It has
been supposed that this substance is snow, produced in the same
way that snow is produced on the earth, by the evaporation of
water.
But there are difficulties in the way of this explanation. The sun
sends less than half as much heat to Mars as to the earth, and it
does not seem likely that the polar regions can ever receive
enough of heat to melt any considerable quantity of snow. Nor does
it seem likely that any clouds from which snow could fall ever
obscure the surface of Mars.
But a very slight change in the explanation will make it tenable.
Quite possibly the white deposits may be due to something like
hoar-frost condensed from slightly moist air, without the actual
production of snow. This would produce the effect that we see.
Even this explanation implies that Mars has air and water, rare
though the former may be. It is quite possible that air as thin as
that of Mars would sustain life in some form. Life not totally
unlike that on the earth may therefore exist upon this planet for
anything that we know to the contrary. More than this we cannot
say.
In the case of the outer planets the answer to our question must
be in the negative. It now seems likely that Jupiter is a body
very much like our sun, only that the dark portion is too cool to
emit much, if any, light.
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