THE RUDDY PLANET
Mars is a planet next in order beyond the earth, and its distance from
the sun averages 141-1/2 million miles. It has a relatively rapid motion
among the stars, its color is reddish, and, when nearest to us, it is
perhaps the most conspicuous object in the sky.
Mars appeared to the ancients just as it does to us to-day. Aristotle
recorded an observation of Mars, 356 B. C., when the moon passed over
the planet, or occulted it, as our expression is. Galileo made the first
observations of Mars with a telescope in 1610, and his little instrument
was powerful enough to enable him to discover that the planet had
phases, though it did not pass through all the phases that Mercury and
Venus do. This was obvious from the fact that Mars is always at a
greater distance from the sun than we are, and the phase can only be
gibbous, or about like the moon when midway between full and quarter.
Many observers in the seventeenth century followed up the planet with
such feeble optical power as the telescopes of that epoch provided:
Fontana (who made the first sketch), Riccioli and Bianchini in Italy,
Cassini in France, Huygens in Holland, and later Sir William Herschel in
England.
It was Cassini who first made out the whitish spots or polar caps of
Mars in 1666, but not until after Huygens had noted the fact that Mars
turned round on an axis in a period but little longer than the earth's.
Cassini followed it up later with a more accurate value; and
observations in our own day, when combined with these early ones, enable
us to say that the Martian day is equal to 24 hours 37 minutes 22.67
seconds, accurate probably to the hundredth part of a second.
When we know that a planet turns round on an axis, we know that it has a
day. When we know the direction of the axis in space or in relation to
the plane of its path round the sun, we know that it has seasons: we can
tell their length and when they begin and end. It did not take many
years of observation to prove that the axis round which Mars turns is
tilted to the plane of its path round the sun by an angle practically
the same as that at which the earth's axis is tilted. So there is the
immediate inference that on Mars the order and perhaps the character of
the seasons is much the same as here on the earth.
At least two things, however, tend to modify them. First, the year of
Mars is not 365 days like ours, but 687 days. Each of the four seasons
on Mars, therefore, is proportionally longer than our seasons are. Then
comes the question of atmosphere--how much of an atmosphere does Mars
really possess in proportion to ours, and how would its lesser amount
modify the blending of the seasons into one another?
All discussion of Mars and the problems of existence of life upon that
planet hinge upon the character and extent of Martian atmosphere. The
planet seems never to be covered, as the earth usually is, with
extensive areas of cloud which to an observer in space would completely
mask its oceans and continents. Nearly all the time Mars in his
equatorial and temperate zones is quite clear of clouds. A few whitish
spots are occasionally seen to change their form and position in both
northern and southern latitudes, and they vary with the progress of the
day on Mars, as clouds naturally would. But Schiaparelli, perhaps the
best of all observers, thought them to be not low-lying clouds of the
nimbus type that would produce rains, but rather a veil of fog, or
perhaps a temporary condensation of vapor, as dew or hoar frost. But the
strongest argument for an atmosphere is based on the temporary darkening
or obscuration of well known and permanent markings on the surface of
Mars. These are more or less frequently observed and clouds afford the
best explanation of their occurrence.
So much for evidence supplied by the telescope alone. When, however, we
employ the spectroscope in conjunction with the telescope, another sort
of evidence is at hand. Several astronomers have reached the conclusion
that watery vapor exists in the atmosphere of Mars, while other
astronomers equipped with equal or superior apparatus, and under equally
favorable or even better conditions, have reached the remarkable
conclusion that the spectra of Mars and the moon are identical in every
particular. From this we should be led to infer that Mars has perhaps no
more atmosphere than the moon has, that is to say, none whatever that
present instruments and methods of investigation have enabled us to
detect.
What then, shall we conclude? Simply that the atmosphere of Mars is
neither very dense nor extensive. Probably its lower strata close to
the planet's surface are about as dense as the earth's atmosphere is at
the summits of our highest mountains.
This conclusion is not unwelcome, if we keep a few fundamental facts in
clear and constant view. Mars is a planet of intermediate size between
the earth and the moon: twice the moon's diameter (2,160 miles) very
nearly equals the diameter of Mars (4,200 miles), and twice the diameter
of Mars does not greatly exceed the earth's diameter (7,920 miles). As
to the weights or masses of these bodies, Mars is about one-ninth, and
the moon one-eightieth of the earth. The atmospheric envelope of the
earth is abundant, the moon has none as far as we can ascertain; so it
seems safe to infer that Mars has an atmosphere of slight density: not
dense enough to be detected by spectroscopic methods, but yet dense
enough to enable us to explain the varying telescopic phenomena of the
planet's disk which we should not know how to account for, if there were
no atmosphere whatever. One astronomer has, indeed, gone so far as to
calculate that in comparison with our planet Mars is entitled to
one-twentieth as much atmosphere as we have, and that the mercurial
barometer at "sea level" would run about five and a half inches, as
against thirty inches on the earth.
