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.

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, therefore, to suppose either that the planet has sources of heat internal or external which are not yet explained, or else, as long ago suggested, that the polar ‘snow’ may possibly be composed of something else than frozen water."[4]

[Footnote 4:  General Astronomy, by Charles A. Young.  Revised edition, 1898, p. 363.]

Even while granting the worst that can be said for the low temperature of Mars, the persistent believer in its habitability could take refuge in the results of recent experiments which have proved that bacterial life is able to resist the utmost degree of cold that can be applied, microscopic organisms perfectly retaining their vitality—­or at least their power to resume it—­when subjected to the fearfully low temperature of liquid air.  But then he would be open to the reply that the organisms thus treated are in a torpid condition and deprived of all activity until revived by the application of heat; and the picture of a world in a state of perpetual sleep is not particularly attractive, unless the fortunate prince who is destined to awake the slumbering beauty can also be introduced into the romance.[5]