THE POLAR CAPS
Almost as soon as magnification gives Mars a disk that disk shows
markings, white spots crowning a globe spread with blue-green patches
on an orange ground. The smallest telescope is capable of this far-off
revelation; while with increased power the picture grows steadily more
articulate and full. With a two and a quarter inch glass the writer saw
them thirty-five years ago.
After the assurance that markings exist the next thing to arrest
attention is that these markings move. The patches of color first made
out by the observer are shortly found by him to have shifted in place
upon the planet. And this not through mistake on his part but through
method in the phenomena; for all do it alike. In orderly rotation the
features make their appearance upon the body’s righthand limb (in the
telescopic image), travel across the central meridian of the disk and
vanish over its lefthand border. One follows another, each rising,
culminating and setting in its turn under the observer’s gaze. A
constantly progressing panorama passes majestically before his sight,
new objects replacing the old with a march so steady and withal so
swift that a few minutes will suffice to mark unmistakably the fact of
such procession. But for all this ceaseless turning under his gaze,
after a certain lapse of time it is evident that the same features are
being shown him over again. With such recognition of recurrence comes
the first advance toward acquaintance with the Martian world. For that
in all their journeying their configuration alters not, proves them
permanent in place, part and parcel of the solid surface of that other
globe. This surface, then, lies exposed to view and by its turning
shows itself subject, like our earth, to the vicissitudes of day and
night.
In such self-exposure Mars differs from all the four great planets,
Jupiter, Saturn, Uranus and Neptune. Features, indeed, are apparent on
the first two of these globes and dimly on the other two as well, but
they lack the stability of the Martian markings. They are forever
exchanging place. In the case of Jupiter what we see is undoubtedly a
cloud-envelop through which occasional glimpses may possibly be caught
of a chaotic nucleus below. With Saturn it is the same; and the
evidence is that the like is true of Uranus and Neptune. What goes on
under their great cloud canopies we can only surmise. With Mars,
however, we are not left to imagination in the matter but so far as our
means permit can actually observe what there takes place. Except for
distance, which, through science, year by year grows less, it is as if
we hovered above the planet in a balloon, with its various features
spread out to our gaze below.
Attention shows these areographic features to be on hand with punctual
precision for their traverse of the disk once every twenty-four hours
and thirty-seven minutes. For over two hundred years this has been the
case, their untiring revolutions having been watched so well that we
know the time they take to the nicety of a couple of hundredths of a
second. We thus become possessed of a knowledge of the length of the
Martian day and it is not a little interesting to find that it very
closely counterparts in duration our own, being only one thirty-fifth
the longer of the two. We also find from the course the markings pursue
the axis about which they turn; and just as the period of the rotation
tells us the length of the Martian day so the tilt of the axis, taken
in connection with the form of the orbit, determines the character of
the Martian seasons. Here again we confront a curious resemblance in
the circumstances of the two planets, for the tilt of the equator to
the plane of the orbit is with Mars almost precisely what it is for the
Earth. The more carefully the two are measured the closer the
similitude becomes. Sir William Herschel made the Martian 28°,
Schiaparelli reduced this to 25°, and later determination by the writer
puts it nearer 24°. The latter is the one now adopted in the British
Nautical Almanac for observers of the planet. This is a very close
parallelism indeed; so that in general character the Martian seasons
are nearly the counterpart of ours. In length, however, they differ;
first because the year of Mars is almost double the length of the
terrestrial one and secondly because from the greater ellipticity of
Mars’ orbit the seasons are more unequal than is the case with us, some
being run through with great haste, others being lingered on a
disproportionate time. It is usual on the Earth to consider spring as
the period from the vernal equinox, about March 21, to the summer
solstice, about June 20; summer as lasting thence to the autumnal
equinox; autumn from this latter date, about September 20, to the
winter solstice on December 21; and winter from that point on to the
next spring equinox again. On this division our seasons in the northern
hemisphere last respectively: spring, 91 days; summer, 92 days; autumn,
92 days; and winter, 90 days. On Mars these become, reckoned in our
days: spring, 199 days; summer 183 days; autumn, 147 days; and winter,
158 days. If we had counted them in Martian days they would have
totaled about one thirty-fifth less in number each.
In its days and seasons, then, Mars is wonderfully like the Earth;
except for the length of the year we should hardly know the difference
in reckoning of time could we some morning wake up there instead of
here. Only in one really unimportant respect should we feel strange; in
months we should find ourselves turned topsy-turvy. But lunations have
nothing to do with climate nor with the alternation between night and
day; and in these two important respects we should certainly feel at
home.
