MARTIAN POLAR EXPEDITIONS
Polar expeditions exert an extreme attraction on certain minds, perhaps
because they combine the maximum of hardship with the minimum of
headway. Inconclusiveness certainly enables them to be constantly
renewed, without loss either of purpose or prestige. The fact that the
pole has never been trod by man constitutes the lodestone to such
undertakings; and that it continues to defy him only whets his endeavor
the more. Except for the demonstration of the polar drift-current
conceived of and then verified by Nansen, very little has been added by
them to our knowledge of the globe. Nor is there specific reason to
suppose that what they might add would be particularly vital. Nothing
out of the way is suspected of the pole beyond the simple fact of being
so positioned. Yet for their patent inconclusion they continue to be
sent in sublime superiority to failure.
Martian polar expeditions, as undertaken by the astronomer, are the
antipodes of these pleasingly perilous excursions in three important
regards, which if less appealing to the gallery commend themselves to
the philosopher. They involve comparatively little hardship; they have
accomplished what they set out to do; and the knowledge they have
gleaned has proved fundamental to an understanding of the present
physical condition of the planet.
The antithesis in pole-pursuing between the two planets manifests
itself at the threshold of the inquiry, in the relative feasibility
with which the phenomena on Mars may be scanned. For, curiously enough,
instead of being the pole and its surrounding paleocrystic ice which
remains hidden on Mars, it is rather the extreme extent of its
extension and the lowest latitudinal deposit of frost which lies
shrouded in mystery. The difficulty there is not to see the pole but to
see in winter the regions from which our own expeditions set out. And
this because the poles are well displayed to us at times which are
neither few nor very far between; while favorable occasions for marking
the edge of the caps when at their greatest have neither proved so
numerous nor so favorable. The tilt of the planet’s axis when
conveniently placed for human observation has been the cause of the one
drawback; the planet’s meteorological condition in those latitudes at
that season the reason for the other.
What knowledge we have of the size of the caps in degrees upon the
surface of the planet at this their extreme equatorward extension has
been given in the last chapter. Their aspect at the time together with
what that aspect betokens was not there touched upon. With it,
therefore, and the peculiarities it presents to view we shall begin our
account of the caps’ annual history.
[Illustration: South Polar Cap in winter.]
When first the hemisphere, the pole of which has for half a Martian
year been turned away from the sun, begins to emerge from its long
hibernation, the snow-cap which covers it down even to temperate
regions presents an undelimited expanse of white, the edges of which
merge indistinguishably into the groundwork color of the regions round
about. Of a dull opaque hue along its border, its contour is not sharp
but fades off in a fleecy fringe without hard and fast line of
demarcation. Such notably was the aspect of the north temperate zone in
1896 when, tilted as it then was away from us into a mere northern
horizon of the planet’s limb, it showed prior to the definite
recognition of the north polar cap in August of that year, and such too
was the look of the disk’s southern edge both before and after the
first certain detection of the southern cap in 1903 and 1905. Each was
then in the depth of winter. For in Martian chronology the season
corresponded in each at the time to what we know in our northern
hemisphere as the latter part of February and the early part of March
and the appearance of the planet’s surface in both was not unlike what
we know at the same season in latitude 45°. Indeed, there is reason to
suppose bad weather there then and the extreme fringe, from the pale
tint it exhibited, to have been cloud rather than snow.
It is quite in keeping with what we know on earth or can conceive of
elsewhere that such aspect should characterize the cap at or near the
attainment of its greatest development. Whether it were not yet quite
arrived at this turning-point of its career or had but slightly passed
it a vagueness of outline would in either event proclaim the fact. For
were the frost still depositing, the cap’s edge would show indefinite;
and on the other hand had it just begun to melt, evaporation would give
it an undefined edge before the melting water had gathered in
sufficient quantities to be itself noticeable.
