_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°
N. to 60° 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.