of the
surface of Mars is that suggested by the American astronomer Mr. W.H.
Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the
'canals' of Mars and the rifts as well as the luminous streaks on the
moon are cracks in the volcanic crust, caused by internal stresses due
to the action of the heated interior. These cracks he considers to be
symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's
'oases') because they have originated from them, just as the white
streaks on the moon radiate from the larger craters as centres. He
further supposes that water and carbon-dioxide issue from the interior
into these fissures, and, in conjunction with sunlight, promote the
growth of vegetation. Owing to the very rare atmosphere, the vapours, he
thinks, would not ascend but would roll down the outsides of the
craterlets and along the borders of the canals, thus irrigating the
immediate vicinity and serving to promote the growth of some form of
vegetation which renders the canals and oases visible.[13]

[Footnote 13: _Nature_, vol. 70, p. 536.]

This opinion is especially important because, next to Mr. Lowell, Mr.
Pickering is perhaps the astronomer who has given most attention to Mars
during the last fifteen years. He was for some time at Flagstaff with
Mr. Lowell, and it was he who discovered the oases or craterlets, and
who originated the idea that we did not see the 'canals' themselves but
only the vegetable growth on their borders. He also observed Mars in the
Southern Hemisphere at Arequipa; and he has since made an elaborate
study of the moon by means of a specially constructed telescope of 135
feet focal length, which produced a direct image on photographic plates
nearly 16 inches in diameter.[14]

[Footnote 14: _Nature_, vol. 70, May 5, p.xi, supplement.]

It is clear therefore that Mr. Lowell's views as to the artificial
nature of the 'canals' of Mars are not accepted by an astronomer of
equal knowledge and still wider experience. Yet Professor Pickering's
alternative view is more a suggestion than an explanation, because there
is no attempt to account for the enormous length and perfect
straightness of the lines on Mars, so different from anything that is
found either on our earth or on the moon. There must evidently be some
great peculiarity of structure or of conditions on Mars to account for
these features, and I shall now attempt to point out what this
peculiarity is and how it may have arisen.

_The Meteoritic Hypothesis._

During the last quarter of a century a considerable change has come over
the opinions of astronomers as regards the probable origin of the Solar
System. The large amount of knowledge of the stellar universe, and
especially of nebulae, of comets and of meteor-streams which we now
possess, together with many other phenomena, such as the constitution of
Saturn's rings, the great number and extent of the minor planets, and
generally of the vast amount of matter in the form of meteor-rings and
meteoric dust in and around our system, have all pointed to a different
origin for the planets and their satellites than that formulated by
Laplace as the Nebular hypothesis.

It is now seen more clearly than at any earlier period, that most of the
planets possess special characteristics which distinguish them from one
another, and that such an origin as Laplace suggested--the slow cooling
and contraction of one vast sun-mist or nebula, besides presenting
inherent difficulties--many think them impossibilities--in itself does
not afford an adequate explanation of these peculiarities. Hence has
arisen what is termed the Meteoritic theory, which has been ably
advocated for many years by Sir Norman Lockyer, and with some
unimportant modifications is now becoming widely accepted. Briefly, this
theory is, that the planets have been formed by the slow aggregation of
solid particles around centres of greatest condensation; but as many of
my readers may be altogether unacquainted with it, I will here give a
very clear statement of what it is, from Professor J.W. Gregory's
presidential address to the Geological Section of the British
Association of the present year. He began by saying that these modern
views were of far more practical use to men of science than that of
Laplace, and that they give us a history of the world consistent with
the actual records of geology. He then continues:

"According to Sir Norman Lockyer's Meteoritic Hypothesis, nebulae,
comets, and many so-called stars consist of swarms of meteorites which,
though normally cold and dark, are heated by repeated collisions, and so
become luminous. They may even be volatilised into glowing meteoric
vapour; but in time this heat is dissipated, and the force of gravity
condenses a meteoritic swarm into a single globe. 'Some of the swarms
are,' says Lockyer, 'truly members of the solar system,' and some of
these travel round the sun in nearly circular orbits, like planets. They
may be regarded as infinitesimal planets, and so Chamberlain calls them
'planetismals.'