Its behavior subsequent to recognition bore out the inference from its
aspect when it first appeared. While for many days prior to its coming
unmistakably into view it was impossible to say whether what was seen
of the southern cap in 1903 and 1905 was cloud or snow; so even after
it had definitely disclosed itself it continued to play at odds with
the observer. Showing sharp at the edges one day it would appear but
hazily defined the next, thus clearly demonstrating itself to be at the
then unstable acme of its spread. Such a state of things we are only
too familiar with in our own March weather when after days of sunshine
that have melted off the winter’s white and fringed it with rivulets
and awakening grass, a snow-storm falling upon it powders the ground
again that was beginning to be bare and at one stroke extends the
domain of the snow while mystifying the actual limits it may be said to
occupy. The same condition of things, then, is not unknown on Mars, and
to fix the precise date of so wavering a phenomenon is not so much
matter of difficult observation as of physical impossibility.
[Illustration: Hellas in winter.]
Nor is the southern cap, at this the height of its winter expansion,
confined strictly to its own proper limits. Faint extensions, now so
connected with its main body as to form part and parcel of it, now so
detached and dull of tint as to make the observer doubtful of the exact
relationship, are generally to be seen attendant on it. Hellas in
winter is much given to such questionable garb, and has in consequence
been mistaken by more than one observer for the cap itself, appearing
as it does well upon the southern limb and being often the only region
to show white. Indeed, frost-bound as it then is, to consider it the
polar cap, though possibly geographically incorrect, may
climatologically be sustainable. Its northern extremity extends down to
latitude 30°, a pretty low latitude for frost. Still such equatorward
extension is not without corroborating parallel. In 1903, at what was
in Martian dates April 26, the whole of the region south of the Solis
Lacus and the Nectar showed white, with a whiteness which may as well
have been hoarfrost as cloud. Now, the Nectar runs east and west in
latitude 28°. So that in this instance, too, it is possible that arctic
conditions knocked at the very doors of the tropics. Encroachment of
the sort is equivalent to snow in Cairo and permanent snow at that; not
an occasional snow flurry, but something to linger on the ground and
stay visible sixty millions of miles away.
[Illustration: White south of Nectar and Solis Lacus.]
Knowledge of either cap in this the midwinter of its year has been a
matter of the most recent oppositions of the planet. Up to within the
last few years our acquaintance with either cap was chiefly confined to
the months,—one might almost say the weeks,—immediately surrounding
the summer solstice of its respective hemisphere. The behavior of the
caps during the rest of their career was largely unknown to us, from
the very disadvantageous positions they occupied at the times the
planet was nearest to the earth. Beginning with 1894, however, our
knowledge of both has been much extended, by a proportionate extension
of the period covered by the observations. It used to be thought
impracticable to observe the planet far on either side of opposition;
now it is observed from as much as four months before that event to the
same period after it. The result is a systematic series of observations
which in many ways has given unexpected insight into Martian
conditions. One of the benefits secured has been the lengthening of the
period of study of the cap’s career, a pushing of inquiry farther back
into its spring history and a longer lingering with it in its autumnal
rebuilding. Yet up to the very last opposition a gap in its chronology
still remained between February 25 and April 1. The opposition of 1905
has bridged this hiatus and brought us down to the latter date, at
which the melting of the cap begins in earnest.
From this point, April 1 on, we have abundant evidence of the cap’s
behavior. Its career now for some time is one long chronicle of
contraction. Like Balzac’s _Peau de Chagrin_ it simply shrinks, giving
out of its virtue in the process. The cap proceeds to dwindle almost
under the observer’s eye till, from an enormous white counterpane
spread over all the polar and a large part of the temperate zone, its
area contracts to but the veriest nightcap of what it was before. From
seventy degrees across it becomes sixty, then fifty, then forty, till
by the middle of the Martian May it has become not more than thirty
degrees in diameter. During this time, from the moment the melting
began in good earnest, the retreating white is girdled by a dark band,
of a blue tint, which keeps pace with the edge of the cap, shrinking as
it shrinks, and diminishing in width as the volume of the melting
decreases.
