MARS, A WORLD MORE ADVANCED THAN OURS
Mars is the fourth planet in the order of distance from the sun, and the
outermost member of the terrestrial group. Its mean distance is
141,500,000 miles, variable, through the eccentricity of its orbit, to
the extent of about 13,000,000 miles. It will be observed that this is
only a million miles less than the variation in Mercury's distance from
the sun, from which, in a previous chapter, were deduced most momentous
consequences; but, in the case of Mars, the ratio of the variation to
the mean distance is far smaller than with Mercury, so that the effect
upon the temperature of the planet is relatively insignificant.
Mars gets a little less than half as much solar light and heat as the
earth receives, its situation in this respect being just the opposite
to that of Venus. Its period of orbital revolution, or the length of its
year, is 687 of our days. The diameter of Mars is 4,200 miles, and its
density is 73 per cent of the earth's density. Gravity on its surface is
only 38 per cent of terrestrial gravity--i.e., a one hundred-pound
weight removed from the earth to Mars would there weigh but thirty-eight
pounds. Mars evidently has an atmosphere, the details of which we shall
discuss later.
The poles of the planet are inclined from a perpendicular to the plane
of its orbit at very nearly the same angle as that of the earth's poles,
viz., 24° 50´. Its rotation on its axis is also effected in almost the
same period as the earth's, viz., 24 hours, 37 minutes.
When in opposition to the sun, Mars may be only about 35,000,000 miles
from the earth, but its average distance when in that position is more
than 48,000,000 miles, and may be more than 60,000,000. These
differences arise from the eccentricities of the orbits of the two
planets. When on the farther side of the sun--i.e., in conjunction with
the sun as seen from the earth--Mars's average distance from us is about
235,000,000 miles. In consequence of these great changes in its
distance, Mars is sometimes a very conspicuous object in the sky, and at
other times inconspicuous.
The similarity in the inclination of the axis of the two planets results
in a close resemblance between the seasons on Mars and on the earth,
although, owing to the greater length of its year, Mars's seasons are
much longer than ours. Winter and summer visit in succession its
northern and southern hemispheres just as occurs on the planet that we
inhabit, and the torrid, temperate, and frigid zones on its surface have
nearly the same angular width as on the earth. In this respect Mars is
the first of the foreign planets we have studied to resemble the earth.
Around each of its poles appears a circular white patch, which visibly
expands when winter prevails upon it, and rapidly contracts, sometimes
almost completely disappearing, under a summer sun. From the time of
Sir William Herschel the almost universal belief among astronomers has
been that these gleaming polar patches on Mars are composed of snow and
ice, like the similar glacial caps of the earth, and no one can look at
them with a telescope and not feel the liveliest interest in the planet
to which they belong, for they impart to it an appearance of likeness to
our globe which at first glance is all but irresistible.
To watch one of them apparently melting, becoming perceptibly smaller
week after week, while the general surface of the corresponding
hemisphere of the planet deepens in color, and displays a constantly
increasing wealth of details as summer advances across it, is an
experience of the most memorable kind, whose effect upon the mind of the
observer is indescribable.
Early in the history of the telescope it became known that, in addition
to the polar caps, Mars presented a number of distinct surface features,
and gradually, as instruments increased in power and observers in
skill, charts of the planet were produced showing a surface diversified
somewhat in the manner that characterizes the face of the earth,
although the permanent forms do not closely resemble those of our
planet.
Two principal colors exist on the disk of Mars--dark, bluish gray or
greenish gray, characterizing areas which have generally been regarded
as seas, and light yellowish red, overspreading broad regions looked
upon as continents. It was early observed that if the dark regions
really are seas, the proportion of water to land upon Mars is much
smaller than upon the earth.
For two especial reasons Mars has generally been regarded as an older or
more advanced planet than the earth. The first reason is that, accepting
Laplace's theory of the origin of the planetary system from a series of
rings left off at the periphery of the contracting solar nebula, Mars
must have come into existence earlier than the earth, because, being
more distant from the center of the system, the ring from which it was
formed would have been separated sooner than the terrestrial ring. The
second reason is that Mars being smaller and less massive than the earth
has run through its developments a cooling globe more rapidly. The
bearing of these things upon the problems of life on Mars will be
considered hereafter.
And now, once more, Schiaparelli appears as the discoverer of surprising
facts about one of the most interesting worlds of the solar system.
