our snow or ice, melts under the rays of the Sun; although it seems
difficult to admit that the Martial snow is identical with our
terrestrial snow, and that it melts at a like temperature. The south
polar spot of Mars entirely disappeared from sight in its summer season
in 1877, although the planet receives less than one-half as much heat as
we receive from the Sun; yet on our globe the arctic or antarctic ices
and snow are perpetual--never melting entirely. An important fact
disclosed by the melting away of the southern polar spot is, that in
melting it is always surrounded with a very dark surface, which takes
the place of the melted portion of the spot, as observed by myself in
1877-78. When the polar spot had entirely disappeared, its place was
occupied by a very dark spot. Now, if the polar spot is really ice, and
the dark spots are actual seas, this polar spot must be situated in
mid-ocean, since, on melting away, it is replaced by a dark spot. If the
polar spots are composed of a white substance melting under the rays of
the Sun, as seems altogether probable, its melting point must be above
that of terrestrial snow.
Many of the dark spots of Mars, and especially those whose northern
border forms an irregular belt upon the equatorial regions of this
planet, are bordered on that side by a white luminous belt, following
all their sinuosities. These white borders are variable. Sometimes they
are very prominent and intensely bright, especially at some points,
which occasionally almost equal the polar spots in brilliancy; while at
other times they are so faint, that they can hardly be distinguished, or
are even invisible; although the atmosphere is clear and the dark spots
appear perfectly well defined. While these white borders were invisible,
I have sometimes watched for several hours at a time to see if I could
detect any traces of them in places where they usually appear the most
prominent, but generally without success. On a few occasions, however, I
had the good fortune to see some of these spots forming gradually in the
course of one or two hours, at places where nothing of them could be
seen before.
These whitish fringes forming and vanishing along the coasts of the
Martial seas have been very little studied by astronomers. From my
observations made during the last seven years, it appears very probable
that this belt and its white spots are mainly due to the condensation of
vapors around, and over high peaks, and extensive mountain chains,
forming the Martial sea-coasts, as the Andes and Rocky Mountains form
the sea-coasts of the Pacific Ocean. These high mountains on Mars,
condensing the vapors into fogs or clouds above them, or at their sides,
as often happens in our mountainous districts, would certainly suffice
to produce the phenomena observed. Some of the highest peaks among these
mountain chains may even have their summits covered with perpetual snow,
or some substance partaking of the nature of snow. The temporary
visibility and invisibility of the white spots seen on Mars, as well as
the rapid transformations they sometimes undergo, may be explained as
caused by clouds having a high reflective power and a liability to form
and disappear quickly.
The assumption that these irregular whitish bands and spots are formed
by the condensation of vapors on mountain chains, and elevated table
lands, is supported by my observations made in 1877 and 1879. When such
white spots were traversing the terminator at sunrise, they very often
projected far into the night side, thus indicating that they were at a
higher level than that of the general surface. Indentations in the
terminator, corresponding to large dark spots crossing its line, also
clearly indicated the depression of the dark spots below the general
surface. The highest mountainous districts thus observed on Mars, are
situated between 60° and 70° of south latitude, towards the western
extremity of Gill Land. The mountain chain, which almost completely
forms the surface of this land, is so elevated at some points, that they
not only change the form of the terminator when they are seen upon it,
but also the limb of the planet, as seen by myself. They then appear so
brilliant, that the principal summit among them has been mistaken by
several observers for the polar spot itself, as proved by the wrong
position assigned to it on their drawings. It seems probable that this
high peak, which appears always white, is constantly covered with snow,
or the similar material replacing it on Mars. This high region is
situated between longitudes 180° and 190°.
The highest mountainous parts belonging to the hemisphere represented on
Plate VIII., which are nearly always more or less visible as whitish
spots and bands, form a coast line along the northern (lower) border of
De La Rue Ocean. This great spot, which is not so simple as it has been
represented by observers, is in fact divided by two narrow isthmuses,
one in the north, the other in the east, both joining, in the interior
of the great ocean, a peninsula heretofore known as Hall Island. Upon
the south-eastern extremity of this peninsula, a white spot, called
Dawes Ice Island, was observed in 1865, but it soon disappeared, and was
after that seen only now and then. It is very probable that this
so-called Ice Island was due to clouds forming around the summit of some
high peak of this peninsula.
