THE BLUE-GREEN AREAS
Descending now equatorwards from the polar regions, and their in part
paleocrystic, in part periodic, coating of ice, we come out upon the
general uncovered expanse of the planet which in winter comprises
relatively less surface than on Earth, but in summer relatively more.
Forty degrees and eighty-six degrees may be taken as the mean hiemal
and æstival limits respectively of the snow on Mars; forty-five and
seventy-five as those of the Earth. Whatever ground is thus bared of
superficial covering on Mars lies fully exposed to view, thanks to the
absence of obscuring cloud; and it is at once evident that the terrane
is diversified, patches of blue-green alternating with stretches of
reddish ochre. Of the two opaline tints the reddish ochre predominates,
fully five eighths of the disk being occupied by it.
It was early evident in the study of the surface of Mars that its
ochreish disk was not spotless. Huyghens in 1659 saw the Syrtis Major.
From this first fruit of areography dates, indeed, the initial
recognition of the planet’s rotation; for on noting that the marking
changed its place, he inferred a turning of the planet upon itself in
about twenty-four hours. Thirteen years later he observed and drew it
again and this time in company with the polar cap. Again, after eleven
more years, he depicted it for the third time, and now so changed
because of the different tilt of the planet toward the earth that it
may be doubted whether Huyghens himself recognized it for the same. But
that he drew it correctly a globe of Mars will at once show.
From such small beginning did areography progress to the perception of
permanent patches of a sombre hue distributed more or less irregularly
over the disk. Impressing the observers simply as dark at first, they
later came to be recognized as possessing color, a blue-green, which
contrasted beautifully with the reddish ochre of the rest of the
surface. Cassini, Maraldi, Bianchini, Herschel, Schroeter, all saw
markings which they reproduced. Finally, with Beer and Maedler, came
the first attempt at a complete geography. In and out through the ochre
was traced the blue; commonly in long Mediterraneans of shade, but here
and there in isolated Caspians of color. With our modern telescopic
means the dark patches are easily visible, the very smallest glass
sufficing to disclose them. When thus shown they much resemble in
contour the dark patches on the face of the Moon as seen with the naked
eye. Now these patches were early taken for lunar seas and received
names in keeping with the conception; as the Sea of Serenity, the Sea
of Vapors, and so forth. Following the recognition of a like
appearance, like appellatives were given to the Martian markings; and
the Mare Sirenum, or Sea of the Sirens, the Mare Cimmerium, and others
sufficiently proclaim what was thought of them at the time they were
thus baptized. Indeed, if a general similarity be any warrant for a
generic name they were not at the time ill-termed. For, common to all
three bodies, the Earth, the Moon, and the planet Mars, is the
figuration of their surfaces into light areas and dark. In the Martian
disk, as in the lunar one, we seem to be looking at a cartographic
presentation of some strange geography suspended in the sky; the first
generic difference between the two being that the chart is done in
chiaroscuro for the Moon, in color for Mars. On mundane maps, we know
the dusky washes for oceans; so on the Moon it was only natural to
consider their counterparts as _maria_; and on Mars as ‘seas.’ Nor did
the blue-green hue of the Martian ones detract from the resemblance.
But in something other than color these markings are alike. In fact,
color could hardly be excuse for considering the lunar _maria_ what
their name implies, for distinctive tint is lacking in them, even to
the naked eye. It was in form that the likeness lay. Their figures were
such as our own oceans show; and allowing for a sisterly contrast amid
a sisterly resemblance, the lunar _maria_ or the Martian seas might
well have been of similar origin to those with which our schoolboy
study of atlases had made us familiar. Thus did similarity in look
suggest similarity in origin, and intuitive recognition clothe its
objects with the same specific name.