"The planetismal theory is a development of the meteoritic theory, and
presents it in an especially attractive guise. It regards meteorites as
very sparsely distributed through space, and gravity as powerless to
collect them into dense groups. So it assigns the parentage of the solar
system to a spiral nebula composed of planetismals, and the planets as
formed from knots in the nebula, where many planetismals had been
concentrated near the intersections of their orbits. These groups of
meteorites, already as dense as a swarm of bees, were then packed closer
by the influence of gravity, and the contracting mass was heated by the
pressure, even above the normal melting-point of the material, which was
kept rigid by the weight of the overlying layers."

Now, adopting this theory as the last word of science upon the subject
of the origin of planets, we see that it affords immense scope for
diversity in results depending on the total _amount_ of matter available
within the range of attraction of an incipient planetary mass, and the
_rates_ at which this matter becomes available. By a special combination
of these two quantities (which have almost certainly been different for
each planet) I think we may be able to throw some light upon the
structure and physical features of Mars.

_The Probable Mode of Origin of Mars._

This planet, lying between two of much greater mass, has evidently had
less material from which to be formed by aggregation; and if we
assume--as in the absence of evidence to the contrary we have a right to
do--that its beginnings were not much later (or earlier) than those of
the earth, then its smaller size shows that it has in all probability
aggregated very much more slowly. But the internal heat acquired by a
planet while forming in this manner will depend upon the rate at which
it aggregates and the velocity with which the planetismals' fall into
it, and this velocity will increase with its mass and consequent force
of gravity. In the early stages of a planet's growth it will probably
remain cold, the small amount of heat produced by each impact being lost
by radiation before the next one occurs; and with a small and slowly
aggregating planet this condition will prevail till it approaches its
full size. Then only will its gravitative force be sufficient to cause
incoming matter to fall upon it with so powerful an impact as to produce
intense heat. Further, the compressive force of a small planet will be a
less effective heat-producing agency than in the case of a larger one.

The earth we know has acquired a large amount of internal heat, probably
sufficient to liquefy its whole interior; but Mars has only one-ninth
part the mass of the earth, and it is quite possible, and even probable,
that its comparatively small attractive force would never have liquefied
or even permanently heated the more central portions of its mass. This
being admitted, I suggest the following course of events as quite
possible, and not even improbable, in the case of this planet. During
the whole of its early growth, and till it acquired nearly its present
diameter, its rate of aggregation was so slow that the planetismals
falling upon it, though they might have been heated and even partially
liquefied by the impact, were never in such quantity as to produce any
considerable heating effect on the whole mass, and each local rise of
temperature was soon lost by radiation. The planet thus grew as a solid
and cold mass, compacted together by the impact of the incoming matter
as well as by its slowly increasing gravitative force. But when it had
attained to within perhaps 100, perhaps 50 miles, or less, of its
present diameter, a great change occurred in the opportunity for further
growth. Some large and dense swarm of meteorites, perhaps containing a
number of bodies of the size of the asteroids, came within the range of
the sun's attraction and were drawn by it into an orbit which crossed
that of Mars at such a small angle that the planet was able at each
revolution to capture a considerable number of them. The result might
then be that, as in the case of the earth, the continuous inpour of the
fresh matter first heated, and later on liquefied the greater part of it
as well perhaps as a thin layer of the planet's original surface; so
that when in due course the whole of the meteor-swarm had been captured,
Mars had acquired its present mass, but would consist of an intensely
heated, and either liquid or plastic thin outer shell resting upon a
cold and solid interior.

The size and position of the two recently discovered satellites of Mars,
which are believed to be not more than ten miles in diameter, the more
remote revolving around its primary very little slower than the planet
rotates, while the nearer one, which is considerably less distant from
the planet's surface than its own antipodes and revolves around it more
than three times during the Martian day, may perhaps be looked upon as
the remnants of the great meteor-swarm which completed the Martian
development, and which are perhaps themselves destined at some distant
period to fall into the planet. Should future astronomers witness the
phenomenon the effect produced upon its surface would be full of
instruction.

