ps of
this planet. Every reader of astronomy at the present time knows that,
during the Martian winter, white caps form around the pole of the
planet which is turned away from the sun, and grow larger and larger
until the sun begins to shine upon them, when they gradually grow
smaller, and perhaps nearly disappear. It seems, therefore, fairly well
proved that, under the influence of cold, some white substance forms
around the polar regions of Mars which evaporates under the influence
of the sun's rays. 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. It is doubtful whether Jupiter has anything in the nature of a
solid surface. Its interior is in all likelihood a mass of molten
matter far above a red heat, which is surrounded by a comparatively
cool, yet, to our measure, extremely hot, vapor. The belt-like clouds
which surround the planet are due to this vapor combined with the rapid
rotation. If there is any solid surface below the atmosphere that we
can see, it is swept by winds such that nothing we have on earth could
withstand them. But, as we have said, the probabilities are very much
against there being anything like such a surface. At some great depth
in the fiery vapor there is a s