Considerable assumption, however, underlay the pleasing simplicity of
the correlation on other grounds, consequent not so much upon any lack
of astronomic knowledge as, curiously, upon a dearth of knowledge of
ourselves. We know how other bodies look to us, but we ignore how we
look to them. It is not so easy to see ourselves as others see us; for
a far view may differ from a near one, and a matter of inclination
greatly alter the result. Owing both to distance and to tilt we lack
that practical acquaintance with the aspect of our own oceans viewed
from above, necessary to definite predication of their appearance
across interplanetary space. Our usual idea is that seas show dark, but
it is also quite evident that under some circumstances they appear the
contrary. It all depends upon the position of the observer and upon the
position of the Sun. Their usual ultramarine may become even as molten
brass from indirect reflection; while on direct mirroring, they give
back the Sun with such scarce perceptible purloining of splendor as to
present a dazzling sheen not to be gazed upon without regret. Canopied
by a welkin they assume its leaden hue, while at the same time, their
shores, less impressionable to borrowed lighting, show several tints
darker than themselves. Surfaces so sensitive to illumination hardly
admit of more accusable tint than a chameleon. Nevertheless, we are
probably justified in our conviction that perpendicularly visaged, they
would on the whole outdo in sombreness land round about them, and so be
evident as dusky patches against a brighter ground.
One phenomenon we might with some confidence look to see exhibited by
them were they oceans, and that is the reflected image of the Sun
visible as a burnished glare at a calculable point. Specular reflection
of the sort was early suggested in the case of Mars, and physical
ephemerides for the planet registered for many years the precise spot
where the starlike image should be sought. But it was never seen. Yet
not till the marine character of the Martian seas had been otherwise
disproved was the futile quest for it abandoned. Indeed, it was a tacit
recognition that our knowledge had advanced when this column in the
ephemeris was allowed to lapse.
On this general marine ascription doubt was first cast in the case of
the Moon. So soon as the telescope came to be pointed at our satellite,
it was evident that the darker washes were not water surfaces at all,
but very palpably plains. Long low ridges of elevation stood out upon
them like prairie swells, which grew in visible relief according to the
slanting character of the illumination. Cracks or rills, too, appeared
near their edges and craters showed in their very midst. Patently solid
they betrayed their constitution not only by diverse topography but by
diversified tint. All manner of shades of neutral tone mottled their
surface, from seeming porphyry to chalk. Belief perforce departed when
the telescope thus pricked the bubble, evaporating as the water itself
had done long before.
So much was known before the Mars’ markings were named. Nevertheless,
humanity, true to its instincts, promptly proceeded to commit again the
same mistake, and, cheerfully undeterred by the exposure of its errors
in the case of the Moon, repeated the christening in the case of Mars.
So sure was it of its ground that what it saw was not ground, that
though the particular appellatives of the several seas were constantly
altered, rebaptisms, while changing the personal, kept the generic
name. Dawes’ Ocean, for example, later became l’Ocean Newton and later
still the Mare Erythraeum, but remained set down as much a sea as
before. About thirteen years ago, however, what had befallen the seas
of the Moon, befell those of Mars: the loss of their character. It was
first recognized through a similar exposure; but the fact was led up to
and might have been realized in consequence of quite a different line
of evidence. The initial thing to cast doubt upon the seas being what
they seemed to be was the detection of change in their aspect. That the
detection was not made much earlier than actually happened shows how a
phenomenon may elude observation if scrutiny be not persistent, and its
results from time to time not carefully compared. Schiaparelli was the
one who first noticed variation in the look of the seas, and the
discovery was as much due to the assiduity with which he followed the
planet opposition after opposition as to the keenness with which he
scanned it. The noting of change in the blue-green areas constituted,
in fact, one of the first fruits of systematic study of the planet.
Change in configuration, that is, alteration of area, preceded in
recognition alteration of tint. Thus the Syrtis Major showed larger to
him in 1879 than it had in 1877. This was natural; difference of degree
being a more delicate matter to perceive than its effect upon extent.