During the exceptionally favorable opposition of Mars in 1877, when an
American astronomer, Asaph Hall, discovered the planet's two minute
satellites, and again during the opposition of 1879, the Italian
observer caught sight of an astonishing network of narrow dark lines
intersecting the so-called continental regions of the planet and
crossing one another in every direction. Schiaparelli did not see the
little moons that Hall discovered, and Hall did not perceive the
enigmatical lines that Schiaparelli detected. Hall had by far the larger
and more powerful telescope; Schiaparelli had much the more steady and
favorable atmosphere for astronomical observation. Yet these differences
in equipment and circumstances do not clearly explain why each observer
should have seen what the other did not.
There may be a partial explanation in the fact that an observer having
made a remarkable discovery is naturally inclined to confine his
attention to it, to the neglect of other things. But it was soon found
that Schiaparelli's lines--to which he gave the name "canals," merely on
account of their shape and appearance, and without any intention to
define their real nature--were excessively difficult telescopic objects.
Eight or nine years elapsed before any other observer corroborated
Schiaparelli's observations, and notwithstanding the "sensation" which
the discovery of the canals produced they were for many years regarded
by the majority of astronomers as an illusion.
But they were no illusion, and in 1881 Schiaparelli added to the
astonishment created by his original discovery, and furnished additional
grounds for skepticism, by announcing that, at certain times, many of
the canals geminated, or became double! He continued his observations at
each subsequent opposition, adding to the number of the canals observed,
and charting them with classical names upon a detailed map of the
planet's surface.
At length in 1886 Perrotin, at Nice, detected many of Schiaparelli's
canals, and later they were seen by others. In 1888 Schiaparelli greatly
extended his observations, and in 1892 and 1894 some of the canals were
studied with the 36-inch telescope of the Lick Observatory, and in the
last-named year a very elaborate series of observations upon them was
made by Percival Lowell and his associates, Prof. William C. Pickering
and Mr. A.E. Douglass, at Flagstaff, Arizona. Mr. Lowell's charts of the
planet are the most complete yet produced, containing 184 canals to
which separate names have been given, besides more than a hundred other
markings also designated by individual appellations.
It should not be inferred from the fact that Schiaparelli's discovery
in 1877 excited so much surprise and incredulity that no glimpse of the
peculiar canal-like markings on Mars had been obtained earlier than
that. At least as long ago as 1864 Mr. Dawes, in England, had seen and
sketched half a dozen of the larger canals, or at least the broader
parts of them, especially where they connect with the dark regions known
as seas, but Dawes did not see them in their full extent, did not
recognize their peculiar character, and entirely failed to catch sight
of the narrower and more numerous ones which constitute the wonderful
network discovered by the Italian astronomer. Schiaparelli found no less
than sixty canals during his first series of observations in 1877.
Let us note some of the more striking facts about the canals which
Schiaparelli has described. We can not do better than quote his own
words:
"There are on this planet, traversing the continents, long dark lines
which may be designated as _canals_, although we do not yet know what
they are. These lines run from one to another of the somber spots that
are regarded as seas, and form, over the lighter, or continental,
regions a well-defined network. Their arrangement appears to be
invariable and permanent; at least, as far as I can judge from four and
a half years of observation. Nevertheless, their aspect and their degree
of visibility are not always the same, and depend upon circumstances
which the present state of our knowledge does not yet permit us to
explain with certainty. In 1879 a great number were seen which were not
visible in 1877, and in 1882 all those which had been seen at former
oppositions were found again, together with new ones. Sometimes these
canals present themselves in the form of shadowy and vague lines, while
on other occasions they are clear and precise, like a trace drawn with a
pen. In general they are traced upon the sphere like the lines of great
circles; a few show a sensible lateral curvature. They cross one another
obliquely, or at right angles. They have a breadth of two degrees, or
120 kilometres [74 miles], and several extend over a length of eighty
degrees, or 4,800 kilometres [nearly 3,000 miles]. Their tint is very
nearly the same as that of the seas, usually a little lighter. Every
canal terminates at both its extremities in a sea, or in another canal;
there is not a single example of one coming to an end in the midst of
dry land.
"This is not all. In certain seasons these canals become double. This
phenomenon seems to appear at a determinate epoch, and to be produced
simultaneously over the entire surface of the planet's continents. There
was no indication of it in 1877, during the weeks that preceded and
followed the summer solstice of that world. A single isolated case
presented itself in 1879. On the 26th of December, this year--a little
before the spring equinox, which occurred on Mars on the 21st of
January, 1880--I noticed the doubling of the Nile [a canal thus named]
between the Lakes of the Moon and the Ceraunic Gulf. These two regular,
equal, and parallel lines caused me, I confess, a profound surprise,
the more so because a few days earlier, on the 23d and the 24th of
December, I had carefully observed that very region without discovering
anything of the kind.
