CLIMATE AND WEATHER
In gazing at the successive phases presented by the polar caps as their
annual history unrolls itself to view, beginning with vast white cloaks
that in winter hide so effectively the planet’s shoulders, to little
round knobs that in summer sit like guardsmen’s caps more or less askew
upon the poles, the bodily eye sees only the glisten of far-off snow.
The mind’s eye, however, perceives something more: the conviction they
carry of the presence of an atmosphere surrounding the planet. Elusive
as water vapor is to sight for its transparency and to spectroscopic
determination for its earthly omnipresence, recognition of its
existence elsewhere by deduction raises such reasoning at once to a
more conspicuous plane than it might otherwise assume. Especially is
this true where the deduction is itself conclusive, as is here the
case. For it depends on phenomena not its own, but which are in their
turn dependent on it. We are not even beholden to any knowledge of the
substance composing the caps for the fundamental inference that an
atmosphere surrounds them. Whatever that substance were, the fact that
the caps dissipate and reform shows us with absolute certainty that
they pass into the gaseous state, to be later solidified afresh. This
gas constitutes of itself an atmosphere; while another phenomenon, to
wit, their blue girdles as they melt, affirming their substance to be
snow and ice, enables us to precise the fact that this gas is water
vapor.
From such premise given us by the polar caps we are able to infer much
more by the help of the kinetic theory of gases. But the speed of
parting by a planet with its gases is conditioned by the mean speed of
each gas. Water vapor will, therefore, go before nitrogen, oxygen or
carbonic acid gas. If, then, we find it present over the surface of a
planet we are assured of the possibility that the other three may be
there too, and from the similarity of matter in space strong reason to
suspect that they actually are.
Corroborative evidence of the accuracy of the deduction as to the
presence of a Martian air is shown in several other ways; in the
existence of clouds to begin with. Rare as they are, these certainly
float at times over parts of the planet, although it is doubtful
whether they can then be seen. Fortunately for assurance we have other
ways of ascertaining their presence than that of obscuration. Nor is it
of account to the argument that they should be few and far between, as
they unquestionably are. One single instance of such mediumistic
support is enough to support the theory of a medium; and that instance
has been more than once observed.
Direct evidence of atmosphere is further forthcoming in the limb-light.
This phenomenon might be described as a brilliant obscuration. It is a
circlet of illumination that swamps the features as they near the full
edge of the disk, the limb of the planet as it is called. Obliteration
of the sort is evident, more or less markedly, at all times, and is not
due to foreshortening, as the broadest areas are affected. The fading
out of the detail at the limb suggests nothing so much as a veil drawn
between us and it, lighter in tint than what it covers. Such a veil can
be none other than air or the haze and cloud that air supports. From
its effect, impartial in place and partial in character, cloud is
inadmissible as a cause and we are left with air charged with dust or
vapor in explanation. Obscuration due to it should prove most dense at
the limb, since there the eye has to penetrate a greater depth of it;
just as on the earth our own air gives azure dimness to the distance in
deepened tinting as the mountains lie remote.
Another bit of evidence lies in the apparent detection of a twilight
arc. In 1894 measures made of the polar and equatorial diameters of the
planet showed certain systematic residuals left after all known
corrections had been applied. The only thing which would account for
them was the supposition that a twilight arc had been unconsciously
seen and as unconsciously measured. In delicate quantities of the sort
too great reliance cannot be put, but if the residuals be not referable
to other cause they give us not only further evidence of an atmosphere,
but at the same time our only hint of that atmosphere’s extent. From
them it would seem that the air must be rare, not more than about four
inches of barometric pressure, as we reckon it, and probably less; a
thin, high air more rarefied than prevails upon our highest mountain
tops.
Corroborative of this is the aspect of the planet. From the general
look of the disk a scant covering of air is inferable. For one of the
striking things about the planet’s features is their patent exposure to
our sight. Except in the winter time of its hemisphere or in the spring
after the greatest melting of the polar cap, nothing seems to stand in
our way of an uninterrupted view of the surface, whether in the arctic,
temperate, or tropic zones. From the openness of its expression,
however, too much case should not be made as we really know but little
of how an atmosphere-enshrouded planet would look. We find no
difficulty in seeing objects a hundred miles away across the surface of
the earth and yet the thickness of the air strata in such horizontal
traversing is many fold what it is when we look directly up. It is also
out of all proportion laden with dust and smoke. In the purer regions
of the earth, a clear air imposes but little bar to sight, and conjures
up far things startlingly distinct.
Nevertheless, every evidence points to a thin air upon Mars: _a priori_
reasoning, indirect deduction and direct sight. Now, from a thinness of
atmosphere it would follow, other things equal, that the climate was
cold. About this there has been much question, but less of answering
reply. From the distance of the planet from the sun it is certain less
heat is received by it than falls upon the earth in something like the
ratio of one to two. But that the amount effective is as the amount
received is far from sure. The available heat is much affected by the
manner of its reception. A blanket of air acts like the glass of a
conservatory, letting the light rays in, but hindering the heat rays
out. The light rays falling on the ground or the air are transformed
into heat rays that, finding the return journey less easy, are
consequently trapped. All substances are thus calorifiers, but water
vapor is many times more potent than ordinary air to heat-ensnaring. A
humid air has a hothouse tang to it most perceptible. Now, what the
relative percentage of water vapor in the Martian atmosphere may be we
do not know.