As the result of such an origin as that suggested, Mars would possess a
structure which, in the essential feature of heat-distribution, would be
the very opposite of that which is believed to characterise the earth,
yet it might have been produced by a very slight modification of the
same process. This peculiar heat-distribution, together with a much
smaller mass and gravitative force, would lead to a very different
development of the surface and an altogether diverse geological history
from ours, which has throughout been profoundly influenced by its heated
interior, its vast supply of water, and the continuous physical and
chemical reactions between the interior and the crust.

These reactions have, in our case, been of substantially the same
nature, and very nearly of the same degree of intensity throughout the
whole vast eons of geological time, and they have resulted in a
wonderfully complex succession of rock-formations--volcanic, plutonic,
and sedimentary--more or less intermingled throughout the whole series,
here remaining horizontal as when first deposited, there upheaved or
depressed, fractured or crushed, inclined or contorted; denuded by rain
and rivers with the assistance of heat and cold, of frost and ice, in an
unceasing series of changes, so that however varied the surface may be,
with hill and dale, plains and uplands, mountain ranges and deep
intervening valleys, these are as nothing to the diversities of interior
structure, as exhibited in the sides of every alpine valley or
precipitous escarpment, and made known to us by the work of the miner
and the well-borer in every part of the world.

_Structural Straight Lines on the Earth._

The great characteristic of the earth, both on its surface and in its
interior, is thus seen to be extreme diversity both of form and
structure, and this is further intensified by the varied texture,
constitution, hardness, and density of the various rocks and debris of
which it is composed. It is therefore not surprising that, with such a
complex outer crust, we should nowhere find examples of those
geometrical forms and almost world-wide straight lines that give such a
remarkable, and as Mr. Lowell maintains, 'non-natural' character to the
surface of Mars, but which, as it seems to me, of themselves afford
_prima facie_ evidence of a corresponding simplicity and uniformity in
its internal structure.

Yet we are not ourselves by any means devoid of 'straight lines'
structurally produced, in spite of every obstacle of diversity of form
and texture, of softness and hardness, of lamination or crystallisation,
which are adverse to such developments. Examples of these are the
numerous 'faults' which occur in the harder rocks, and which often
extend for great distances in almost perfect straight lines. In our own
country we have the Tyneside and Craven faults in the North of England,
which are 30 miles long and often 20 yards wide; but even more striking
is the great Cleveland <DW18>--a wall of volcanic rock dipping slightly
towards the south, but sometimes being almost vertical, and stretching
across the country, over hill and dale, in an almost perfect straight
line from a point on the coast ten miles north of Scarborough, in a
west-by-north direction, passing about two miles south of Stockton and
terminating about six miles north-by-east of Barnard Castle, a distance
of very nearly 60 miles. The great fault between the Highlands and
Lowlands of Scotland extends across the country from Stonehaven to near
Helensburgh, a distance of 120 miles; and there are very many more of
less importance.

Much more extensive are some of the great continental dislocations,
often forming valleys of considerable width and length. The Upper Rhine
flows in one of these great valleys of subsidence for about 180 miles,
from Mulhausen to Frankfort, in a generally straight line, though
modified by denudation. Vaster still is the valley of the Jordan through
the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the
Gulf of Akaba, believed to form one vast geological depression or
fracture extending in a straight line for 400 miles.

Thousands of such faults, <DW18>s, or depressions exist in every part of
the world, all believed to be due to the gradual shrinking of the heated
interior to which the solid crust has to accommodate itself, and they
are especially interesting and instructive for our present purpose as
showing the tendency of such fractures of solid rock-material to extend
to great lengths in straight lines, notwithstanding the extreme
irregularity both in the surface contour as well as in the internal
structures of the varied deposits and formations through which they
pass.

_Probable Origin of the Surface-features of Mars._

Returning now to Mars, let us consider the probable course of events
from the point at which we left it. The heat produced by impact and
condensation would be likely to release gases which had been in
combination with some of the solid matter, or perhaps been itself in a
solid state due to intense cold, and these, escaping outwards to the
surface, would produce on a small scale a certain amount of upheaval and
volcanic disturbance; and as an outer crust rapidly formed, a number of
vents might remain as craters or craterlets in a moderate state of
activity. Owing to the comparatively small force of gravity, the outer
crust would become scoriaceous and more or less permeated by the gases,
which would continue to escape through it, and this would facilitate the
cooling of the whole of the heated outer crust, and allow it to become
rather densely compacted. When the greater portion of the gases had thus
escaped to the outer surface and assisted to form a scanty atmosphere,
such as now exists, there would be no more internal disturbance and the
cooling of the heated outer coating would steadily progress, resulting
at last in a slightly heated, and later in a cold layer of moderate
thickness and great general uniformity. Owing to the absence of rain and
rivers, denudation such as we experience would be unknown, though the
superficial scoriaceous crust might be partially broken up by expansion
and contraction, and suffer a certain amount of atmospheric erosion.