"I awaited with curiosity the return of the planet in 1881, to see if an
analogous phenomenon would present itself in the same place, and I saw
the same thing reappear on the 11th of January, 1882, one month after
the spring equinox--which occurred on the 8th of December, 1881. The
duplication was still more evident at the end of February. On this same
date, the 11th of January, another duplication had already taken place,
that of the middle portion of the canal of the Cyclops, adjoining
Elysium. [Elysium is a part of one of the continental areas.]
"Yet greater was my astonishment when, on the 19th of January, I saw the
canal Jamuna, which was then in the center of the disk, formed very
rigidly of two parallel straight lines, crossing the space which
separates the Niliac Lake from the Gulf of Aurora. At first sight I
believed it was an illusion, caused by fatigue of the eye and some new
kind of strabismus, but I had to yield to the evidence. After the 19th
of January I simply passed from wonder to wonder; successively the
Orontes, the Euphrates, the Phison, the Ganges, and the larger part of
the other canals, displayed themselves very clearly and indisputably
duplicated. There were not less than twenty examples of duplication, of
which seventeen were observed in the space of a month, from the 19th of
January to the 19th of February.
"In certain cases it was possible to observe precursory symptoms which
are not lacking in interest. Thus, on the 13th of January, a light,
ill-defined shade extended alongside the Ganges; on the 18th and the
19th one could only distinguish a series of white spots; on the 20th the
shadow was still indecisive, but on the 21st the duplication was
perfectly clear, such as I observed it until the 23d of February. The
duplication of the Euphrates, of the canal of the Titans, and of the
Pyriphlegethon also began in an uncertain and nebulous form.
"These duplications are not an optical effect depending on increase of
visual power, as happens in the observation of double stars, and it is
not the canal itself splitting in two longitudinally. Here is what is
seen: To the right or left of a pre-existing line, without any change in
the course and position of that line, one sees another line produce
itself, equal and parallel to the first, at a distance generally varying
from six to twelve degrees--i.e., from 350 to 700 kilometres (217 to 434
miles); even closer ones seem to be produced, but the telescope is not
powerful enough to distinguish them with certainty. Their tint appears
to be a quite deep reddish brown. The parallelism is sometimes
rigorously exact. There is nothing analogous in terrestrial geography.
Everything indicates that here there is an organization special to the
planet Mars, probably connected with the course of its seasons."[1]
[Footnote 1: L'Astronomie, vol. i, 1882, pp. 217 _et seq._]
Schiaparelli adds that he took every precaution to avoid the least
suspicion of illusion. "I am absolutely sure," he says, "of what I have
observed."
I have quoted his statement, especially about the duplication of the
canals, at so much length, both on account of its intrinsic interest and
because it has many times been argued that this particular phenomenon
must be illusory even though the canals are real.
One of the most significant facts that came out in the early
observations was the evident connection between the appearance of the
canals and the seasonal changes on Mars. It was about the time of the
spring equinox, when the white polar caps had begun to melt, that
Schiaparelli first noticed the phenomenon of duplication. As the season
advanced the doubling of the canals increased in frequency and the lines
became more distinct. In the meantime the polar caps were becoming
smaller. Broadly speaking, Schiaparelli's observation showed that the
doubling of the canals occurred principally a little after the spring
equinox and a little before the autumn equinox; that the phenomenon
disappeared in large part at the epoch of the winter solstice, and
disappeared altogether at the epoch of the summer solstice. Moreover, he
observed that many of the canals, without regard to duplication, were
invisible at times, and reappeared gradually; faint, scarcely visible
lines and shadows, deepened and became more distinct until they were
clearly and sharply defined, and these changes, likewise, were evidently
seasonal.
The invariable connection of the canals at their terminations with the
regions called seas, the fact that as the polar caps disappeared the
sealike expanses surrounding the polar regions deepened in color, and
other similar considerations soon led to the suggestion that there
existed on Mars a wonderful system of water circulation, whereby the
melting of the polar snows, as summer passed alternately from one
hemisphere to the other, served to reenforce the supply of water in the
seas, and, through the seas, in the canals traversing the broad
expanses of dry land that occupy the equatorial regions of the planet.
The thought naturally occurred that the canals might be of artificial
origin, and might indicate the existence of a gigantic system of
irrigation serving to maintain life upon the globe of Mars. The
geometrical perfection of the lines, their straightness, their absolute
parallelism when doubled, their remarkable tendency to radiate from
definite centers, lent strength to the hypothesis of an artificial
origin. But their enormous size, length, and number tended to stagger
belief in the ability of the inhabitants of any world to achieve a work
so stupendous.