On the opposite hemisphere to that represented on Plate VIII., the white
fringes bordering the dark spots are much more conspicuous than they are
on this side. On the eastern side of a remarkable dark spot called
Kaiser Sea, they are very bright, and almost always present, although
they vary considerably, both in brightness and in extent. To the south
of Kaiser Sea, they are very conspicuous on the eastern border of
Lockyer Land, forming an elevated and deeply indented coast-line along
Lambert Sea. There the white spots never disappear entirely, being
always visible on the north side, where they turn westward along Dawes'
Ocean--the mountain chain attaining there its greatest altitude. Very
frequently Lockyer Land, which seems to be a vast plateau, appears
throughout white and brilliant, this occurring usually towards the
sunrise or sunset of that region, probably from the condensation of
vapors and the formation of fogs, but generally this whiteness gradually
disappears with the progress of the sun above this plateau. Inside of
the great continents of Mars these temporary white spots are not so
frequent, but when visible they occupy always the same positions--a fact
which probably indicates that they occupy the culminating points of
these continents. One of these temporary white spots inside of the
continents is represented on Plate VIII., on the left-hand side, below
De La Rue Ocean, on Maedler Continent.
Although large, opaque, cumulus-shaped, cloud-like forms are seen in the
polar regions of Mars, such forms are very seldom seen in the tropical
zones, or, at least, it appears so, from the fact that my observations,
continued during the last seven years, have disclosed no real opaque
cloudy forms there. Although the Martial sky is frequently overcast by
dense vapors or thick fogs in these regions, yet no real opaque clouds
were ever seen; the most prominent among the dusky spots being faintly
visible through the vapory veil, when they approached the centre of the
disk.
Besides these phenomena, which prove that Mars is surrounded by an
atmosphere having a great deal of similarity to our own, a further proof
is afforded by the fact that the dark spots, which appear sharply
defined and black when they are seen near the centre, become less and
less visible as they advance towards the limb, and are totally invisible
before they reach it. Moreover, the spectroscope also indicates the
existence of an atmosphere, and even the presence of watery vapor in it.
A very curious state of the Martial atmosphere is revealed by my
observations of 1877-78. During eight consecutive weeks, from December
12th to February 6th, a whole hemisphere of the planet--precisely that
represented on Plate VIII.--was completely covered by dense vapors, or a
thick fog which barely allowed the dark spots to be seen through it,
even when they were in the centre of the disk. The opposite hemisphere
of Mars appeared just as clear and calm as possible; there all the spots
and their minutest details could be seen, and when the planet was
observed at the proper time, the line separating the foggy from the
clear side was plainly visible.
The reddish tint observed on the continents of Mars has been supposed by
some astronomers to be the real color of the atmosphere of this planet.
But, for many reasons, this explanation is not acceptable. Besides the
fact that the border of the planet appears white, while it should be
more red than the other part, owing to the greater depth of atmosphere
there presented to us, the polar spots, the white bands along the
sea-coasts, and the cloud-like forms appear perfectly white, not the
slightest tint of red being visible on them, as would be the case if
these objects were seen through an atmosphere tinted red. Other
astronomers have supposed that the vegetation of this planet has a
reddish color; but this is not supported by observation. It has been
again supposed, with much more probability, that the surface of Mars is
composed of an ochreous material which gives the planet its predominant
ruddy color.
Until lately Mars was supposed to be without a satellite, but in August,
1877, Professor Hall, of the Washington Observatory, made one of the
most remarkable discoveries of the time, and found two satellites
revolving around this planet. These satellites are among the smallest
known heavenly bodies, their diameter having been estimated at from 6 to
10 miles for the outer satellite, and from 10 to 40 miles for the inner
one.
