The planet Venus, the bright globe which is known to all as the morning
and evening "star," seems at first sight more promising as regards the
possibility of life. It is of nearly the same size as the earth, and it
has a good atmosphere, but there are many astronomers who believe that,
like Mercury, it always presents the same face to the sun, and it would
therefore have the same disadvantage--a broiling heat on the sunny side
and the cold of space on the opposite side. We are not sure. The
surface of Venus is so bright--the light of the sun is reflected to us
by such dense masses of cloud and dust--that it is difficult to trace
any permanent markings on it, and thus ascertain how long it takes to
rotate on its axis. Many astronomers believe that they have succeeded,
and that the planet always turns the same face to the sun. If it does,
we can hardly conceive of life on its surface, in spite of the
cloud-screen.
[Illustration: FIG. 14.--THE MOON
Showing a great plain and some typical craters. There are thousands of
these craters, and some theories of their origin are explained on page
34.]
[Illustration: FIG. 15.--MARS
1} Drawings by Prof. Lowell to accompany actual photographs of Mars
showing many of the
2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
3 Drawing by Prof. Lowell made January 6, 1914.
4 Drawing by Prof. Lowell made January 21, 1914.
Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
depict quite different sections. Note the change in the polar snow-caps
in the last two.]
[Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS
Note the mysterious "rays" diverging from the almost perfectly circular
craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
also the mountains to the right with the lunar dawn breaking on them.]
We turn to Mars; and we must first make it clear why there is so much
speculation about life on Mars, and why it is supposed that, if there
_is_ life on Mars, it must be more advanced than life on the earth.
Is there Life on Mars?
The basis of this belief is that if, as we saw, all the globes in our
solar system are masses of metal that are cooling down, the smaller will
have cooled down before the larger, and will be further ahead in their
development. Now Mars is very much smaller than the earth, and must have
cooled at its surface millions of years before the earth did. Hence, if
a story of life began on Mars at all, it began long before the story of
life on the earth. We cannot guess what sort of life-forms would be
evolved in a different world, but we can confidently say that they would
tend toward increasing intelligence; and thus we are disposed to look
for highly intelligent beings on Mars.
But this argument supposes that the conditions of life, namely air and
water, are found on Mars, and it is disputed whether they are found
there in sufficient quantity. The late Professor Percival Lowell, who
made a lifelong study of Mars, maintained that there are hundreds of
straight lines drawn across the surface of the planet, and he claimed
that they are beds of vegetation marking the sites of great channels or
pipes by means of which the "Martians" draw water from their polar
ocean. Professor W. H. Pickering, another high authority, thinks that
the lines are long, narrow marshes fed by moist winds from the poles.
There are certainly white polar caps on Mars. They seem to melt in the
spring, and the dark fringe round them grows broader.
Other astronomers, however, say that they find no trace of water-vapour
in the atmosphere of Mars, and they think that the polar caps may be
simply thin sheets of hoar-frost or frozen gas. They point out that, as
the atmosphere of Mars is certainly scanty, and the distance from the
sun is so great, it may be too cold for the fluid water to exist on the
planet.
If one asks why our wonderful instruments cannot settle these points,
one must be reminded that Mars is never nearer than 34,000,000 miles
from the earth, and only approaches to this distance once in fifteen or
seventeen years. The image of Mars on the photographic negative taken in
a big telescope is very small. Astronomers rely to a great extent on the
eye, which is more sensitive than the photographic plate. But it is easy
to have differences of opinion as to what the eye sees, and so there is
a good deal of controversy.
In August, 1924, the planet will again be well placed for observation,
and we may learn more about it. Already a few of the much-disputed
lines, which people wrongly call "canals," have been traced on
photographs. Astronomers who are sceptical about life on Mars are often
not fully aware of the extraordinary adaptability of life. There was a
time when the climate of the whole earth, from pole to pole, was
semi-tropical for millions of years. No animal could then endure the
least cold, yet now we have plenty of Arctic plants and animals. If the
cold came slowly on Mars, as we have reason to suppose, the population
could be gradually adapted to it. On the whole, it is possible that
there is advanced life on Mars, and it is not impossible, in spite of
the very great difficulties of a code of communication, that our "elder
brothers" may yet flash across space the solution of many of our
problems.
§ 2