After a time a change of view occurred concerning the nature of the
expanses called seas, and Mr. Lowell, following his observations of
1894, developed the theory of the water circulation and irrigation of
Mars in a new form. He and others observed that occasionally canals were
visible cutting straight across some of the greenish, or bluish-gray,
areas that had been regarded as seas. This fact suggested that, instead
of seas, these dark expanses may rather be areas of marshy ground
covered with vegetation which flourishes and dies away according as the
supply of water alternately increases and diminishes, while the reddish
areas known as continents are barren deserts, intersected by canals; and
as the water released by the melting of the polar snows begins to fill
the canals, vegetation springs up along their sides and becomes visible
in the form of long narrow bands.
According to this theory, the phenomena called canals are simply lines
of vegetation, the real canals being individually too small to be
detected. It may be supposed that from a central supply canal irrigation
ditches are extended for a distance of twenty or thirty miles on each
side, thus producing a strip of fertile soil from forty to sixty miles
wide, and hundreds, or in some cases two or three thousands, of miles in
length.
The water supply being limited, the inhabitants can not undertake to
irrigate the entire surface of the thirsty land, and convenience of
circulation induces them to extend the irrigated areas in the form of
long lines. The surface of Mars, according to Lowell's observation, is
remarkably flat and level, so that no serious obstacle exists to the
extension of the canal system in straight bands as undeviating as arcs
of great circles.
Wherever two or more canals meet, or cross, a rounded dark spot from a
hundred miles, or less, to three hundred miles in diameter, is seen. An
astonishing number of these appear on Mr. Lowell's charts. Occasionally,
as occurs at the singular spot named Lacus Solis, several canals
converging from all points of the compass meet at a central point like
the spokes of a wheel; in other cases, as, for instance, that of the
long canal named Eumenides, with its continuation Orcus, a single
conspicuous line is seen threading a large number of round dark spots,
which present the appearance of a row of beads upon a string. These
circular spots, which some have regarded as lakes, Mr. Lowell believes
are rather oases in the great deserts, and granting the correctness of
his theory of the canals the aptness of this designation is apparent.[2]
[Footnote 2: The reader can find many of these "canals" and "oases," as
well as some of the other regions on Mars that have received names, in
the frontispiece.]
Wherever several canals, that is to say, several bands of vegetation or
bands of life, meet, it is reasonable to assume that an irrigated and
habitable area of considerable extent will be developed, and in such
places the imagination may picture the location of the chief centers of
population, perhaps in the form of large cities, or perhaps in groups of
smaller towns and villages. The so-called Lacus Solis is one of these
localities.
So, likewise, it seems but natural that along the course of a broad,
well-irrigated band a number of expansions should occur, driving back
the bounds of the desert, forming rounded areas of vegetation, and thus
affording a footing for population. Wherever two bands cross such areas
would be sure to exist, and in almost every instance of crossing the
telescope actually shows them.
As to the gemination or duplication of many of the lines which, at the
beginning of the season, appear single, it may be suggested that, in the
course of the development of the vast irrigation system of the planet
parallel bands of cultivation have been established, one receiving its
water supply from the canals of the other, and consequently lagging a
little behind in visibility as the water slowly percolates through the
soil and awakens the vegetation. Or else, the character of the
vegetation itself may differ as between two such parallel bands, one
being supplied with plants that spring up and mature quickly when the
soil about their roots is moistened, while the plants in the twin band
respond more slowly to stimulation.
Objection has been made to the theory of the artificial origin of the
canals of Mars on the ground, already mentioned, that the work required
to construct them would be beyond the capacity of any race of creatures
resembling man. The reply that has been made to this is twofold. In the
first place, it should be remembered that the theory, as Mr. Lowell
presents it, does not assert that the visible lines are the actual
canals, but only that they are strips of territory intersected, like
Holland or the center of the plain of Lombardy, by innumerable
irrigation canals and ditches. To construct such works is clearly not an
impossible undertaking, although it does imply great industry and
concentration of effort.
In the second place, since the force of gravity on Mars is in the ratio
of only 38 to 100 compared with the earth's, it is evident that the
diminished weight of all bodies to be handled would give the inhabitants
of Mars an advantage over those of the earth in the performance of
manual labor, provided that they possess physical strength and activity
as great as ours. But, in consequence of this very fact of the slighter
force of gravity, a man upon Mars could attain a much greater size, and
consequently much greater muscular strength, than his fellows upon the
earth possess without being oppressed by his own weight. In other words,
as far as the force of gravity may be considered as the decisive factor,
Mars could be inhabited by giants fifteen feet tall, who would be
relatively just as active, and just as little impeded in their movements
by the weight of their bodies, as a six-footer is upon the earth. But
they would possess far more physical strength than we do, while, in
doing work, they would have much lighter materials to deal with.
