LIFE IN OTHER WORLDS
If life was not brought to the earth from another planet, then life
was created, or originated, on the earth. Some of the conditions
which attended its birth have been considered, and they amount to
this: that the temperature of the earth, the elements of the earth,
the conditions of the earth's surface, oceans, and atmosphere were
exactly those which favoured the origination, the continuance, and
the development of living things. The earth, among all the heavenly
bodies which we can examine at all closely, is probably the only one
on which life, as we know it, would have much chance of survival.
The Sun as an abode of life we may at once put out of the question.
Taking the planets in order of distance from the Sun: of Mercury we
know very little, but astronomers like Schiaparelli and Lowell have
pronounced it to be an airless, dead planet with a surface cracked in
cooling untold ages ago. Venus is believed to be much like the earth,
not differing greatly in size, and probably having an atmosphere of
considerable extent;[10] but its appearance is so bright, viewed
from the earth, that it has been surmised that what we see is not
the planet itself but its atmosphere always charged with clouds or
possibly snow. Of Mars as an abode of life, and of the Moon, which
is the body nearest to us, we shall speak more fully in the present
chapter. Coming to the outer major planets, the giant Jupiter--with
a bulk more than a thousand times as great as the earth--has a
constitution by no means so solid. For many reasons the belief seems
justified that Jupiter must be a still hot, and almost gaseous body,
without a solid crust. If Jupiter's comparatively small weight for
its size and its wonderful and varying system of cloud coverings
are evidence of an early stage of development and a high internal
temperature, still more is this the case with Saturn. In bulk it is
not far inferior to Jupiter; but it is so much lighter than water
that if some of its fragments fell into one of the earth's oceans
they would float there. Its outer coverings or envelopes must consist
of heated gases in active circulation. Of Uranus and Neptune, still
farther off, we know very little, and the progress of knowledge
concerning them is very slow; but it is more probable that they are
in the early stage of development attributed to Jupiter and Saturn
than in the solid stage of planets like the earth. So that we may
fairly dismiss the probability of the existence of life as we know it
on any of them--and neglect incidentally, therefore, any possibility
that life could have been carried in a meteorite from them to us.
Whether there are other forms of "life" than any we know is a
question hardly needing discussion.
[Footnote 10: W. W. Bryant, _A History of Astronomy_ (Methuen),
1907.]
There remains the question of the probability of life on the Moon or
on Mars; and the question of possible life on the Moon is specially
worth consideration, because the earth's satellite was once part of
the earth's mass. We may first repeat briefly the explanation which
Sir G. H. Darwin has given of the Moon's separation from the earth.
If a flexible hoop or ring is spun very rapidly it will be seen to
flatten itself at top and bottom (or, as we might say, at its poles)
and broaden itself out at its middle or equator. The semi-liquid
earth once rotated so rapidly that its swelling equatorial belt was
almost at the point of separation from the parent body. 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 a considerable distance from it. There
are several estimates of the bulk of the earth thus shot off; but
we may assume that about one-fiftieth of the earth escaped thus. It
must have consisted of a considerable portion of the earth's solid
crust, and a much larger quantity of the molten rocks of the earth's
interior.
The Moon is much lighter than the earth. The earth taken as a whole
weighs about five and a half times as much as water. If we consider
its surface alone, this weighs rather more than two and a half times
the weight of water--from which it can be seen that the interior of
the earth is very much denser than the earth's surface crust.[11]
The Moon weighs rather less than three and a half times its bulk
in water. This shows clearly that the Moon is composed of material
scraped off from the outer surface of the earth, rather than of
matter obtained from a considerable depth. At the same time the layer
of material removed had an appreciable thickness. The volume of the
Moon is equivalent to a solid body whose surface is equal to the
area of all the earth's oceans, and whose depth would be thirty-six
miles. It seems probable, therefore, that at the time when the Moon
was torn off, or shot off, from the earth, the parent body had a
solid crust averaging at least thirty-six miles in thickness, while
beneath this crust the temperature was so high that the materials
underneath were molten or liquid, and in other places were only kept
solid by the enormous pressure of the material above them. When
the Moon separated from the earth three-quarters of this crust was
carried away. It has sometimes been supposed that the remainder was
torn into two parts, one of which formed the great land area of the
Eastern Hemisphere and the other the great land area of which North
and South America are the relics in the Western Hemisphere. These two
great areas, at that time, floated on the semi-liquid surface like
two large ice-floes. But they were, of course, a good deal heavier
than ice, and the molten stuff on which they floated was a good deal
heavier than water. Later on this liquid stuff cooled and hardened.