The thinness of the Martian air has caused it to be likened to that
upon our highest mountain peaks which are in large part covered with
perpetual snow. But the comparison is not well founded. A peak differs
materially from a plateau in the countenance it gives to the heat
falling upon it. On a plateau each warmed acre of ground helps the
retention of heat by its neighbor; while in addition to being destitute
of side support the higher winds generated about an isolated peak blow
its own caloric away. Still less does any analogy hold between the two
when the plateau is a world-wide one.
From these considerations it is evident glosses are possible upon the
bald idea of a much lower temperature prevailing on the Martian surface
than on the earth’s. Doubtless the theoretic cold has been greatly
overdone. Reversely, recent observations tend to lower the apparent
temperature disclosed by the features of the disk, and between the
rising of the theoretic and the falling of the observed we are left
with a very reasonable compromise and reconcilement as the result.
The various look and behavior of the surface of Mars point to a mean
temperature colder than that of the earth, but above the freezing-point
of water; for regions, at least, outside of the polar caps and during
all but the winter months. Except at certain special spots, and
possibly even there, frost is unknown at all times within the tropics
and except in winter in temperate latitudes. These anomalous
localities, mentioned in the preceding chapter, may be said to be the
exceptions that prove the rule of general non-glaciation. For if they
be snow, they stand witness to its absence elsewhere upon the disk, and
if they are not, they testify the more emphatically to the same effect.
As between different parts of the surface, the tilt of the Martian axis
and the greater length of the Martian seasons, the one the same as, the
other the double of, our own, tend to an accentuation of the heat in
the temperate and arctic or antarctic zones. The greatest insolation on
earth is not, as we might suppose, at the equator, but at the parallels
of 43.°5 north and south; even the poles themselves receiving a quarter
as much heat again on midsummer day as ever falls to the lot of the
line. This broad physical fact is equally true of Mars, while in the
matter of consecutive exposure Mars in summer outdoes the earth. For
the longer the seasons, the more nearly does the effective heat
approach the received amount. Thus both on the score of heat received
and of heat husbanded these zones must be relatively warm. And this
shows itself in the look of the surface. In summer it is clearly warmer
within the polar regions than is the case on earth, to judge by the
effect. In winter the cold is doubtless proportionately severe.
For the diurnal range of temperature we have less data. There is
evidence pointing to chilly nights, but it is meagre, and we are left
to fall back on the cold of our deserts at night for analogic condition
of the state of things over the Martian desert levels after the sun
goes down.
If we are uncertain of the precise character of the Martian climate, we
know on the other hand a good deal about the Martian weather. A
pleasing absence of it over much of the planet distinguishes Martian
conditions from our own. That we can scan the surface as we do without
practical interruption day in and day out proves the weather over it to
be permanently fair. In fact a clear sky, except in winter, and in many
places even then, is not only the rule, but the rule almost without
exceptions. In the early days of Martian study cases of obscuration
were recorded from time to time by observers, in which portions of the
disk were changed or hidden as if clouds were veiling them from view.
More modern observations fail to support this deduction, partly by
absence of instances, partly by other explanation of the facts.
Certainly the recorded instances are very rare. Indeed, occasions of
the sort must to any Martians be events, since only one possible
example has presented itself to me during the course of my
observations, extending more or less over eleven years. Even in this
case there was no obliteration, though a certain whiteness overspread
an area near the equator temporarily. Position seemed to point to its
identity with a cloud which made its appearance about that time upon
the terminator, and lasted for some thirty-six hours. The cloud,
however, showed evidence of being, not the kind with which we are
familiar, but a dust-storm, in keeping, indeed, with the desert region
(Chryse) in which it originated.
With the exception of sporadic disturbance of the sort the whole
surface of the planet outside the immediate vicinity of the polar caps
seems free from cloud or mist and to lie perpetually unveiled to space.