Whether the theory that the canals of Mars really are canals is true or
not, at any rate there can now be no doubt as to the existence of the
strange lines which bear that designation. The suggestion has been
offered that their builders may no longer be in existence, Mars having
already passed the point in its history where life must cease upon its
surface. This brings us to consider again the statement, made near the
beginning of this chapter, that Mars is, perhaps, at a more advanced
stage of development than the earth. If we accept this view, then,
provided there was originally some resemblance between Mars's life forms
and those of the earth, the inhabitants of that planet would, at every
step, probably be in front of their terrestrial rivals, so that at the
present time they should stand well in advance. Mr. Lowell has, perhaps,
put this view of the relative advancement in evolution of Mars and its
inhabitants as picturesquely as anybody.
"In Mars," he says, "we have before us the spectacle of a world
relatively well on in years, a world much older than the earth. To so
much about his age Mars bears witness on his face. He shows unmistakable
signs of being old. Advancing planetary years have left their mark
legible there. His continents are all smoothed down; his oceans have all
dried up.... Mars being thus old himself, we know that evolution on his
surface must be similarly advanced. This only informs us of its
condition relative to the planet's capabilities. Of its actual state our
data are not definite enough to furnish much deduction. But from the
fact that our own development has been comparatively a recent thing, and
that a long time would be needed to bring even Mars to his present
geological condition, we may judge any life he may support to be not
only relatively, but really older than our own. From the little we can
see such appears to be the case. The evidence of handicraft, if such it
be, points to a highly intelligent mind behind it. Irrigation,
unscientifically conducted, would not give us such truly wonderful
mathematical fitness in the several parts to the whole as we there
behold.... Quite possibly such Martian folk are possessed of inventions
of which we have not dreamed, and with them electrophones and
kinetoscopes are things of a bygone past, preserved with veneration in
museums as relics of the clumsy contrivances of the simple childhood of
the race. Certainly what we see hints at the existence of beings who are
in advance of, not behind us, in the journey of life."[3]
[Footnote 3: Mars, by Percival Lowell, p. 207 _et seq._]
Granted the existence of such a race as is thus described, and to them
it might not seem a too appalling enterprise, when their planet had
become decrepit, with its atmosphere thinned out and its supply of water
depleted, to grapple with the destroying hand of nature and to prolong
the career of their world by feats of chemistry and engineering as yet
beyond the compass of human knowledge.
It is confidence, bred from considerations like these, in the superhuman
powers of the supposed inhabitants of Mars that has led to the popular
idea that they are trying to communicate by signals with the earth.
Certain enigmatical spots of light, seen at the edge of the illuminated
disk of Mars, and projecting into the unilluminated part--for Mars,
although an outer planet, shows at particular times a gibbous phase
resembling that of the moon just before or just after the period of full
moon--have been interpreted by some, but without any scientific
evidence, as of artificial origin.
Upon the assumption that these bright points, and others occasionally
seen elsewhere on the planet's disk, are intended by the Martians for
signals to the earth, entertaining calculations have been made as to the
quantity of light that would be required in the form of a "flash signal"
to be visible across the distance separating the two planets. The
results of the calculations have hardly been encouraging to possible
investors in interplanetary telegraphy, since it appears that
heliographic mirrors with reflecting surfaces measured by square miles,
instead of square inches, would be required to send a visible beam from
the earth to Mars or _vice versa_.
The projections of light on Mars can be explained much more simply and
reasonably. Various suggestions have been made about them; among others,
that they are masses of cloud reflecting the sunshine; that they are
areas of snow; and that they are the summits of mountains crowned with
ice and encircled with clouds. In fact, a huge mountain mass lying on
the terminator, or the line between day and night, would produce the
effect of a tongue of light projecting into the darkness without
assuming that it was snow-covered or capped with clouds, as any one may
convince himself by studying the moon with a telescope when the
terminator lies across some of its most mountainous regions. To be sure,
there is reason to think that the surface of Mars is remarkably flat;
yet even so the planet may have some mountains, and on a globe the
greater part of whose shell is smooth any projections would be
conspicuous, particularly where the sunlight fell at a low angle across
them.
Another form in which the suggestion of interplanetary communication has
been urged is plainly an outgrowth of the invention and surprising
developments of wireless telegraphy. The human mind is so constituted
that whenever it obtains any new glimpse into the arcana of nature it
immediately imagines an indefinite and all but unlimited extension of
its view in that direction. So to many it has not appeared unreasonable
to assume that, since it is possible to transmit electric impulses for
considerable distances over the earth's surface by the simple
propagation of a series of waves, or undulations, without connecting
wires, it may also be possible to send such impulses through the ether
from planet to planet.
