Indeed, it may be merely a
temporary condensation of vapor under the form of dew or hoar frost.
Accordingly, as far as we may be permitted to argue from the observed
facts, the climate of Mars must resemble that of a clear day upon a high
mountain. By day a very strong solar radiation, hardly mitigated at all
by mist or vapor; by night a copious radiation from the soil toward
celestial space, and because of that a very marked refrigeration. Hence
a climate of extremes, and great changes of temperature from day to
night, and from one season to another. And as on the earth at altitudes
of 5,000 and 6,000 meters (17,000 to 20,000 feet) the vapor of the
atmosphere is condensed only into the solid form, producing those
whitish masses of suspended crystals which we call cirrus clouds, so in
the atmosphere of Mars it would be rarely possible (or would even be
impossible) to find collections of cloud capable of producing rain of
any consequence. The variation of the temperature from one season to
another would be notably increased by their long duration, and thus we
can understand the great freezing and melting of the snow which is
renewed in turn at the poles at each complete revolution of the planet
around the sun.
As our chart demonstrates, in its general topography Mars does not
present any analogy with the earth. A third of its surface is occupied
by the great Mare Australe, which is strewn with many islands, and the
continents are cut up by gulfs, and ramifications of various forms.
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