The most extraordinary feature of these bodies is the proximity of the
inner satellite to the planet, and the consequent rapidity of its
motion. The distance of the inner satellite from the centre of Mars is
about 6,000 miles, and from surface to surface it is less than 4,000
miles, or a little more than the distance from New York to San
Francisco. The shortest period of revolution of any satellite previously
known, is that of the inner satellite of Saturn, which is a little more
than 22½ hours; but the inner satellite of Mars accomplishes its
revolution in 7h. 38m., or in 17 hours less than the period of rotation
of the planet upon its axis. The period of revolution of the outer
satellite is greater, of course, and equals 30h. 7m.
From this rapidity of motion of the inner satellite of Mars, a very
curious result follows, which at first sight may appear in contradiction
with the fact that this body has a direct motion, like that of all the
planets of the solar system, and moves around Mars from west to east.
While the outer satellite of this planet, in company with all the stars
and planets, rises in the east and sets in the west, the inner
satellite, on the contrary, rises in the west and sets in the east.
Since the period of rotation of Mars is greater than is the period of
revolution of this satellite, it necessarily follows that this last body
must constantly be gaining on the rotation, and, consequently, that the
satellite sets in the east and rises in the west, compassing the whole
heavens around Mars three times a day, passing through all its phases in
11 hours, each quarter of this singular Moon lasting less than 3 hours.
It has been shown above that Mars has many points of resemblance to the
Earth. It has an atmosphere constituted very nearly like ours; it has
fogs, clouds, rains, snows, and winds. It has water, or at least some
liquids resembling it; it has rivers, lakes, seas and oceans. It has
also islands, peninsulas, continents, mountains and valleys. It has two
Moons, which must create great and rapid tides in the waters of its seas
and oceans. It has its days and nights, its warm and cold seasons, and
very likely its vegetation, its prairies and forests, like the Earth. On
the other hand, its year and seasons are double those of the Earth, and
its distance from the Sun is greater.
Is this planet, which is certainly constituted very nearly like our
globe, and seems so nearly fitted for the wants of the human race,
inhabited by animals and intelligent beings?
To answer this question, either in the negative or in the affirmative,
would be to step out of the pure province of science, and enter the
boundless domain of speculation, since no observer has ever seen
anything indicating that animal life exists on Mars, or on any other
planet or satellite. So far as observation goes, Mars seems to be a
planet well suited to sustain animal life, and we may reason from
analogy that if animal life can exist at all outside of the Earth, Mars
must have its flora and fauna; it must have its fishes and birds, its
mammalia and men; although all these living beings must inevitably be
very different in appearance from their representatives on the Earth, as
can easily be imagined from the differences existing between the two
planets. Although all this is possible, and even very probable, yet it
must be remembered that we have not the slightest evidence that it is
so; and until we have acquired this evidence, we may only provisionally
accept this idea as a pleasing hypothesis, which, after all, may be
wrong and totally unfounded.
THE PLANET JUPITER
PLATE IX
Jupiter, the giant of the planetary world, is the fifth in order of
distance from the Sun, and is next to Mars, our ruddy neighbor. To the
naked eye, Jupiter appears as a very brilliant star, whose magnitude,
changing with the distance of this planet from the Earth, sometimes
approaches that of Venus, our bright morning and evening star.
The mean distance of Jupiter from the Sun is 475,000,000 miles, but
owing to the eccentricity of its orbit, its distance varies from 452 to
498 millions of miles. The distance of this planet from the Earth varies
still more. When nearest to our globe, or in opposition, its distance is
reduced to 384,000,000 miles, and its apparent diameter increased to
50"; while when it is farthest, or in conjunction, its distance is
increased to 567,000,000 miles, and its apparent diameter reduced to
30"; Jupiter being thus 183,000,000 miles nearer our globe while in
opposition than when it is in conjunction.
This planet revolves around the Sun in 11 years, 10 months and 17 days,
or in only 50 days less than 12 terrestrial years. Such is the year of
this planet. The plane of its orbit is inclined 1° 19' to the ecliptic.
No planet, except Uranus, has an orbit exhibiting a smaller inclination.