The fact that the electric undulations employed in wireless telegraphy
pass between stations connected by the crust of the earth itself, and
immersed in a common atmospheric envelope, is not deemed by the
supporters of the theory in question as a very serious objection, for,
they contend, electric waves are a phenomenon of the ether, which
extends throughout space, and, given sufficient energy, such waves could
cross the gap between world and world.
But nobody has shown how much energy would be needed for such a purpose,
and much less has anybody indicated a way in which the required energy
could be artificially developed, or cunningly filched from the stores of
nature. It is, then, purely an assumption, an interesting figment of
the mind, that certain curious disturbances in the electrical state of
the air and the earth, affecting delicate electric instruments,
possessing a marked periodicity in brief intervals of time, and not yet
otherwise accounted for, are due to the throbbing, in the all-enveloping
ether, of impulses transmitted from instruments controlled by the
_savants_ of Mars, whose insatiable thirst for knowledge, and presumably
burning desire to learn whether there is not within reach some more
fortunate world than their half-dried-up globe, has led them into a
desperate attempt to "call up" the earth on their interplanetary
telephone, with the hope that we are wise and skilful enough to
understand and answer them.
In what language they intend to converse no one has yet undertaken to
tell, but the suggestion has sapiently been made that, mathematical
facts being invariable, the eternal equality of two plus two with four
might serve as a basis of understanding, and that a statement of that
truth sent by electric taps across the ocean of ether would be a
convincing assurance that the inhabitants of the planet from which the
message came at least enjoyed the advantages of a common-school
education.
But, while speculation upon this subject rests on unverified, and at
present unverifiable, assumptions, of course everybody would rejoice if
such a thing were possible, for consider what zest and charm would be
added to human life if messages, even of the simplest description, could
be sent to and received from intelligent beings inhabiting other
planets! It is because of this hold that it possesses upon the
imagination, and the pleasing pictures that it conjures up, that the
idea of interplanetary communication, once broached, has become so
popular a topic, even though everybody sees that it should not be taken
too seriously.
The subject of the atmosphere of Mars can not be dismissed without
further consideration than we have yet given it, because those who think
the planet uninhabitable base their opinion largely upon the assumed
absence of sufficient air to support life. It was long ago recognized
that, other things being equal, a planet of small mass must possess a
less dense atmosphere than one of large mass. Assuming that each planet
originally drew from a common stock, and that the amount and density of
its atmosphere is measured by its force of gravity, it can be shown that
Mars should have an atmosphere less than one fifth as dense as the
earth's.
Dr. Johnstone Stoney has attacked the problem of planetary atmospheres
in another way. Knowing the force of gravity on a planet, it is easy to
calculate the velocity with which a body, or a particle, would have to
start radially from the planet in order to escape from its gravitational
control. For the earth this critical velocity is about seven miles per
second; for Mars about three miles per second. Estimating the velocity
of the molecules of the various atmospheric gases, according to the
kinetic theory, Dr. Stoney finds that some of the smaller planets, and
the moon, are gravitationally incapable of retaining all of these gases
in the form of an atmosphere. Among the atmospheric constituents that,
according to this view, Mars would be unable permanently to retain is
water vapor. Indeed, he supposes that even the earth is slowly losing
its water by evaporation into space, and on Mars, owing to the slight
force of gravity there, this process would go on much more rapidly, so
that, in this way, we have a means of accounting for the apparent drying
up of that planet, while we may be led to anticipate that at some time
in the remote future the earth also will begin to suffer from lack of
water, and that eventually the chasms of the sea will yawn empty and
desolate under a cloudless sky.
But it is not certain that the original supply of atmospheric elements
was in every case proportional to the respective force of gravity of a
planet. The fact that Venus appears to have an atmosphere more extensive
and denser than the earth's, although its force of gravity is a little
less than that of our globe, indicates at once a variation as between
these two planets in the amount of atmospheric material at their
disposal. This may be a detail depending upon differences in the mode,
or in the stage, of their evolution. Thus, after all, Dr. Stoney's
theory may be substantially correct and yet Mars may retain sufficient
water to form clouds, to be precipitated in snow, and to fill its canals
after each annual melting of the polar caps, because the original supply
was abundant, and its escape is a gradual process, only to be completed
by age-long steps.
Even though the evidence of the spectroscope, as far as it goes, seems
to lend support to the theory that there is no water vapor in the
atmosphere of Mars, we can not disregard the visual evidence that,
nevertheless, water vapor exists there.
