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° 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.

Professor Young says: "As to the temperature of Mars we have no certain
knowledge. On the one hand, we know that on account of the planet's
distance from the sun the intensity of solar radiation upon its surface
must be less than here in the ratio of 1 to (1.524)^2--i.e., only about
43 per cent as great as with us; its 'solar constant' must be less than
13 calories against our 30. Then, too, the low density of its
atmosphere, probably less at the planet's surface than on the tops of
our highest mountains, would naturally assist to keep down the
temperature to a point far below the freezing-point of water. But, on
the other hand, things certainly _look_ as if the polar caps were really
masses of _snow_ and _ice_ deposited from vapor in the planet's
atmosphere, and as if these actually melted during the Martian summer,
sending floods of water through the channels provided for them, and
causing the growth of vegetation along their banks. We are driven,