The very careful
comparisons of the spectra of Mars and of the moon made by
Campbell at the Lick Observatory failed to show the slightest
difference in the two. If Mars had an atmosphere as dense as ours,
the result could be seen in the darkening of the lines of the
spectrum produced by the double passage of the light through it.
There were no lines in the spectrum of Mars that were not seen
with equal distinctness in that of the moon. But this does not
prove the entire absence of an atmosphere. It only shows a limit
to its density. It may be one-fifth or one-fourth the density of
that on the earth, but probably no more.
That there must be something in the nature of vapor at least seems
to be shown by the formation and disappearance of the white polar
caps of this planet. Every reader of astronomy at the present time
knows that, during the Martian winter, white caps form around the
pole of the planet which is turned away from the sun, and grow
larger and larger until the sun begins to shine upon them, when
they gradually grow smaller, and perhaps nearly disappear. It
seems, therefore, fairly well proved that, under the influence of
cold, some white substance forms around the polar regions of Mars
which evaporates under the influence of the sun's rays.
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