What are the polar caps if they are not snow? Frozen carbon dioxide, it
has been suggested; but this is hardly satisfactory, for it offers no
explanation of the fact that when the polar caps diminish, and in
proportion as they diminish, the "seas" and the canals darken and
expand, whereas a reasonable explanation of the correlation of these
phenomena is offered if we accept the view that the polar caps consist
of snow.
Then there are many observations on record indicating the existence of
clouds in Mars's atmosphere. Sometimes a considerable area of its
surface has been observed to be temporarily obscured, not by dense
masses of cloud such as accompany the progress of great cyclonic storms
across the continents and oceans of the earth, but by comparatively thin
veils of vapor such as would be expected to form in an atmosphere so
comparatively rare as that of Mars. And these clouds, in some instances
at least, appear, like the cirrus streaks and dapples in our own air, to
float at a great elevation. Mr. Douglass, one of Mr. Lowell's associates
in the observations of 1894 at Flagstaff, Arizona, observed what he
believed to be a cloud over the unilluminated part of Mars's disk,
which, by micrometric measurement and estimate, was drifting at an
elevation of about fifteen miles above the surface of the planet. This
was seen on two successive days, November 25th and November 26th, and it
underwent curious fluctuations in visibility, besides moving in a
northerly direction at the rate of some thirteen miles an hour. But,
upon the whole, as Mr. Lowell remarks, the atmosphere of Mars is
remarkably free of clouds.
The reader will remember that Mars gets a little less than half as much
heat from the sun as the earth gets. This fact also has been used as an
argument against the habitability of the planet. In truth, those who
think that life in the solar system is confined to the earth alone
insist upon an almost exact reproduction of terrestrial conditions as a
_sine qua non_ to the habitability of any other planet. Venus, they
think, is too hot, and Mars too cold, as if life were rather a happy
accident than the result of the operation of general laws applicable
under a wide variety of conditions. All that we are really justified in
asserting is that Venus may be too hot and Mars too cold for _us_. Of
course, if we adopt the opinion held by some that the temperature on
Mars is constantly so low that water would remain perpetually frozen, it
does throw the question of the kind of life that could be maintained
there into the realm of pure conjecture.
The argument in favor of an extremely low temperature on Mars is based
on the law of the diminution of radiant energy inversely as the square
of the distance, together with the assumption that no qualifying
circumstances, or no modification of that law, can enter into the
problem. According to this view, it could be shown that the temperature
on Mars never rises above -200° F. But it is a view that seems to be
directly opposed to the evidence of the telescope, for all who have
studied Mars under favorable conditions of observation have been
impressed by the rapid and extensive changes that the appearance of its
surface undergoes coincidently with the variation of the planet's
seasons. It has its winter aspect and its summer aspect, perfectly
distinct and recognizable, in each hemisphere by turns, and whether the
polar caps be snow or carbon dioxide, at any rate they melt and
disappear under a high sun, thus proving that an accumulation of heat
takes place.
Professor Young says: "As to the temperature of Mars we have no certain
knowledge. On the one hand, we know that on account of the planet's
distance from the sun the intensity of solar radiation upon its surface
must be less than here in the ratio of 1 to (1.524)^2--i.e., only about
43 per cent as great as with us; its 'solar constant' must be less than
13 calories against our 30. Then, too, the low density of its
atmosphere, probably less at the planet's surface than on the tops of
our highest mountains, would naturally assist to keep down the
temperature to a point far below the freezing-point of water. But, on
the other hand, things certainly _look_ as if the polar caps were really
masses of _snow_ and _ice_ deposited from vapor in the planet's
atmosphere, and as if these actually melted during the Martian summer,
sending floods of water through the channels provided for them, and
causing the growth of vegetation along their banks. We are driven,
therefore, to suppose either that the planet has sources of heat
internal or external which are not yet explained, or else, as long ago
suggested, that the polar 'snow' may possibly be composed of something
else than frozen _water_."[4]
[Footnote 4: General Astronomy, by Charles A. Young. Revised edition,
1898, p. 363.]