After the melting has been for some time under way and the cap has
become permanently bordered by its dark blue band a peculiar phenomenon
makes its appearance in the cap itself. This is its fission into one or
more parts. The process begins by the appearance of dark rifts which,
starting in from the cap’s exterior, penetrate into its heart until at
last they cleave it in two. Rifts have been seen by several observers
and in both caps; and what is most suggestive they always appear in the
same places, year after year. Sometimes oppositions elapse between
their several detections for they are not the least difficult of
detail; but when they are caught, they prove to lie just where they did
before.
The permanency in place of the rifts, a characteristic true of them
all, shows them to be of local habit. Thus the rift of 1884 and 1897
reappeared again to another observer in the same position in 1901. They
are, therefore, features of, or directly dependent on, the surface of
the planet. But it will not do from this fact to infer that they are
expressive of depressions there. The evidence is conclusive that great
irregularities of surface do not exist on Mars. As we shall see when we
come to consider the orology of the planet it is certain that
elevations there of over two or three thousand feet in altitude are
absent. Differences of temperature, able to explain a melting of the
ice in one locality coincidentally with its retention in an adjacent
one, must in consequence be unknown. And this much more conclusively
than at first appears, for the reason that the smaller the planet’s
mass the less rapidly does its blanket of air thin out in ascent above
the surface. This is in consequence of the greater pull the larger body
exerts and the greater density it imparts to a compressible gas like
our atmosphere. Gravity acts like any force producing pressure and by
it the envelope of air is squeezed into a smaller compass. But as this
is done throughout the atmospheric layer it means a more rapid
rarefaction as one leaves the body. The action is such that the height
necessary to reach an analogic density varies inversely as the gravity
of the mass. In consequence of this, to compass a relative thermometric
fall for which a moderate difference of elevation would suffice on
Earth, an immoderate one must be made on Mars. For gravity there being
but three eighths what it is here, eight thirds the rise must be made
to attain a proportionate lowering of temperature. This fact renders
the above argument against elevation and depression being the cause of
the phenomenon three times as cogent as it otherwise would be.
With so gradual a gradient in barometric pressure there and so low a
set of contour lines, altitude must be a negligible factor in Martian
surface meteorologic phenomena. Both density and temperature can be but
little affected by such cause, and we must search elsewhere for
explanation of what surface peculiarities we detect.
Meanwhile the rifts themselves, from being lines which penetrate the
cap from its periphery in toward its centre, end by traversing it in
its entirety and separating portions which, becoming outlying
subsidiary patches, themselves proceed to dwindle and eventually
disappear. The rifts usually take their rise from such broader parts of
the cap-encircling blue belt as make beads upon that cordon and are
clearly spots where the product of the melting of the cap is either
specially collected, or produces its most visible effect.
So far the description might apply with substantial accuracy to either
cap. Yet the conduct of the two is in some ways diverse and begins to
accentuate itself from this point on.
[Illustration: Northern Cap hooded with vapor.]
From the time that the north polar cap reaches a diameter of about
twenty-five degrees, a singular change steals over it. From having been
up to then of a well-defined outline it now proceeds to grow hazy and
indistinct all along its edge. This change in its character at the same
period of its career has been quite noticeable at each of the three
last oppositions, so that small doubt remains that the metamorphosis is
a regularly recurrent one in the history of the cap. Coincident with
the obliteration of its contour, its dimensions seemingly enlarge. It
is as if a hood had been drawn over the cap of a dull white different
from the dazzling brilliance of the cap itself and covering more
ground. Such is probably what occurs; with vapor for veil. The
excessive melting of the cap produces an extensive evaporation which
then in part condenses to be deposited afresh, in part remains as a
covering, shutting off from our view the outlines of the cap itself. It
would seem that at this time the cap melts faster than the air can
carry it off. A sort of steaming appears to be going on, taking place
_in situ_. For it clearly is not wafted away. The time of its coming
too is significant. For the season is May 15, the height of time for a
spring haze to set in. Then later it dissipates with the same quiet
indefiniteness with which it gathered.