In general, then, the climate of Mars is probably very much like that of
a clear season on a very high terrestrial table land or mountain--a
climate of wide extremes, with great changes of temperature from day to
night. The inequality of Martian seasons is such that in his northern
hemisphere the winter lasts 381 days and the summer only 306 days.
Now, the polar caps of Mars, which are reasonably assumed to be due to
snow or hoar frost, attain their maximum three or four months after the
winter solstice, and their minimum about the same length of time after
the summer solstice. This lagging should be interpreted as an argument
for a Martian atmosphere with heat-storing qualities, similar to that
possessed by the earth.
Upon this characteristic, indeed, depends the climate at the surface of
Mars: whether it is at all similar to our own, and whether fluid water
is a possibility on Mars or not. While the cosmic relations of the
planet in its orbit are quite the same as ours, nevertheless the greater
distance of Mars diminishes his supply of direct solar heat to about
half what we receive. On the other hand, his distance from the sun
during his year of motion around it varies much more widely than ours,
so that he receives when nearest the sun about one-half more of solar
heat than he does when farthest away.
Southern summers on Mars, therefore, must be much hotter, and southern
winters colder than the corresponding seasons of his northern
hemisphere. Indeed, the length of the southern summer, nearly twice that
of the terrestrial season, sometimes amply suffices to melt all the
polar ice and snow, as in October, 1894, when the southern polar cap of
Mars dwindled rapidly and finally vanished completely.
Very interesting in this connection are the researches of Stoney on the
general conditions affecting planetary atmospheres and their
composition. According to the kinetic theory, if the molecules of gases
which are continually in motion travel outward from the center of a
planet, as they frequently must, and with velocities surpassing the
limit that a planet's gravity is capable of controlling, these molecules
will effect a permanent escape from the planet, and travel through space
in orbits of their own.
So the moon is wholly without atmosphere because the moon's gravity is
not powerful enough to retain the molecules of its component gases. So
also the earth's atmosphere contains no helium or free hydrogen. So,
too, Mars is possessed of insufficient force of gravity to retain water
vapor, and the Martian atmosphere may therefore consist mainly of
nitrogen, argon, and carbon dioxide.
As everyone knows, the axis of the earth if extended to the northern
heavens would pass very near the north polar star, which on that account
is known as Polaris. In a similar manner the axis of Mars pierces the
northern heavens about midway between the two bright stars Alpha Cephei
and Alpha Cygni (Deneb). The direction of this axis is pretty accurately
known, because the measurement of the polar caps of the planet as they
turn round from night to night, year in and year out, has enabled
astronomers to assign the inclination of the axis with great precision.
These caps are a brilliant white, and they are generally supposed to be
snow and ice. They wax and wane alternately with the seasons on Mars,
being largest at the end of the Martian winter and smallest near the end
of summer. The existence of the polar caps together with their seasonal
fluctuations afford a most convincing argument for the reality of a
Martian atmosphere, sufficiently dense to be capable of diffusing and
transporting vapor.
The northern cap is centered on the pole almost with geometric
exactness, and as far as the 85th parallel of latitude. On the other
hand, the south polar cap is centered about 200 miles from the true
pole, and this distance has been observed to vary from one season to
another. No suggestion has been made to account for this singular
variation. On one occasion it stretched down to Martian latitude 70
degrees and was over 1,200 miles in diameter.
Pickering watched the changing conditions of shrinking of the south
polar cap in 1892 with a large telescope located in the Andes of Peru.
Mars was faithfully followed on every night but one from July 13 to
September 9, and the apparent alterations in this cap were very marked,
even from night to night. As the snows began to decrease, a long dark
line made its appearance near the middle of the cap, and gradually grew
until it cut the cap in two. This white polar area (and probably also
the northern one in similar fashion) becomes notched on the edge with
the progress of its summer season; dark interior spots and fissures
form, isolated patches separate from the principal mass, and later seem
to dissolve and disappear. Possibly if one were located on Mars and
viewing our earth with a big telescope, the seasonal variation of our
north and south polar caps might present somewhat similar phenomena. All
the recent oppositions of Mars have been critically observed by
Pickering from an excellent station in Jamaica.
Quite obviously the fluctuations of the polar caps are the key to the
physiographic situation on Mars, and they are made the subject of the
closest scrutiny at every recurring opposition of the planet. Several
observers, Lowell in particular, record a bluish line or a sort of
retreating polar sea, following up the diminishing polar cap as it
shrinks with the advance of summer. It is said that no such line is
visible during the formation of the polar cap with the approach of
winter. All such results of critical observation, just on the limit of
visibility, have to be repeated over and over again before they become
part of the body of accepted scientific fact. And in many instances the
only sure way is to fall back on the photographic record, which all
astronomers, whether prejudiced or not, may have the opportunity to
examine and draw their individual conclusions.
Already the approaching opposition of 1924, the most favorable since the
invention of the telescope, is beginning to attract attention, and
preparations are in progress, of new and more powerful instruments, with
new and more sensitive photographic processes, by means of which many of
the present riddles of Mars may be solved.