The final result of this mode of aggregation would be, that the planet
would consist of an outer layer of moderate thickness as compared with
the central mass, which outer layer would have cooled from a highly
heated state to a temperature considerably below the freezing-point, and
this would have been all the time _contracting upon a previously cold,
and therefore non-contracting nucleus._ The result would be that very
early in the process great superficial tensions would be produced, which
could only be relieved by cracks or fissures, which would initiate at
points of weakness--probably at the craterlets already referred to--from
which they would radiate in several directions. Each crack thus formed
near the surface would, as cooling progressed, develop in length and
depth; and owing to the general uniformity of the material, and possibly
some amount of crystalline structure due to slow and continuous cooling
down to a very low temperature, the cracks would tend to run on in
straight lines and to extend vertically downwards, which two
circumstances would necessarily result in their forming portions of
'great circles' on the planet's surface--the two great facts which Mr.
Lowell appeals to as being especially 'non-natural.'

_Symmetry of Basaltic Columns._

We have however one quite natural fact on our earth which serves to
illustrate one of these two features, the direction of the downward
fissure. This is, the comparatively common phenomenon of basaltic
columns and 'Giant's Causeways.' The wonderful regularity of these, and
especially the not unfrequent upright pillars in serried ranks, as in
the palisades of the Hudson river, must have always impressed observers
with their appearance of artificiality. Yet they are undoubtedly the
result of the very slow cooling and contraction of melted rocks under
compression by strata _below and above them_, so that, when once
solidified, the mass was held in position and the tension produced by
contraction could only be relieved by numerous very small cracks at
short distances from each other in every direction, resulting in five,
six, or seven-sided polygons, with sides only a few inches long. This
contraction began of course at the coolest surface, generally the upper
one; and observation of these columns in various positions has
established the rule that their direction lengthways _is always at right
angles to the cooling surface_, and thus, whenever this surface was
horizontal, the columns became almost exactly vertical.

_How this applies to Mars._

One of the features of the surface of Mars that Mr. Lowell describes
with much confidence is, that it is wonderfully uniform and level, which
of course it would be if it had once been in a liquid or plastic state,
and not much disturbed since by volcanic or other internal movements.
The result would be that cracks formed by contraction of the hardened
outer crust would be vertical; and, in a generally uniform material at a
very uniform temperature, these cracks would continue almost
indefinitely in straight lines. The hardened and contracting surface
being free to move laterally on account of there being a more heated and
plastic layer below it, the cracks once initiated above would
continually widen at the surface as they penetrated deeper and deeper
into the slightly heated substratum. Now, as basalt begins to soften at
about 1400 deg. F. and the surface of Mars has cooled to at least the
freezing-point--perhaps very much below it--the contraction would be so
great that if the fissures produced were 500 miles apart they might be
three miles wide at the surface, and, if only 100 miles apart, then
about two-thirds of a mile wide.[15] But as the production of the
fissures might have occupied perhaps millions of years, a considerable
amount of atmospheric denudation would result, however slowly it acted.
Expansion and contraction would wear away the edges and sides of the
fissures, fill up many of them with the debris, and widen them at the
surfaces to perhaps double their original size.[16]

[Footnote 15: The coefficient of contraction of basalt is 0.000006 for
1 deg. F., which would lead to the results given here.]

[Footnote 16: Mr. W.H. Pickering observed clouds on Mars 15 miles high;
these are the 'projections' seen on the terminator when the planet is
partially illuminated. They were at first thought to be mountains; but
during the opposition of 1894, more than 400 of them were seen at
Flagstaff during nine months' observation. Usually they are of rare
occurrence. They are seen to change in form and position from day to
day, and Mr. Lowell is strongly of opinion that they are dust-storms,
not what we term clouds. They were mostly about 13 miles high,
indicating considerable aerial disturbance on the planet, and therefore
capable of producing proportional surface denudation.]