[Illustration: Northern Cap unmasked.]
It is some time in Martian June before the spring haze clears away, and
when it does go, only a tiny polar cap stands revealed beneath it, from
six to eight degrees across, or from a tenth to a fifteenth of what it
was when it passed into its curious spring chrysalis. The date of
emergence varies. In 1903 it occurred early, the haze not being marked
after June 3, though recurring again at intervals for a day or so. In
1905 it was later; perceptibly thin after June 21 it did not certainly
clear away till June 9 and came back again on July 16 and possibly on
the 25th.
These vicissitudes of aspect give us glimpses into a sweet
unreasonableness in Martian weather which makes it seem more akin to
our own. And this on two counts, diurnal and annual. From day to day
atmospheric conditions shift for purely local cause; while,
furthermore, successive Martian years are not alike. In some the season
is early; in others late. So that Mars is no more exempt than are we
from the wantonness of weather.
Clearly disclosed thus reduced to its smallest possible terms it
remains for some months of our days, for six weeks of its own. During
that period it continues practically unchanged, neither increasing nor
decreasing significantly in size, nor altering notably in aspect.
Measures of the drawings of it then make it from five to eight degrees
across and it is possible that it really fluctuates between narrow
limits, though its clear-cut outline at all times renders the variation
difficult to explain. We are not so near it as we could wish; for on
these occasions even at their best it is over two hundred times as
distant as the moon and the greatest magnification possible still
leaves it a hundred thousand miles away.
To the south polar cap a somewhat similar history attaches, but with a
difference. In its case no such regularly recurrent spring haze has yet
been noted. The melting of this cap would seem to be of a more orderly
nature than its fellow and not to outdo what can conveniently be
carried off.
That an excess of evaporation should not take place is the more
peculiar from the fact that at its maximum it is the larger of the two
and therefore has the greater quantity of matter to get rid of. Its
summer, also, is shorter than the arctic one, so that it has the less
time to dispose of its accumulations. The only other respect in which
it seems to be differently circumstanced from its antipodes is in the
character of its surroundings. About it are large blue-green areas
which with intermissions stretch down in places to within less than ten
degrees of the equator; whereas the other pole is continuously
encircled for long distances by practically uninterrupted ochre. The
character of the environment seems thus the only thing that can account
for the difference in behavior and this proves the more plausible when
we come to consider what those two classes of regions respectively
represent.
In other ways as well the southern cap is the more self-contained. The
rifts, indeed, break it up into separate portions and these in part
remain as outlying detachments of the main body, as was notably the
case in 1877 and in 1894, but they hardly have the permanency and
importance of those similarly formed about the arctic pole. Nothing
antarctic for instance compares with the subsidiary patch of the north
polar regions lying in longitude 206°, which both in Schiaparelli’s
time, and during the late oppositions as well was almost as fixed a
feature of the arctic zone as the cap proper. Not quite so constant,
however, and not so solid-looking a landmark is this patch for all its
extent, which nearly equals the area of the more legitimate portion. It
bears on its face a more pallid complexion as if it were thinner, and
this is borne out by the fact that it occasionally disappears, an event
which so far at least has never befallen the northern cap itself.
Less constant the southern one is to its own minimum than the northern.
In some seasons, in most in fact, it reaches like the other a more or
less definite limit of diminution which it does not pass. But this is
not always the case. In 1894 it disappeared entirely at the height of
its midsummer. The season was probably unusually hot then in the
southern hemisphere of Mars.
In position the caps have something to say about physiographic
conditions. Both caps at their minima are then irregular and the centre
of the south one is markedly eccentric to the areographic pole. It lies
some six degrees north along the thirtieth meridian. The northern one
is also probably eccentric, but much less so, with a divergence not
much exceeding a degree and of doubtful orientation. Not only are both
caps not upon their respective poles but they are not opposite each
other, the one lying in longitude 30°, the other in 290°. This speaks,
of course, for local action. In some wise this must depend on the
configuration of the surface, yet so far as markings go there is
nothing to show what the dependence is.