But its bottom was still a good deal lower than the surface of the
great areas of land which had "floated" on it; and therefore it
formed great depressions all about and surrounding them. Thus the
depressions were there ready formed, into which the waters of the
earth, beginning as rain and ending as rivers or lakes, could flow as
into reservoirs. On the whole scientific men incline to believe that,
according to a popular tradition, the Moon may have been torn from
the earth where the Pacific Ocean now lies, and may have left that
hollow behind it when it went.
[Footnote 11: The figures are: earth's specific gravity = 5·6. The
specific gravity of the surface material ranges from, in general,
between 2·2 and 3·2, with an average of 2·7. The specific gravity of
the Moon is 3·4.]
Many people, scientific men and astronomers among them, have imagined
the possibilities of life on the Moon. In his clever romance, _The
First Man on the Moon_, Mr. H. G. Wells has gathered together all
the more reasoned speculations on the subject. They all turn on one
point--Is there an atmosphere on the Moon which would support life?
There are gases of some kind on the Moon. There must be gases, for
example, shut up in the moon's rocks; there may be gases in the
Moon's interior. Mr. Wells imagined that there was a good deal of gas
inside the Moon; indeed, he went so far as to suppose that the Moon
was partly hollow. If it were we should perhaps be able to accept
that as an explanation of the fact that the Moon is, bulk for bulk,
considerably lighter than the earth, and is, in short, rather lighter
than we should expect it to be. If the Moon were hollow there might
be an atmosphere and water inside it, and a race of beings might
live there--in the underworld of the Moon. The "Selenites," as Mr.
Wells called them, would probably be not in the least like human
beings, though they might be immeasurably more intelligent, because,
seeing that the earth cooled at a later period than the moon, life
might have begun earlier on the Moon, and would have had, perhaps,
many hundreds of thousands of years in which to develop. Mr. Wells
therefore imagined the "Selenites" to resemble in some respects a
race of very large insects with enormous brains. However, we need
pursue these romantic speculations no further, but must turn to
inquire not whether life might exist in the interior of the Moon
(which we can never see), but what would be the kind of life that
could exist on the part of the Moon that we can see.
In the first place, we believe that the atmosphere there would be
very, very thin; as thin as the atmosphere which is left in the
bell-receiver of an ordinary air-pump when the experimenter has done
the best he can to exhaust it of air altogether.[12] In the second
place, the atmosphere would not be one of oxygen and nitrogen as
that of the earth is, but of some heavy gas like carbon-dioxide
(which will not support animal life). The question is whether it
would support vegetable life. Several astronomers (no less eminent
a one than W. H. Pickering, of Harvard University, among them)
have supposed that it might, and they have imagined great jungles
of vegetation springing up on the surface of the Moon under the
influence of the Sun's rays--jungles which are stricken down again
when they are four days old under the oncoming of night. For the
Moon's day is equal to several of ours, and when night comes there
the temperature sinks to a level colder far than that of the earth's
Arctic regions; so cold, in fact, that even gases would be turned
to liquid and then frozen solid. It is by no means certain that the
gases we have mentioned would support vegetable life, but assuming
that they would, we should then expect the vegetation to spring up
with extraordinary rapidity--because it would be so little hampered
by its own weight--when the vertical rays of the Sun were beating
down on the Moon. When that was the case the temperature there would
be from 500° (F.) to 600° (F.) higher than during the night.