Though the axis could be determined by the daily march of any marking
and thus the planet’s tropic, temperate and polar regions marked out,
the process is made easier by the presence of white patches covering
the planet’s poles and known, in consequence, as the polar caps. It is
from measures of the patches that the position of the Martian poles has
actually been determined. These polar caps are exactly analogous in
general position to those which bonnet our own Earth. They reproduce
the appearance of the ice and snow of our arctic and antarctic regions
seen from space, in a very remarkable manner. In truth they are things
of note in more ways than one and would claim precedence on many
counts. Priority of recognition, however, alone entitles them to
premier consideration. Among the very first of the disk’s detail to be
made out by man, they justly demand description first.
With peculiar propriety the polar caps have thus the _pas_. Not only do
they stand first in order of visibility, but they prove to occupy a
like position logically when it comes to an explanation of the planet’s
present physical state. It is not matter of hazard that the most
evident of all the planet’s markings should also be the most
fundamental, the fountainhead from which everything else flows. It is
of the essence of the planet’s condition and furnishes the key to its
comprehension. The steps leading to this conclusion are as interesting
as they are cogent. They start at the polar caps’ visibility. For their
size first riveted man’s attention and then attention to them disclosed
that most vital of the characteristics of the planet’s surface: change.
Just as almost all of the features we note are permanent in place,
showing that they belong to the surface, so are they all impermanent in
character. Change is the only absolutely unchanging thing except
position about the features the planet presents to view. It was in the
aspect of the polar caps that this important fact first came to light.
Not only did they thus initially instance a general law, they have
turned out to make it; for by themselves changing they largely cause
change in all the rest. But for a long time they alone exemplified its
workings. To Sir William Herschel we owe the first study of their
change in aspect. This eminent observer noted that their varying size
was subject to a regular rhythmic wax and wane timed to the course of
the seasons of the planet’s year. The caps increased in the winter of
their hemisphere and decreased in its summer and being situate in
opposite hemispheres they did this alternately with pendulum-like
precision. His observations were soon abundantly confirmed, for the
phenomena take place upon a vast scale and are thus easy of
recognition. At their maximum spread the caps cover more than one
hundred times as much ground as when they have shrunk to their minimum.
In the depth of winter they stretch over much more than the polar zone,
coming down to 60° and even 50° of latitude north or south as the case
may be, thence melting till by midsummer they span only five or six
degrees across.
In this they bear close analogue to the behavior of our own. Ours would
show not otherwise were they viewed from the impersonal standpoint of
space. Very little telescopic aid suffices to disclose the Martian
polar phenomena in this their more salient characteristics and convince
an observer of their likeness to those of the earth. Any one may note
what is there going on by successive observations of the planet with a
three-inch glass. Nor is the change by any means slow. A few days at
the proper Martian season, or at most a couple of weeks, produces
conspicuous and conclusive alterations in the size of these nightcaps
of the planet’s winter sleep. Resembling our own so well they were
early surmised to be of like constitution and composed, therefore, of
ice and snow. Plausible on its face, this view of them was generally
adopted and common sense has held to it ever since. It has encountered,
of course, opposition, partly from very proper conservatism, but
chiefly from that earth-centred philosophy which has doubted most
advances since Galileo’s time, and carbonic acid has been put forward
by this school of sceptics to take its place. We shall critically
examine both objections; the latter first, because a certain physical
fact enables us to dispose of it at once. In casual appearance there is
not much to choose between the rival candidates of common sense and
uncommon subtlety, water and frozen carbonic acid gas, both being
suitably white and both going and coming with the temperature. But,
upon closer study, in one point of behavior the two substances act
quite unlike, and had half the ingenuity been expended in testing the
theory as in broaching it this fact had come to light to the suggestors
as it did upon examination to the writer and had served as a touchstone
in the case. At pressures of anything like one atmosphere or less
carbonic acid passes at once from the solid to the gaseous state.
Water, on the other hand, lingers in the intermediate stage of a
liquid. Now, as the Martian cap melts it shows surrounded by a deep
blue band which accompanies it in its retreat, shrinking to keep pace
with the shrinkage in the cap. This is clearly the product of the
disintegration since it waits so studiously upon it. The substance
composing the cap, then, does not pass instantaneously or anything like
it from the solid to the gaseous condition.
This badge of blue ribbon about the melting cap, therefore,
conclusively shows that carbonic acid is not what we see and leaves us
with the only alternative we know of: water.