In size Mars is one of the smallest members of our solar family. Its
mass is a little more than one-ninth that of the earth, and its entire
surface is only about one-third as great as ours. It is the merest
trifle more dense than Mercury, but only about sixty-six one-hundredths
as dense as the earth. Its force of gravity is about thirty-six
one-hundredths as powerful as that of the earth. A man weighing
two hundred pounds here would be relieved of about one hundred and
twenty-four pounds of his weight if transported to Mars, weighing there
only seventy-six pounds, which would greatly increase his efficiency if
he were in other respects the same.
It would necessarily follow that Mars, having such small force of
gravity, could not long retain a heavy atmosphere, even if it had set
out with such a one. No molecule of gas moving at a greater speed than
three and thirteen-hundredths miles a second could be held by Mars in
its atmosphere, and so much as it may have had of the rarer gases
which move with great rapidity must have escaped long ago. But it did
not begin life with an atmosphere heavy in proportion to that which the
larger planets have. We have seen, in the case of Mercury, that being
one of the small planets entails many restrictions in development. Such
planets not only lose their atmosphere more quickly than the larger
ones, but it is less dense to begin with. The atmosphere of Mars is
probably no denser than we have at the tops of our highest mountains,
more than likely not even so dense as that. There is some water vapor,
and there are a few clouds most of the time; but in the main the
atmosphere is so clear and thin that we can without any doubt see the
actual surface of the planet. It is not certain that the clouds we
see are formed from water vapor, as clouds of the ordinary kind are.
It has been suggested that they may be simply dust-clouds. But this
is as yet not much more than a suggestion, and nothing convincing has
been offered to substantiate the idea. Even dust-clouds would need
currents of air to create and carry them; so, whether dust or vapor,
the presence of clouds implies an atmosphere.
The famous white polar caps, which furnish so many news items to the
journals, are also of uncertain origin, and their true nature can be
determined only by a fuller knowledge of the atmosphere of Mars. They
appear in the winter season on the planet and disappear in its summer,
so there seems to be no doubt that they are dependent in some way on
the temperature in the polar regions of Mars. If they are hoar-frost
or snow, they are condensations of water vapor; and, in that case,
when they disappear there must be sufficient heat to melt them. It has
been contended that the sun’s rays fall too obliquely on the poles of
Mars to melt more than a few inches of snow, but that the caps may be
light snow or frost, and thus capable of being dissolved by even such
oblique rays of sunlight as they receive. Also it has been suggested
that the deposit resembling snow may be carbon dioxide, which condenses
into a white substance at a temperature more than a hundred degrees
(-109° Fahr.) lower than is necessary to produce snow and melts at a
correspondingly low temperature. What the nature of the phenomenon seen
at the poles of Mars is depends largely upon what the temperature is;
and the temperature in turn is dependent in some measure on the density
and constitution of the atmosphere, as well as the planet’s distance
from the sun.
The normal temperature of an unprotected body at the distance of Mars
from the sun is about thirty-two degrees blow zero (Fahrenheit); and
since we know Mars has no dense atmosphere to retain the heat it
acquires, it is natural to suppose the existence there of a very low
temperature, and one incompatible with our ideas of life and growth.
The most favorable conclusions do not place the mean temperature higher
than forty-eight degrees Fahrenheit. It is certain that the planet
must be subjected to great extremes of temperature within its range,
since its filmy robe of atmosphere cannot protect it to any extent from
the direct rays of the sun during the day, nor prevent the heat from
escaping with great rapidity at night; so that, whatever heat it may
gain in the daytime, it probably loses much of it during the night.
Until we know more of the constitution of the atmosphere of Mars we can
know nothing certainly about its temperature beyond the fact that it is
much colder than ours and more subject to variations. Anything much
more definite than this is speculative at present. But with all the
observation that is now given to Mars, and with the always increasing
facilities for the work, many uncertainties regarding the planet are
likely to be made clear before long. The spectroscope will probably be
the final resort for facts concerning the atmosphere.