ridges is the Mare Humorum, in the southeastern quadrant. Here, if
it be observed under a rising sun, a number of these objects will be
seen extending from the region north of the ring-mountain Vitello in
long undulating lines, roughly concentric with the western border
of the “sea,” and gradually diminishing in altitude as they spread
out, with many ramifications, to a distance of 200 miles or more
toward the north. At this stage of illumination they are strikingly
beautiful in a good telescope, reminding one of the ripple-marks left
by the tide on a soft, sandy beach. Like most other objects of their
class, they are very evanescent, gradually disappearing as the sun
rises higher in the lunar firmament, and ultimately leaving nothing
to indicate their presence beyond here and there a ghostly streak or
vein of a somewhat lighter hue than that of the neighboring surface.
The Maria, like almost every other part of the visible surface,
abound in craters of a minute type, which are scattered here
and there without any apparent law or ascertained principle of
arrangement.
Walled-plains, approximating more or less to the circular form,
though frequently deviating considerably from it, are among the
largest inclosures on the moon. They vary from upward of 150 to 160
miles or under in diameter, and are often encircled by a complex
rampart of considerable breadth, rising in some instances to a height
of 12,000 feet or more above the inclosed plain. This rampart is
rarely continuous, but is generally interrupted by gaps, crossed by
transverse valleys and passes and broken by more recent craters and
depressions. As a rule, the area within the circumvallation (usually
termed “the floor”) is only slightly, if at all, lower than the
region outside: it is very generally of a dusky hue, similar to that
of the gray plains of Maria, and, like them, is usually variegated
by the presence of hills, ridges, and craters, and is sometimes
traversed by delicate furrows, termed clefts or rills.
Ptolemæus, in the third quadrant and not far removed from the
centre of the disk, may be taken as a typical example of the class.
Here we have a vast plain, 115 miles from side to side, encircled
by a massive but much broken wall, which at one peak towers more
than 9,000 feet above a level floor, which includes details of a
very remarkable character. The adjoining Alphonsus is another, but
somewhat smaller object of the same type, as are also Albategnius and
Arzachel; and Plato, in a high northern latitude, with its noble,
many-peaked rampart and its variable steel-gray interior, Grimaldi,
near the eastern limb (perhaps the darkest area on the moon),
Schickard, nearly as big on the southeastern limb, and Bailly, larger
than either (still further south in the same quadrant), although they
approach some of the smaller “seas” in size, are placed in the same
category. The conspicuous central mountain, so frequently associated
with other types of ringed inclosures, is by no means invariably
found within the walled-plains; though, as in the case of Petavius,
Langrenus, Gassendi, and several other noteworthy examples, it is
very prominently displayed. The progress of sunrise on all these
objects affords a magnificent spectacle. Very often when the rays
infringe on their apparently level floor at an angle of from 1° to
2°, it is seen to be coarse, rough grained, and covered with minute
elevations, although an hour or so afterward it appears as smooth as
glass.
The more massive and extended mountain ranges of the moon are found
in the Northern Hemisphere, and (what is significant) in that
portion of it which exhibits few indications of other superficial
disturbances. The most prominently developed systems, the Alps, the
Caucasus, and the Apennines, forming a mighty western rampart to the
Mare Imbrium and giving it all the appearance of a vast walled-plain,
present few points of resemblance to any terrestrial chain. The
former include many hundred peaks, among which Mont Blanc rises to a
height of 12,000 feet, and a second, some distance west of Plato, to
nearly as great an altitude; while others ranging from 5,000 to 8,000
feet are common. They extend in a southwest direction from Plato to
the Caucasus, terminating somewhat abruptly, a little west of the
central meridian in about N. lat. 42°. One of the most interesting
features associated with this range is the so-called great Alpine
valley, which cuts through it west of Plato.
