Venus, they
think, is too hot, and Mars too cold, as if life were rather a happy
accident than the result of the operation of general laws applicable
under a wide variety of conditions. All that we are really justified in
asserting is that Venus may be too hot and Mars too cold for _us_. Of
course, if we adopt the opinion held by some that the temperature on
Mars is constantly so low that water would remain perpetually frozen, it
does throw the question of the kind of life that could be maintained
there into the realm of pure conjecture.
The argument in favor of an extremely low temperature on Mars is based
on the law of the diminution of radiant energy inversely as the square
of the distance, together with the assumption that no qualifying
circumstances, or no modification of that law, can enter into the
problem. According to this view, it could be shown that the temperature
on Mars never rises above -200 deg. F. But it is a view that seems to be
directly opposed to the evidence of the telescope, for all who have
studied Mars under favorable conditions of observation have been
impressed by the rapid and extensive changes that the appearance of its
surface undergoes coincidently with the variation of the planet's
seasons. It has its winter aspect and its summer aspect, perfectly
distinct and recognizable, in each hemisphere by turns, and whether the
polar caps be snow or carbon dioxide, at any rate they melt and
disappear under a high sun, thus proving that an accumulation of heat
takes place.
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