Page:Mars - Lowell.djvu/71

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49
ATMOSPHERE

limb. The effect we are familiar with on Earth in the haze that always borders the horizon,—a haze most noticeable in places where there is dust, or ice, or water in the air. Here, then, we have a hint of the state of things on Mars. Ice particles both are probable and would give the brilliancy required.

This first hint receives independent support from another Martian phenomenon. Contrary to what the distance of the planet from the Sun and the thinness of its atmospheric envelope would lead us to expect, the climate of Mars appears to be astonishingly mild. Whereas calculation from distance and atmospheric density would put its average temperature below freezing, thus relegating it to perpetual ice, the planet’s surface features imply that the temperature is relatively high. Observation gives every evidence that the mean temperature must actually be above that of the Earth; for not only is there practically no sign of snow or ice outside the frigid zone at any time, but the polar snow-caps melt to a minimum quite beyond that of our own, affording rare chance for quixotic polar expeditions. Such plea8ing amelioration of the climate must be accounted for, and aqueous vapor seems the most likely thing to do it; for aqueous vapor is quite specific as a planetary comforter, being the very best of blankets. It acts, indeed, like the glass