[Footnote 12: The exhaustion produced by an ordinary air-pump is
never a complete vacuum. Exhaustion which leaves only 1/2000th of the
original air is unusually efficient.]
Perhaps we may here explain some of the reasons why vegetation would
be little hampered by its own weight on the Moon. It is similar to
the reasons why light gases escape from the Moon. The mass of the
Moon--that is to say, the amount of matter it contains--is 1/80th
that of the earth. Therefore since the weight of a body means the
measure of the force by which gravity attracts it (to the earth or
to the Moon), bodies on the Moon's surface are much lighter than
they are here. The ratio is almost exactly one-sixth; consequently a
man weighing 180 lbs. on the earth if transplanted to the Moon would
find that he only weighed 30 lbs. there, and could carry two men at
once on his back for twenty miles much more easily than he could walk
that distance without a load here. He could throw a stone six times
as far as on the earth, and jump six times as high. Indeed, jumping
over a moderate-sized house would be a gymnastic feat scarcely worth
mentioning on the Moon.
After consideration of all these facts; and despite the belief of
some distinguished astronomers that changes are sometimes perceptible
on the Moon's surface; and that hoar frost can be perceived there;
and perhaps volcanic eruptions--the general conclusion arrived
at by astronomical authority is that organic life does not exist
either on the Moon or in it; and we may conclude this outline of
the speculations concerning it by quoting the American astronomer,
Professor N. S. Shaler: "It is naturally painful to conclude that the
Moon is and always has been deprived of those features of existence
which we deem the nobler; that it has never known the stir of air
or water or the higher life of beings who inherit the profit of
experience, and thereby climb the way that has led upward to man.
That these large gifts have been denied to the nearest companion of
the earth has its lesson for the naturalist. How vast are the effects
arising from the interrelation of actions.... If the gases could
have been retained in the Moon (by its attractive force) there is no
reason why it should not have had the history of a miniature earth.
As it is, from the beginning it appears to have been determined
that the Sun should not warm it in the same way as the earth; that
rain should not fall on it, nor the stirrings of life and energy be
visible on it. There is no imaginable accident that can alter its
state. Just as it is, our Moon is likely to see the Sun go out."
This chapter may be ended by a brief application of some of these
considerations to the case of the planet Mars. Next to the Moon Mars
is our nearest neighbour, and the erection of great telescopes in
America, one of them at Flagstaff Observatory, Arizona, where the air
is extraordinarily clear and telescopic vision unusually penetrating,
has stimulated the observation of the planet to a very great degree
during recent years. Mars has an atmosphere not nearly so dense as
that of the earth, but still dense enough in all probability to
support some form of organic life. It may, for example, support
vegetation. In some other respects Mars resembles the earth. It has
arctic circles; it has clouds, though whether these are of vapour or
of dust is not quite certain; and it has a less variable temperature
by far than that of the Moon. There are, at any rate, some of the
conditions to support and perhaps to encourage life; and if we could
be certain that the atmosphere in Mars more nearly resembles that of
the earth, and that its temperature was such as to be sometimes above
that of our Arctic regions, then it would be difficult to deny that
life, and probably intelligent beings, existed there. One very able
and intelligent astronomer is convinced that life and intelligent
beings do exist there. This is Professor Lowell, of Flagstaff
Observatory, who has devoted a number of years and a great deal
of money to the careful observation of the planet. He has brought
forward many cogent arguments to show that Mars might be inhabited,
and that great telescopes can discover signs on it, and may discover
further signs, which are a reason for supposing it to be so. It is
not, however, within the range of this book to examine these reasons
in detail; and we need only say that in the first decade of the
twentieth century most astronomers, despite the close examination
of Mars and its markings, which had been conducted for more than a
generation, were still not convinced that life as we know it could
exist there.