Even while granting the worst that can be said for the low temperature
of Mars, the persistent believer in its habitability could take refuge
in the results of recent experiments which have proved that bacterial
life is able to resist the utmost degree of cold that can be applied,
microscopic organisms perfectly retaining their vitality--or at least
their power to resume it--when subjected to the fearfully low
temperature of liquid air. But then he would be open to the reply that
the organisms thus treated are in a torpid condition and deprived of all
activity until revived by the application of heat; and the picture of a
world in a state of perpetual sleep is not particularly attractive,
unless the fortunate prince who is destined to awake the slumbering
beauty can also be introduced into the romance.[5]
[Footnote 5: Many of the present difficulties about temperatures on the
various planets would be beautifully disposed of if we could accept the
theory urged by Mr. Cope Whitehouse, to the effect that the sun is not
really a hot body at all, and that what we call solar light and heat are
only local manifestations produced in our atmosphere by the
transformation of some other form of energy transmitted from the sun;
very much as the electric impulses carried by a wire from the
transmitting to the receiving station on a telephone line are translated
by the receiver into waves of sound. According to this theory, which is
here mentioned only as an ingenuity and because something of the kind so
frequently turns up in one form or another in popular semi-scientific
literature, the amount of heat and light on a planet would depend mainly
upon local causes.]
To an extent which most of us, perhaps, do not fully appreciate, we are
indebted for many of the pleasures and conveniences and some of the
necessities of life on our planet to its faithful attendant, the moon.
Neither Mercury nor Venus has a moon, but Mars has two moons. This
statement, standing alone, might lead to the conclusion that, as far as
the advantages a satellite can afford to the inhabitants of its master
planet are concerned, the people of Mars are doubly fortunate. So they
would be, perhaps, if Mars's moons were bodies comparable in size with
our moon, but in fact they are hardly more than a pair of very
entertaining astronomical toys. The larger of the two, Phobos, is
believed to be about seven miles in diameter; the smaller, Deimos, only
five or six miles. Their dimensions thus resemble those of the more
minute of the asteroids, and the suggestion has even been made that they
may be captured asteroids which have fallen under the gravitational
control of Mars.
The diameters just mentioned are Professor Pickering's estimates, based
on the amount of light the little satellites reflect, for they are much
too small to present measurable disks. Deimos is 14,600 miles from the
center of Mars and 12,500 miles from its surface. Phobos is 5,800 miles
from the center of the planet and only 3,700 from the surface. Deimos
completes a revolution about the planet in thirty hours and eighteen
minutes, and Phobos in the astonishingly short period--although, of
course, it is in strict accord with the law of gravitation and in that
sense not astonishing--of seven hours and thirty-nine minutes.
Since Mars takes twenty-four hours and thirty-seven minutes for one
rotation on its axis, it is evident that Phobos goes round the planet
three times in the course of a single Martian day and night, rising,
contrary to the general motion of the heavens, in the west, running in a
few hours through all the phases that our moon exhibits in the course of
a month, and setting, where the sun and all the stars rise, in the east.
Deimos, on the other hand, has a period of revolution five or six hours
longer than that of the planet's axial rotation, so that it rises, like
the other heavenly bodies, in the east; but, because its motion is so
nearly equal, in angular velocity, to that of Mars's rotation, it shifts
very slowly through the sky toward the west, and for two or three
successive days and nights it remains above the horizon, the sun
overtaking and passing it again and again, while, in the meantime, its
protean face swiftly changes from full circle to half-moon, from
half-moon to crescent, from crescent back to half, and from half to
full, and so on without ceasing.
And during this time Phobos is rushing through the sky in the opposite
direction, as if in defiance of the fundamental law of celestial
revolution, making a complete circuit three times every twenty-four
hours, and changing the shape of its disk four times as rapidly as
Deimos does! Truly, if we were suddenly transported to Mars, we might
well believe that we had arrived in the mother world of lunatics, and
that its two moons were bewitched. Yet it must not be supposed that all
the peculiarities just mentioned would be clearly seen from the surface
of Mars by eyes like ours. The phases of Phobos would probably be
discernible to the naked eye, but those of Deimos would require a
telescope in order to be seen, for, notwithstanding their nearness to
the planet, Mars's moons are inconspicuous phenomena even to the
Martians themselves. Professor Young's estimate is that Phobos may shed
upon Mars one-sixtieth and Deimos one-twelve-hundredth as much reflected
moonlight as our moon sends to the earth. Accordingly, a "moonlit night"
on Mars can have no such charm as we associate with the phrase. But it
is surely a tribute to the power and perfection of our telescopes that
we have been able to discover the existence of objects so minute and
inconspicuous, situated at a distance of many millions of miles, and
half concealed by the glaring light of the planet close around which
they revolve.
If Mars's moons were as massive as our moon is they would raise
tremendous tides upon Mars, and would affect the circulation of water in
the canals, but, in fact, their tidal effects are even more
insignificant than their light-giving powers. But for astronomers on
Mars they would be objects of absorbing interest.
Upon quitting Mars we pass to the second distinctive planetary group of
the solar system, that of the asteroids.