_Suggested Explanation of the 'Oases.'_

The numerous round dots seen upon the 'canals,' and especially at points
from which several canals radiate and where they intersect--termed
'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explained
in two ways. Those from which several canals radiate may be true craters
from which the gases imprisoned in the heated surface layers have
gradually escaped. They would be situated at points of weakness in the
crust, and become centres from which cracks would start during
contraction. Those dots which occur at the crossing of two straight
canals or cracks may have originated from the fact that at such
intersections there would be four sharply-projecting angles, which,
being exposed to the influence of alternate heat and cold (during day
and night) on the two opposite surfaces, would inevitably in time become
fractured and crumbled away, resulting in the formation of a roughly
circular chasm which would become partly filled up by the debris. Those
formed by cracks radiating from craterlets would also be subject to the
same process of rounding off to an even greater extent; and thus would
be produced the 'oases' of various sizes up to 50 miles or more in
diameter recorded by Mr. Lowell and other observers.

_Probable Function of the Great Fissures._

Mr. Pickering, as we have seen, supposes that these fissures give out
the gases which, overflowing on each side, favour the growth of the
supposed vegetation which renders the course of the canals visible, and
this no doubt may have been the case during the remote periods when
these cracks gave access to the heated portions of the surface layer.
But it seems more probable that Mars has now cooled down to the almost
uniform mean temperature it derives from solar heat, and that the
fissures--now for the most part broad shallow valleys--serve merely as
channels along which the liquids and heavy gases derived from the
melting of the polar snows naturally flow, and, owing to their nearly
level surfaces, overflow to a certain distance on each side of them.

_Suggested Origin of the Blue Patches._

These heavy gases, mainly perhaps, as has been often suggested,
carbon-dioxide, would, when in large quantity and of considerable depth,
reflect a good deal of light, and, being almost inevitably dust-laden,
might produce that blue tinge adjacent to the melting snow-caps which
Mr. Lowell has erroneously assumed to be itself a proof of the presence
of liquid water. Just as the blue of our sky is undoubtedly due to
reflection from the ultra-minute dust particles in our higher
atmosphere, similar particles brought down by the 'snow' from the higher
Martian atmosphere might produce the blue tinge in the great volumes of
heavy gas produced by its evaporation or liquefaction.

It may be noted that Mr. Lowell objects to the carbon-dioxide theory of
the formation of the snow-caps, that this gas at low pressures does not
liquefy, but passes at once from the solid to the gaseous state, and
that only water remains liquid sufficiently long to produce the blue
colour' which plays so large a part in his argument for the mild climate
essential for an inhabited planet. But this argument, as I have already
shown, is valueless. For only very deep water can possibly show a blue
colour by reflected light, while a dust-laden atmosphere--especially
with a layer of very dense gas at the bottom of it, as would be the case
with the newly evaporated carbon-dioxide from the diminishing snow-cap
--would provide the very conditions likely to produce this blue tinge of
colour.

It may be considered a support to this view that carbonic-acid gas
becomes liquid at--140 deg. F. and solid at--162 deg. F., temperatures far
higher than we should expect to prevail in the polar and north temperate
regions of Mars during a considerable part of the year, but such as
might be reached there during the summer solstice when the `snows' so
rapidly disappear, to be re-formed a few months later.

_The Double Canals._

The curious phenomena of the 'double canals' are undoubtedly the most
difficult to explain satisfactorily on any theory that has yet been
suggested. They vary in distance apart from about 100 to 400 miles. In
many cases they appear perfectly parallel, and Mr. Lowell gives us the
impression that they are almost always so. But his maps show, in some
cases, decided differences of width at the two extremities, indicating
considerable want of parallelism. A few of the curved canals are also
double.