The Caucasus consist of a massive wedge-shaped mountain land,
projecting southward, and partially dividing the Mare Imbrium from
the Mare Serenitatis, both of which they flank. Though without peaks
so lofty as those pertaining to the Alps, there is one, immediately
east of the ring-plain Calippus, which, towering to 19,000 feet,
surpasses any of which the latter system can boast. The Apennines,
however, are by far the most magnificent range on the visible
surface, including as they do some 3,000 peaks, and extending in an
almost continuous curve of more than 400 miles in length from Mount
Hadley, on the north, to the fine ring-plain Eratosthenes, which
forms a fitting termination, on the south. The great headland Mount
Hadley rises more than 15,000 feet, while a neighboring promontory on
the southeast of it is fully 14,000 feet, and another, close by, is
still higher above the Mare. Mount Huyghens, again in N. lat. 20°,
and the square-shaped mass Mount Wolf, near the southern end of the
chain, include peaks standing 18,000 and 12,000 feet respectively
above the plain to which their flanks descend with a steep declivity.
The counterscarp of the Apennines, in places 160 miles in width from
east to west, runs down to the Mare Vaporum, with a comparatively
gentle inclination. It is everywhere traversed by winding valleys
of a very intricate type, all trending toward the southwest, and
includes some very bright craters and mountain-rings.
Whether variations in the visibility of lunar details, when observed
under apparently similar conditions, actually occur from time to
time from some unknown cause, is one of those vexed questions which
will only be determined when the moon is systematically studied by
experienced observers using the finest instruments at exceptionally
good stations; but no one who examines existing records of rills by
Gruithuisen, Lohrmann, Mädler, Schmidt, and other observers, can well
avoid the conclusion that the anomalies brought to light therein
point strongly to the probability of the existence of some agency
which occasionally modifies their appearance or entirely conceals
them from view. In short, the more direct telescopic observations
accumulate, and the more the study of minute detail is extended, the
stronger becomes the conviction that, in spite of the absence of an
appreciable atmosphere, there may be something resembling low-lying
exhalations from some parts of the surface which from time to time
are sufficiently dense to obscure, or even obliterate, the region
beneath them.
Sir John Herschel maintained that “the actual illumination of the
lunar surface is not much superior to that of weathered sandstone
rock in full sunshine. I have,” he says, “frequently compared the
moon setting behind the gray perpendicular façade of the Table
Mountain, illumined by the sun just risen in the opposite quarter of
the horizon when it has been scarcely distinguishable in brightness
from the rock in contact with it. The sun and moon being at nearly
equal altitudes, and the atmosphere perfectly free from cloud or
vapor, its effect is alike on both luminaries.” Zöllner’s elaborate
researches on this question are closely in accord with the above
observational result. Though he considers that the brightest parts
of the surface are as white as the whitest objects with which we are
acquainted, yet, taking the reflected light as a whole, he finds that
the moon is more nearly black than white. The most brilliant object
on the surface is the central peak of the ring-plain Aristarchus, the
darkest the floor of Grimaldi, or perhaps a portion of that of the
neighboring Riccioli. Between these extremes there is every gradation
of tone. Proctor, discussing this question on the basis of Zöllner’s
experiments respecting the light reflected by various substances,
concludes that the dark area just mentioned must be notably darker
than the dark gray syenite which figures in his tables, while the
floor of Aristarchus is as white as newly fallen snow.
MARS.—AGNES M. CLERKE
The furthest terrestrial planet from the sun is Mars, the “star of
strength.” No other heavenly body, except the moon, is so well placed
for observation from our position in space.
The diameter of Mars is 4,200 miles; its surface is equal to
two-sevenths, its volume to one-seventh those of the earth. But, in
consequence of its inferior mean density, nine such spheres would go
to make up the mass of our world. The superficial force of gravity
on Mars, compared with its terrestrial value, is as thirty-eight to
a hundred. A man could leap there a wall eight feet four inches in
height with no more effort than it would cost him here to spring over
a two-foot fence.
The planet’s rotation is performed in 24 hours, 37 minutes, on an
axis deviating from the vertical by 24° 50′. Hence its seasons
resemble our own, except in being nearly twice as long, for the
Martian year is of 687 days.
