In
addition to this, the rarity of its atmosphere would naturally be
expected to decrease the effective temperature at the planet's
surface, since an atmosphere acts somewhat like the glass cover of a
hot-house in retaining the solar heat which has penetrated it. It has
been calculated that, unless there are mitigating circumstances of
which we know nothing, the average temperature at the surface of Mars
must be far below the freezing-point of water. To this it is replied
that the possible mitigating circumstances spoken of evidently exist
in fact, because we can see that the watery vapor condenses into snow
around the poles in winter, but melts again when summer comes. The
mitigating agent may be supposed to exist in the atmosphere where the
presence of certain gases would completely alter the temperature
gradients.
It might also be objected that it is inconceivable that the Martian
engineers, however great may be their physical powers, and however
gigantic the mechanical energies under their control, could force
water in large quantities from the poles to the equator. This is an
achievement that measures up to the cosmical standard. It is admitted
by the champions of the theory that the difficulty is a formidable
one; but they call attention to the singular fact that on Mars there
can be found no chains of mountains, and it is even doubtful if ranges
of hills exist there.
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