There is one drawing in Mr. Lowell's book (p. 219) of the mouths, or
starting points, of the Euphrates and Phison, two widely separated
double canals diverging at an angle of about 40 deg. from the same two
oases, so that the two inner canals cross each other. Now this suggests
two wide bands of weakness in the planet's crust radiating probably from
within the dark tract called the 'Mare Icarium,' and that some
widespread volcanic outburst initiated diverging cracks on either side
of these bands. Something of this kind may have been the cause of most
of the double canals, or they may have been started from two or more
craterlets not far apart, the direction being at first decided by some
local peculiarity of structure; and where begun continuing in straight
lines owing to homogeneity or uniform density of material. This is very
vague, but the phenomena are so remarkable, and so very imperfectly
known at present, that nothing but suggestion can be attempted.

_Concluding Remarks on the 'Canals.'_

In this somewhat detailed exposition of a possible, and, I hope, a
probable explanation of the surface-features of Mars, I have
endeavoured to be guided by known facts or accepted theories both
astronomical and geological. I think I may claim to have shown that
there are some analogous features of terrestrial rock-structure to
serve as guides towards a natural and intelligible explanation of the
strange geometric markings discovered during the last thirty years, and
which have raised this planet from comparative obscurity into a position
of the very first rank both in astronomical and popular interest.

This wide-spread interest is very largely due to Mr. Lowell's devotion
to its study, both in seeking out so admirable a position as regards
altitude and climate, and in establishing there a first-class
observatory; and also in bringing his discoveries before the public in
connection with a theory so startling as to compel attention. I venture
to think that his merit as one of our first astronomical observers will
in no way be diminished by the rejection of his theory, and the
substitution of one more in accordance with the actually observed facts.



APPENDIX.

_A Suggested Experiment to Illustrate the 'Canals' of Mars._

If my explanation of the 'canals' should be substantially correct--that
is, if they were produced by the contraction of a heated outward crust
upon a cold, and therefore non-contracting interior, the result of such
a condition might be shown experimentally.

Several baked clay balls might be formed to serve as cores, say of 8 to
10 inches in diameter. These being fixed within moulds of say half an
inch to an inch greater diameter, the outer layer would be formed by
pouring in some suitable heated liquid material, and releasing it from
the mould as soon as consolidation occurs, so that it may cool rapidly
from the _outside._ Some kinds of impure glass, or the brittle metals
bismuth or antimony or alloys of these might be used, in order to see
what form the resulting fractures would take. It would be well to have
several duplicates of each ball, and, as soon as tension through
contraction manifests itself, to try the effect of firing very small
charges of small shot to ascertain whether such impacts would start
radiating fractures. When taken from the moulds, the balls should be
suspended in a slight current of air, and kept rotating, to reproduce
the planetary condition as nearly as possible.

The exact size and material of the cores, the thickness of the heated
outer crust, the material best suited to show fracture by contraction,
and the details of their treatment, might be modified in various ways as
suggested by the results first obtained. Such a series of experiments
would probably throw further light on the physical conditions which have
produced the gigantic system of fissures or channels we see upon the
surface of Mars, though it would not, of course, prove that such
conditions actually existed there. In such a speculative matter we can
only be guided by probabilities, based upon whatever evidence is
available.



CHAPTER VIII.


SUMMARY AND CONCLUSION.

This little volume has necessarily touched upon a great variety of
subjects, in order to deal in a tolerably complete manner with the very
extraordinary theories by which Mr. Lowell attempts to explain the
unique features of the surface of the planet, which, by long-continued
study, he has almost made his own. It may therefore be well to sum up
the main points of the arguments against his view, introducing a few
other facts and considerations which greatly strengthen my argument.

The one great feature of Mars which led Mr. Lowell to adopt the view of
its being inhabited by a race of highly intelligent beings, and, with
ever-increasing discovery to uphold this theory to the present time, is
undoubtedly that of the so-called 'canals'--their straightness, their
enormous length, their great abundance, and their extension over the
planet's whole surface from one polar snow-cap to the other. The very
immensity of this system, and its constant growth and extension during
fifteen years of persistent observation, have so completely taken
possession of his mind, that, after a very hasty glance at analogous
facts and possibilities, he has declared them to be 'non-natural'--
therefore to be works of art--therefore to necessitate the
presence of highly intelligent beings who have designed and constructed
them. This idea has  or governed all his writings on the
subject. The innumerable difficulties which it raises have been either
ignored, or brushed aside on the flimsiest evidence. As examples, he
never even discusses the totally inadequate water-supply for such
worldwide irrigation, or the extreme irrationality of constructing so
vast a canal-system the waste from which, by evaporation, when exposed
to such desert conditions as he himself describes, would use up ten
times the probable supply.