The disk of Mars is diversified with three shades of color—reddish,
or dull orange, dark grayish-green, and pure white. The last shows
mainly in two diametrically opposite patches. Each pole is surrounded
by a brilliant cap, suggesting the deposition of ice or snow over
the chilly spaces corresponding to our arctic and antarctic regions.
Nor is this all. Each of the polar hoods shrinks to a mere remnant
as the local summer advances, but regains its original size when
wintry influences are again in the ascendant. Here, and nowhere
else in the planetary system, we meet evidence of seasonal change;
and seasonal change is associated with vital possibilities. Again,
a globe upon which snow visibly melts must contain water; hence
the green markings can not but image to our minds seas and inlets
subdividing continents, the blond complexion of which may be caused
by some native peculiarity of the soil. It is in no way connected
with vegetation, since it neither fades nor flushes with the advent
of spring; and an atmospheric origin is excluded by the circumstance
that it becomes effaced by a whitish haze near the limb, just where
the densest atmospheric strata are traversed by the line of sight.
The spots on Mars are by no means so sharply defined as lunar craters
and _maria_; yet they are fundamentally permanent. Some can be
recognized from drawings made over two hundred years ago; and these
antique records have served modern astronomers to determine with
minute accuracy the rotation-period of the planet. Continents are
somewhat vaguely outlined. Great tracts of them are of an uncertain
and variable hue, as if subject to inundations. This peculiarity,
thoroughly certified during the favorable opposition of 1892, makes
a strong distinction between Mars and the Earth. Terrestrial oceans
keep within the limits assigned to them. On the neighboring planet—as
M. Faye observed in 1892—“water seems to march about at its ease,”
flooding from time to time regions as wide as France. The imperfect
separation of the two elements recalls the conditions prevailing
during the terrestrial carboniferous era.
The main part of the land of Mars is situated in the Northern
Hemisphere. It covers two-thirds of the entire globular surface.
Rather than land, indeed, it should be called a network of land and
water. The great continental block—so its orange tint declares it to
be—is cut up in all possible directions by an intricate system of
what appear to be waterways, running in perfectly straight lines—that
is, along great circles of the globe—for distances varying from 350
to upward of 4,000 miles. They are frequently seen in duplicate,
strictly parallel companions developing thirty to three hundred miles
apart from the original formations. This mysterious phenomenon is
evanescent, or rather periodical.
The canals invariably connect two bodies of water; hence they need
no locks or hydraulic machinery; their course is on a dead level.
The broadest of them are comparable with the Adriatic; those at the
limit of visibility, stretching like the finest spider-threads across
the disk, have a width of eighteen miles. “The canals,” Schiaparelli
says, “may intersect among themselves at all possible angles, but
by preference they converge toward the small spots to which we have
given the name of lakes. For example, seven are seen to converge in
Lacus Phœnicis, eight in Trivium Charontis, six in Lunæ Lacus, and
six in Ismenius Lacus.”
These “lakes” evidently form an integral part of the canal system.
They resemble huge railway junctions; and the largest of them—the
“Eye of Mars” (Schiaparelli’s Lacus Solis)—seems, in Mr. Lowell’s
phrase, like the hub of a five-spoked wheel. Mr. W. H. Pickering
in 1892, and Mr. Percival Lowell in 1894, were amazed at their
extraordinary abundance.
“Scattered over the orange-ochre groundwork of the continental
regions of the planet,” the latter wrote, “are any number of dark,
round spots. How many there may be it is not possible to state, as
the better the seeing, the more of them there seem to be. In spite,
however, of their great number, there is no instance of one occurring
unconnected with a canal. What is more, there is apparently none
which does not lie at the junction of several canals. Reversely, all
the junctions appear to be provided with spots.”
Most of these foci are about 120 miles in diameter, and appear most
precisely circular when most clearly seen. “Plotted upon a globe,”
Mr. Lowell continues, “they and their connecting canals make a
most curious network over all the orange-ochre equatorial parts of
the planet, a mass of lines and knots, the one marking being as
omnipresent as the other. Indeed, the spots are as peculiar and
distinctive a feature of Mars as the canals themselves.”
