Then there are many observations on record indicating the existence of
clouds in Mars's atmosphere. Sometimes a considerable area of its
surface has been observed to be temporarily obscured, not by dense
masses of cloud such as accompany the progress of great cyclonic storms
across the continents and oceans of the earth, but by comparatively thin
veils of vapor such as would be expected to form in an atmosphere so
comparatively rare as that of Mars. And these clouds, in some instances
at least, appear, like the cirrus streaks and dapples in our own air, to
float at a great elevation. Mr. Douglass, one of Mr. Lowell's associates
in the observations of 1894 at Flagstaff, Arizona, observed what he
believed to be a cloud over the unilluminated part of Mars's disk,
which, by micrometric measurement and estimate, was drifting at an
elevation of about fifteen miles above the surface of the planet. This
was seen on two successive days, November 25th and November 26th, and it
underwent curious fluctuations in visibility, besides moving in a
northerly direction at the rate of some thirteen miles an hour. But,
upon the whole, as Mr. Lowell remarks, the atmosphere of Mars is
remarkably free of clouds.
The reader will remember that Mars gets a little less than half as much
heat from the sun as the earth gets. This fact also has been used as an
argument against the habitability of the planet. In truth, those who
think that life in the solar system is confined to the earth alone
insist upon an almost exact reproduction of terrestrial conditions as a
_sine qua non_ to the habitability of any other planet.
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