Again, he urges the 'purpose' displayed in these 'canals.' Their being
_all_ so straight, _all_ describing great circles of the 'sphere,' all
being so evidently arranged (as he thinks) either to carry water to some
'oasis' 2000 miles away, or to reach some arid region far over the
equator in the opposite hemisphere! But he never considers the
difficulties this implies. Everywhere these canals run for thousands of
miles across waterless deserts, forming a system and indicating a
purpose, the wonderful perfection of which he is never tired of dwelling
upon (but which I myself can nowhere perceive).

Yet he never even attempts to explain how the Martians could have lived
_before_ this great system was planned and executed, or why they did not
_first_ utilise and render fertile the belt of land adjacent to the
limits of the polar snows--why the method of irrigation did not, as with
all human arts, begin gradually, at home, with terraces and channels to
irrigate the land close to the source of the water. How, with such a
desert as he describes three-fourths of Mars to be, did the inhabitants
ever get to _know_ anything of the equatorial regions and its needs, so
as to start right away to supply those needs? All this, to my mind, is
quite opposed to the idea of their being works of art, and altogether in
favour of their being natural features of a globe as peculiar in origin
and internal structure as it is in its surface-features. The explanation
I have given, though of course hypothetical, is founded on known
cosmical and terrestrial facts, and is, I suggest, far more scientific
as well as more satisfactory than Mr. Lowell's wholly unsupported
speculation. This view I have explained in some detail in the preceding
chapter.

Mr. Lowell never even refers to the important question of loss by
evaporation in these enormous open canals, or considers the undoubted
fact that the only intelligent and practical way to convey a limited
quantity of water such great distances would be by a system of
water-tight and air-tight tubes laid _under the ground._ The mere
attempt to use open canals for such a purpose shows complete ignorance
and stupidity in these alleged very superior beings; while it is certain
that, long before half of them were completed their failure to be of any
use would have led any rational beings to cease constructing them.

He also fails to consider the difficulty, that, if these canals are
necessary for existence in Mars, how did the inhabitants ever reach a
sufficiently large population with surplus food and leisure enabling
them to rise from the low condition of savages to one of civilisation,
and ultimately to scientific knowledge? Here again is a dilemma which is
hard to overcome. Only a _dense_ population with _ample_ means of
subsistence could possibly have constructed such gigantic works; but,
given these two conditions, no adequate motive existed for the
conception and execution of them--even if they were likely to be of any
use, which I have shown they could not be.

_Further Considerations on the Climate of Mars._

Recurring now to the question of climate, which is all-important, Mr.
Lowell never even discusses the essential point--the temperature that
must _necessarily_ result from an atmospheric envelope one-twelfth (or
at most one-seventh) the density of our own; in either case
corresponding to an altitude far greater than that of our highest
mountains.[17] Surely this phenomenon, everywhere manifested on the
earth even under the equator, of a regular decrease of temperature with
altitude, the only cause of which is a less dense atmosphere, should
have been fairly grappled with, and some attempt made to show why it
should not apply to Mars, except the weak remark that on a level surface
it will not have the same effect as on exposed mountain heights. But it
_does_ have the same effect, or very nearly so, on our lofty plateaux
often hundreds of miles in extent, in proportion to their altitude.
Quito, at 9350 ft. above the sea, has a mean temperature of about 57 deg.
F., giving a lowering of 23 deg. from that of Manaos at the mouth of the Rio
<DW64>. This is about a degree for each 400 feet, while the general fall
for isolated mountains is about one degree in 340 feet according to
Humboldt, who notes the above difference between the rate of cooling for
altitude of the plains--or more usually sheltered valleys in which the
towns are situated--and the exposed mountain sides. It will be seen that
this lower rate would bring the temperature of Mars at the equator down
to 20 deg. F. below the freezing point of water from this cause alone.

[Footnote 17: A four inches barometer is equivalent to a height of
40,000 feet above sea-level with us.]