Like the canals, too, they emerge periodically, and in the same but
a retarded succession. They “are, therefore, in the first place,
seasonal phenomena, and, in the second place, phenomena that depend
for their existence upon the prior existence of the canals.”
Mr. Lowell terms them “oases,” and does not shrink from the full
implication of the term.
The most important result of the numerous observations of Mars,
made during the oppositions of 1892 and 1894, was the recognition
of a regular course of change dependent upon the succession of
its seasons. Schiaparelli had long anticipated this result; he is
commonly in advance of his time. These changes, moreover, when
closely watched, are really self-explanatory. The alternate melting
of the northern and southern snow-caps initiates and to some extent
determines them. As summer advances in either hemisphere, the wasting
of the corresponding white calotte can be followed in every minute
particular. “The snowy regions are then seen to be successively
notched at their edges; black holes and huge fissures are formed
in their interiors; great isolated fragments many miles in extent
stand out from the principal mass, dissolve, and disappear a little
later. In short, the same divisions and movements of these icy fields
present themselves to us at a glance that occur during the summer of
our own arctic regions.”
Indeed, glaciation on Mars is much less durable than on the earth. In
1894 the southern snow-cap vanished to the last speck 59 days after
the solstice and the remnant usually left looks scarcely enough to
make a comfortable cap for Ben Nevis. An immense quantity of water
is thus set free. The polar seas overflow; gigantic inundations
reinforced, doubtless, from other sources, spread to the tropics;
Syrtis regions of marsh or bog deepen in hue, and become distinctly
aqueous; canals dawn on the sight, and grow into undeniable
realities. We seem driven to believe that they discharge the function
of flood-emissaries.
Mr. Lowell does not hesitate to pronounce them of artificial
formation, and, on that large assumption, the purpose of their
connection with his “oases” becomes transparently clear. They bring
to these Tadmors in the wilderness the water supply by which they
are made to “blossom as the rose.” The junction-spots, we are told,
do not enlarge when the vernal freshet reaches them; they only
darken through the sudden development of vegetation. These circular
“districts, artificially fertilized by the canal system,” are strewn
broadcast over vast desert areas, the orange-ochreous sections of
Mars, covering the greater part of its surface, but deep buried in
the millennial dust of disintegrated red sandstone strata.
“Here, then,” Mr. Lowell remarks, “we have an end and reason for the
existence of canals, and the most natural conceivable—namely, that
the canals are constructed for the express purpose of fertilizing the
oases. When we consider the amazing system of the canal lines, we are
carried to this conclusion as forthright as is the water itself; what
we see being not the canal itself, indeed, but the vegetation along
its banks.”
The proportion of water to land is much smaller on Mars than on
the earth. Only two-sevenths of the disk are covered by the dusky
areas, and of late the aqueous nature of some, if not all, of
these has been seriously called in question. Professor Pickering
was convinced by his observations, in 1892 and 1894, “that the
permanent water area upon Mars, if it exist at all, is extremely
limited in its dimensions.” He estimated it at about half the size
of the Mediterranean. Professor Schaeberle is similarly incredulous.
If the dark markings are seas, he asks, how explain the irregular
gradations of shade in them? How, above all, explain their apparent
intersection by well-marked canals? Professor Barnard, observing
with the Lick thirty-six inch in 1894, discerned on the Martian
surface an astonishing wealth of detail, “so intricate, small, and
abundant, that it baffled all attempts to properly delineate it.”
It was embarrassing to find these minute features belonging more
characteristically to the “seas” than to the “continents.” Under the
best conditions, the dark regions lost all trace of uniformity.
Their appearance resembled that of a mountainous country, broken by
cañon, rift, and ridge, seen from a great elevation. These effects
were especially marked in the “ocean” area of the Hour-Glass Sea.
Evidently the relations of solid and liquid in that remote orb
are abnormal; they can not be completely explained by terrestrial
analogies. Yet a series of well-attested phenomena are intelligible
only on the supposition that Mars is, in some real sense, a
terraqueous globe. Where snows melt there must be water; and the
origin of the Rhone from a great glacier is scarcely more evident to
our senses than the dissolution of Martian ice-caps into pools and
streams.