The eccentring of the caps is paralleled by the like state of things on
earth. The pole of cold does not coincide in either hemisphere with the
geographic pole. On the earth its position is largely determined by the
distribution of the land-masses. Continents are not such equalizers of
heat as oceans because of their conductivity on the one hand and their
immobility on the other. In winter they part with their heat more
quickly and convection currents cannot supply the loss. This accounting
for thermal pole eccentricity is inapplicable to Mars because of the
absence there of bodies of water. And it is significant that the degree
the earthly poles of cold are out much exceeds what is the case on
Mars. Possibly areas of vegetation there replace to some effect areas
of water. It is certainly in favor of this view that the arctic regions
there are more desert than the antarctic and that the north pole of
cold occupies more squarely the geographic pole.
Not till 1903 did the actual starting again of either cap chance to be
seen. Nor was this, indeed, a matter of hazard but of persistent
inquiry by observation prolonged after the planet had got so far away
that its scanning had hitherto been discontinued. Such search beyond
the customary limits of observation was essential to success, because
of the relation of the axial tilt to the position of the planet in its
orbit. At an opposition well placed for nearness, the tilt is such as
largely to hide the pole and to present the polar regions too obliquely
to view for effective scanning. This is true both of the arctic and the
antarctic regions in turn. For the Martian axis being inclined somewhat
as our own is to the plane of the planet’s orbit, we at times see well
and at times but poorly the arctic or antarctic zones.
The cap, the starting to form of which was thus caught, was the arctic
one; the date 128 days after the northern summer solstice, or
thereabouts, for as is perhaps natural the advent of the phenomenon
partook of the wavelike advance of such things familiar on earth, an
advance succeeded by a recession and then followed by another advance.
So much is proof of local weather there as here. Hoar-frost was
successively deposited and then melted off.
[Illustration: Deposition of frost.]
What is significant, the deposition of the frost took place
simultaneously over large areas. The very first patch of it, in about
longitude 320°, extended at one stroke down to latitude 55°. For it
actually crossed the Pierius somewhat to the south. A second patch
stretched to the east of the cap. Two wings these made to the kernel of
cap itself. Through the wings could be marked the line of the canal:
the Pierius upon the one side, the Enipeus upon the other. Such
visibility of the canals through the white stretches proved the white
not to be due to cloud suspended between us and them, but a surface
deposit which found no lodgment upon the canals themselves. The same
avoidance of dark markings was evidenced by the showing of the dark rim
round the cap’s kernel. Now, if the deposit were indeed hoarfrost, this
failure to find permanent foothold on the dark markings is what we
should expect to witness. For whether they were vegetation or water,
equally in either case the frost would melt from them first. Probably
they were both vegetal, though some doubt might exist about the latter,
the band around the kernel. It was then August 20 in that hemisphere.
Such deposition over great stretches of country is perhaps not so
surprising as it appears at first sight when seen from without in its
totality. After all, something not unlike it occurs in our snow-storms
when hundreds of square miles are whitened at once. Furthermore, with
an atmosphere as thin as Mars seems to possess the temperature must be
perilously near the freezing-point in the arctic and subarctic regions
at the close of summer.
Steadily, with intermissions, the white sheet increased until even the
dark border to the cap became obliterate, the kernel showing at first
through the veil like the ghost of what it had been, and then ceasing
to be visible at all, its delimitations being buried under deeper and
deeper depositions of frost.
[Illustration: First northern snow.]
The perennial portion of the cap was thus merged in the new-fallen
snow. This marked the on-coming of the arctic winter in full force and
happened even before the polar sun had wholly set. For the pole did not
enter into the shadow till two of our months later, the autumnal
equinox occurring 183 days after the summer solstice or 55 days after
the first fall of frost. Then the pole passed into its star-strewn
arctic night, a polar night of twice the duration of our own and the
circumpolar regions entered upon their long hibernation of ten of our
months.