From change of area his perception went on to change of tone. In his
own words, what he noticed was this: _Memoria_, VI, 1888, “No less
certain is it that, from one opposition to another, one notices in the
seas, very remarkable alterations of tone. Thus the regions called Mare
Cimmerium, Mare Sirenum, and Solis Lacus, which during the years 1877
to 1879 could be numbered among the most sombre on the planet, during
the succeeding oppositions became less and less black, and in 1888 were
of a light gray hardly sufficient to render them visible in the oblique
position in which all three found themselves.... On the other hand, at
the very same moment, the Mare Acidalium and the Lacus Hyperboreus
showed very dark; the latter indeed appeared nearly black, although
seen as tilted as the Syrtis and the equatorial bays. The condition of
the regions called seas is therefore not constant: so much is
unquestionable. Perhaps the change produced in them has to do with the
season of the planet’s year.”
Holding as he did the then prevailing view that the blue-green regions
were bodies of water, he regarded those of intermediate tint as vast
marshes or swamps, and he accounted for change of hue in them as due to
inundations and occasions of drying up. In consequence of losing their
water, the seas, he thought, had in places become so shallow that the
bottom showed through.
Plausible on the surface, this theory breaks down so soon as it is
subjected to quantitative criticism. For the moment we try to track the
water, we detect the inadequacy of the clew. The enormous areas over
which the phenomenon occurs necessitates the establishing an alibi for
all the lost water that has gone. Drying up on such a scale would mean
the removal of many feet of liquid over hundreds of thousands of miles
in extent. To produce any such change in appearance as we witness, even
on the supposition that these seas were none too deep to start with,
would involve lowering the level of the water by from five to twenty
feet throughout two thirds of the whole surface of the southern
hemisphere. This would leave a heap of waters to be accounted for,
bewildering in its immensity. The myriad tons of it must be disposed
of; either by drainage into other regions or by being caught up into
the sky.
In this emergency it might seem at first as if the polar cap of the
opposite hemisphere offered itself as a possible reservoir for the
momentarily superfluous fluid. But such hoped-for outlet to the problem
is at once closed by the simple fact that when the lightening of the
dark regions of the southern hemisphere takes place, the opposite polar
cap has already attained its maximum; in fact, has already begun to
melt. It, therefore, absolutely refuses to lend itself to any such
service. This was not known to Schiaparelli’s time, the observations
which have established it, by recording more completely the history of
the cap, having since been made. Indeed, it was not known even at the
time when the writer, in 1894, showed the impossibility of the transfer
on other grounds; to wit, on the fact of no commensurate concomitant
darkening of the surface elsewhere and on the manifest non-complicity,
if not impotency, of the Martian atmosphere in the process. The
transference of the water to other dark patches in the northern
hemisphere fails of sufficiency of explanation because of the limited
extent of such areas on that side of the globe; while the air is quite
as incapable of carrying away any such body of liquid, though the whole
of it were at the saturation-point, not to mention that there exists no
sign of the attempt. The reader will find this reasoning set forth in
_Mars_, published eleven years ago. He will now note, from what has
been said above about the northern polar cap, that continued
observations since have resulted in opening up another line of proof
which has only strengthened the conclusion there reached.
[Illustration: Lines in dark area.]
The _coup de grâce_, however, to the old belief was given when the
surface of the dark areas was found to be traversed by permanent lines
by Pickering and Douglass. Continued observation showed these lines to
be unchangeable in place. Now permanent lines cannot exist on bodies of
water, and in consequence the idea that what we looked on there were
water surfaces had to be abandoned.
Thus we now know that the markings on both the Moon and Mars which have
been called _maria_ are not in reality seas. Yet we shall do well still
to keep the old-fashioned sonorous names, Mare Erythraeum, Mare
Sirenum, and their fellows, because it is inconvenient to change;
while, if we please, we may see in their consecrated Latin couching the
fit embalming in a dead language of a conception that itself is dead.