The testimony of the spectroscope is to the same effect. Dr. Huggins
found, in 1867, the spectrum of Mars impressed with the distinct
traces of aqueous absorption, and the fact, although called in
question by Professor Campbell of Lick, in 1894, has been reaffirmed
both at Tulse Hill and at Potsdam. That clouds form and mists rise
in the thin Martian air, admits of doubt. During the latter half
of October, 1894, an area much larger than Europe remained densely
obscured. Whether or no actual rain was at that time falling over
the Maraldi Sea and the adjacent continent it would be useless to
conjecture. We only know that with the low barometric pressure at the
surface of Mars, the boiling point of water must be proportionately
depressed (Flammarion puts it at 115° Fahrenheit), which implies that
it evaporates rapidly, and can be transported easily.
If the Martian atmosphere be of the same proportionate mass as that
of our earth, it can possess no more than one-seventh its superficial
density. That is to say, it is more than twice as tenuous as the
air at the summits of the Himalayas. The corresponding height of
a terrestrial barometer would be four and a half inches. Owing,
however, to the reduced strength of gravity on Mars, this slender
envelope is exceedingly extensive. In the pure sky scarcely veiled by
it, the sun, diminished to less than half his size at our horizons,
probably exhibits his coronal streamers and prominences as a regular
part of his noontide glory; atmospheric circulation proceeds so
tranquilly as not to trouble the repose of a land “in which it
seemeth always afternoon”; no cyclones traverse its surface, only
mild trade-winds flow toward the equator, to supply for the volumes
of air gently lifted by the power of the sun, to carry reinforcements
of water-vapor north and south. Aerial movements are, in fact, by a
very strong presumption, of the terrestrial type, but executed with
greatly abated vigor.
Brilliant projections above the terminator of Mars were first
distinctly perceived at the Lick Observatory in 1890. They have
been reobserved at Nice, Arequipa, and Flagstaff (Mr. Lowell’s
observatory), coming into view, as a rule, when circumstances concur
to favor their visibility. They strictly resemble lunar peaks and
craters, catching the first rays of the sun, while the ground about
them is still immersed in darkness; and Professor Campbell connects
them with “mountain chains lying _across_ the terminator of the
planet,” and in some cases possibly snow-covered. He calculates their
height at about ten thousand feet. Their presence was unlooked for,
since a flat expanse is a condition _sine quâ non_ for the minute
intersection of land by water, which seems to prevail on Mars.
Although the sun is less than half as powerful on Mars as it is here,
the Martian climate, to outward appearance, compares favorably with
our own. Polar glaciation is less extensive and more evanescent, and
little snow falls outside the arctic and antarctic regions. Yet the
theoretical mean temperature is minus 4° C., or 61° of Fahrenheit
below freezing. This means a tremendous ice-grip. The coldest spot on
the earth’s surface is considerably warmer than this cruel average.
Fortunately, it exists only on paper. Some compensatory store of
warmth must then be possessed by Mars, and it can scarcely be
provided by its attenuated air. Possibly, internal heat may still be
effective, and we see exemplified in Mars the geological period when
vines and magnolias flourished in Greenland, and date-palms ripened
their fruit on the coast of Hampshire.
The climate of Mars, according to Schiaparelli, “must resemble that
of a clear day upon a high mountain. By day a very strong solar
radiation hardly at all mitigated by mist or vapor; by night a
copious radiation from the soil toward celestial space, and hence a
very marked refrigeration; consequently, a climate of extremes, and
great changes of temperature from day to night, and from one season
to another. And as on the earth, at altitudes of from 17,000 to
20,000 feet, the vapor of the atmosphere is condensed only into the
solid form, producing those whitish masses of suspended crystals
which we call cirrus-clouds, so in the atmosphere of Mars it would
be rarely possible to find collections of cloud capable of producing
rain of any consequence. The variation of temperature from one season
to another would be notably increased by their long duration, and
thus we can understand the great freezing and melting of the snow,
renewed in turn at the poles at each complete revolution of the
planet round the sun.”