But all enquirers have admitted, that if conditions as to atmosphere
were the same as on the earth, its greater distance from the sun would
reduce the temperature to-31 deg. F., equal to 63 deg. below the freezing
point. It is therefore certain that the combined effect of both causes
must bring the temperature of Mars down to at least 70 deg. or 80 deg.below the
freezing point.

The cause of this absolute dependence of terrestrial temperatures upon
density of the air-envelope is seldom discussed in text-books either of
geography or of physics, and there seems to be still some uncertainty
about it. Some impute it wholly to the thinner air being unable to
absorb and retain so much heat as that which is more dense; but if this
were the case the soil at great altitudes not having so much of its heat
taken up by the air should be warmer than below, since it undoubtedly
_receives_ more heat owing to the greater transparency of the air above
it; but it certainly does not become warmer. The more correct view seems
to be that the loss of heat by radiation is increased so much through
the rarity of the air above it as to _more_ than counterbalance the
increased insolation, so that though the surface of the earth at a given
altitude may receive 10 per cent. more direct sun-heat it loses by
direct radiation, combined with diminished air and cloud-radiation,
perhaps 20 or 25 per cent. more, whence there is a resultant cooling
effect of 10 or 15 per cent. This acts by day as well as by night, so
that the greater heat received at high altitudes does not warm the soil
so much as a less amount of heat with a denser atmosphere.

This effect is further intensified by the fact that a less dense cannot
absorb and transmit so much heat as a more dense atmosphere. Here then
we have an absolute law of nature to be observed operating everywhere on
the earth, and the mode of action of which is fairly well understood.
This law is, that reduced atmospheric pressure increases radiation, or
loss of heat, _more rapidly_ than it increases insolation or gain of
heat, so that the result is _always_ a considerable _lowering_ of
temperature. What this lowering is can be seen in the universal fact,
that even within the tropics perpetual snow covers the higher mountain
summits, while on the high plains of the Andes, at 15,000 or 16,000 feet
altitude, where there is very little or no snow, travellers are often
frozen to death when delayed by storms; yet at this elevation the
atmosphere has much more than double the density of that of Mars!

The error in Mr. Lowell's argument is, that he claims for the scanty
atmosphere of Mars that it allows more sun-heat to reach the surface;
but he omits to take account of the enormously increased loss of heat by
direct radiation, as well as by the diminution of air-radiation, which
together necessarily produce a great reduction of temperature.

It is this great principle of the prepotency of radiation over
absorption with a diminishing atmosphere that explains the excessively
low temperature of the moon's surface, a fact which also serves to
indicate a very low temperature for Mars, as I have shown in Chapter VI.
These two independent arguments--from alpine temperatures and from those
of the moon--support and enforce each other, and afford a conclusive
proof (as against anything advanced by Mr. Lowell) that the temperature
of Mars must be far too low to support animal life.

A third independent argument leading to the same result is Dr. Johnstone
Stoney's proof that aqueous vapour cannot exist on Mars; and this fact
Mr. Lowell does not attempt to controvert.

To put the whole case in the fewest possible words:

All physicists are agreed that, owing to the distance of Mars from the
sun, it would have a mean temperature of about-35 deg. F. (= 456 deg. F. abs.)
even if it had an atmosphere as dense as ours.

(2) But the very low temperatures on the earth under the equator, at a
height where the barometer stands at about three times as high as on
Mars, proves, that from scantiness of atmosphere alone Mars cannot
possibly have a temperature as high as the freezing point of water; and
this proof is supported by Langley's determination of the low _maximum_
temperature of the full moon.

The combination of these two results must bring down the temperature of
Mars to a degree wholly incompatible with the existence of animal life.

(3) The quite independent proof that water-vapour cannot exist on Mars,
and that therefore, the first essential of organic life--water--is
non-existent.

The conclusion from these three independent proofs, which enforce each
other in the multiple ratio of their respective weights, is therefore
irresistible--that animal life, especially in its higher forms, cannot
exist on the planet.

Mars, therefore, is not only uninhabited by intelligent beings such as
Mr. Lowell postulates, but is absolutely UNINHABITABLE.





End of Project Gutenberg's Is Mars Habitable?, by Alfred Russel Wallace

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