In the neighborhood of the caps, however, and especially round about
their edge, a very distinct pearly appearance is presented during the
months at which the cap is at its maximum, or in other words, in the
depth of its winter. Of a dull white hue and indefinite contour the
phenomenon suggests cloud. Where it lies spread no markings are
visible; an absence explicable by obscuration due an interposed medium,
but equally well by seasonal non-existence of the markings themselves,
which from the general behavior of these markings we know to be to some
extent certainly the fact. Of the regions where the effect is
noticeable, Hellas is the most striking. So conspicuously white during
the winter of the southern hemisphere as to have been often mistaken
for the polar cap, its ghost shows thus almost regularly every Martian
year. What is as suggestive as it is striking, the blanching is
confined to the solid circle constituting Hellas and does not extend
into the dark regions by which it is circumscribed. Hellas is as
self-contained when thus powdered as when, in its normal ochre or
abnormal red, it stretches like a broad buckler across the body of the
disk. That the land there lies at a higher level than its surroundings
is pretty certain, but that the difference can amount to enough to
explain its silveriness as ice is improbable. In latitude Hellas is
distinctly temperate, lying between the parallels of 55° and 30°; but
on Mars this is no warrant of a like climate. Again, though close on
the south to what constitutes the polar cap, it does not strictly form
part of that cap, but occupies both in position and in kind a something
intermediary between the frost-bound regions of periodic snow and the
warmer ones of perpetual sunshine. It seems to be afflicted with the
winter weather of the north of Europe, and to owe its pearly look at
such times to the same cloud canopy that then distressingly covers
those inclement lands.
Similar in behavior to it is the long chain of so-called islands that,
beginning southwest of Thaumasia, runs thence westward even to the
eastern edge of Hellas. These belt the planet in a west-northwesterly
direction by a strip of territory from ten to fifteen degrees wide, the
medial line of which begins at 55° south and ends in about 40°. They
are parted from the main bright areas by blue-green ‘seas’ of about the
same width as themselves, the Mare Sirenum, the Mare Cimmerium and Mare
Tyrrhenum. These ‘seas’ the white that covers the ‘islands’ never
crosses; though the continent, as we may call it for convenience,
descends at the east to 30° south. Since the ‘seas’ are not seas, the
cause which might bound the snow, were they such, cannot be the cause
here. Nevertheless, they have an effect of some sort on the isothermal
lines as is shown not only by latitudinal comparison with the state of
things in Hellas, but with that in Thaumasia as well. For 30° south is
also the limit apparently of the white on Thaumasia, where ochre desert
stretches ten degrees farther south still; the region in its southern
part being white-mantled, in its northern part not. Here again, then,
the ochre areas make exception to what affects the blue-green ones.
Clearly the blue-green regions temper the action of what gives them
wintry cloak. But why they should do this is not easy to explain on any
supposition terrestrial or marine. Bodies of water tend to foster the
formation of clouds; so, less markedly, do areas of vegetation. Neither
the old ideas, then, nor the new lend themselves in explanation. It may
be that while we here seem to be envisaging cloud we are in reality
looking at hoar-frost. On the other hand, light cloud would show less,
superposed over a dark background, than over an ochre one; and this,
the simplest of all explanations, may be the true one. It is facts like
these that intrigue us in the study of the Martian surface by revealing
conditions which render offhand analogy with the earth unsafe. Indeed,
we are more sure of some things which appear too strange to be true
than of others so simple on their face as to enlist belief. Among the
most difficult and perplexing are meteorological problems like the
above. Here we can only say provisionally that while cloud best answers
to the appearance, frost best fits the cause. For vegetation might melt
frost, yet not dissipate cloud. By raising our conception of the mean
temperature the facts can, however, be reconciled and this is probably
the solution of the difficulty after all.
As we saw in the annual history of the polar caps a dimness somewhat
different affects the northern cap in May and June. After the melting
of the cap is well under way a haziness sets in along its edge which
befuddles its outline and effectually hides what is going on within it.
When at last the screen clears away the cap is found to be reduced to
its least dimensions. Such obstructing sheet looks to be more of the
nature of mist caused by the excessive melting of the cap.
Unfortunately, there are here no patches of blue-green to test a
possible partiality in its behavior over such tracts; nor has similar
action ever yet been remarked in the case of the cap of the southern
hemisphere.
Regular recurrence at the appropriate season of the planet’s year,
together with extensive action at the time, takes this springtide mist
to some extent out of the domain of weather into that of climate. For
it prevails all round the cap and repeats itself in place as each fresh
spring comes on. At least it has done so for the past three oppositions
at which it has been possible to observe well the arctic zones. It is
thus both general in its application and fixed in its behavior.
Nevertheless, it betrays something of the fickleness which
characterizes that more inconstant thing: weather. For it comes and
goes, one thinks for good, only to find it there again some days later.
Not less captious is the meteorologic action shown in the making of the
new polar cap. When the northern one starts to form, vast areas of
frost are deposited in a single night. These, however, are not
permanent. The ground thus covered is during the next few days again
partially laid bare. Then a new fall occurs, hiding the surface a
little more completely than before, and the lost domain is more than
regained. By such wavelike advance and recession the tide of frost
creeps over and submerges the arctic regions as the late summer passes
into the autumn. In this alternate coming and going with succeeding
days, we have an unsteadfastness of action most fittingly paralleled by
our own weather. It would seem that local causes there as here are
superposed upon the orderly progress of the seasons and though at the
on-coming of the autumn the cold is steadily gathering strength,
nevertheless warm days occur now and then to stay its hand, only to be
succeeded in their turn by frosts more biting than before. Even on Mars
nothing in the way of weather is absolutely predicable but
impredicability.