The planet advances in its orbit at the mean rate of 8 miles a second;
which is a little less than half the orbital velocity of the Earth.
Jupiter is of enormous proportions. Its equatorial diameter measures
88,000 miles, and its circumference no less than 276,460 miles, these
dimensions being 11 times greater than those of the Earth. This planet,
notwithstanding its huge size, rotates on its axis in not far from 9h.
55m. 36s., which period constitutes its day. Owing, however, to the
changeable appearance of its surface, this period cannot be ascertained
with very great exactitude. In consequence of its rapid rotation, the
planet is far from spherical, its polar diameter being shorter than the
equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124
times the surface of the earth; while its volume is 1,387 times as
great. If Jupiter occupied the place of our satellite in the sky, it
would appear 40 times as large as the Moon appears to us, and would
cover a surface of the heavens 1,600 times that covered by the full
Moon, and would subtend an angle of 21°. Jupiter's mass does not
correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the
mass of the Sun, and 310 times the mass of the earth; its density being
only ¼ of that of our globe. The force of gravitation at the surface of
this planet is over 2½ times what it is on the Earth, so that a
terrestrial object carried to the surface of Jupiter would weigh over
two and a half times as much as on our globe.
Observed with a telescope, even of moderate aperture, Jupiter, with its
four attending satellites and its dazzling brilliancy, appears as one of
the most magnificent objects in the sky. The general appearance of the
disk is white; but unlike that of Mars, it is brightest towards its
central parts, and a little darker around the limb, especially on the
side opposite to the Sun. Although an exterior planet, and so far from
us, Jupiter shows faint traces of phases when observed near its
quadratures, but this gibbosity of its disk is very slight, and is
indicated only by a kind of penumbral shadow on the limb.
When observed with adequate power, the disk of Jupiter is found to be
highly diversified. The principal features consist of a series of
alternate light and dark streaks or bands, disposed most of the time
parallel with the Jovian equator. These bands differ from each other in
intensity as well as in breadth; those near the equator being usually
much more prominent than those situated in higher latitudes north and
south.
The equatorial zone of Jupiter is occupied most of the time by a broad,
prominent belt 20° or 30° wide, limited on each side by a very dark
narrow streak. Between these two dark borders, but seldom occupying the
whole space between them, appears an irregular white belt, apparently
composed of dense masses of clouds strongly reflecting the Sun's light,
some of these cloudy masses being very brilliant. The spaces left
between the cloudy belt and the dark borders, usually exhibit a delicate
pink or rosy color, which produces a very harmonious effect with the
varying grayish and bluish shades of some of the belts and streaks seen
on the disk. Quite often the cloudy belt is broken up, and consists of
independent cloudy masses, separated by larger or smaller intervals,
these intervals disclosing the rosy background of this zone.
On each side of the equatorial belt there is usually a broad whitish
belt, succeeded by a narrow gray band; the space left on each hemisphere
between these last bands and the limb being usually occupied by two or
three alternate white and gray bands. A uniform gray segment usually
forms a sort of polar cap to Jupiter.
When observed under very favorable conditions, all the lighter belts
appear as if composed of masses of small cloudlets, resembling the white
opaque clouds seen in our atmosphere. This, as already stated, is
particularly noticeable on the equatorial belt. It is not unusual, when
Jupiter is in quadrature, to see some of the most conspicuous white
spots casting a shadow opposite to the Sun; a fact which sufficiently
indicates that these spots are at different levels. They probably form
the summits of vast banks of clouds floating high up in the atmosphere
of Jupiter.
What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our
sight penetrates anywhere deeper into the interior, it can only be
through the narrow fissures of this envelope, which appear as gray or
dark streaks or spots. That most of the visible surface of Jupiter is
simply a cloudy covering, is abundantly proved by the proper motion of
its spots, which sometimes becomes very great.
In periods of calm, very few changes are noticeable in the markings of
the planet, except, perhaps, some slight modifications of form in the
cloudy, equatorial belt which, in general, is much more liable to
changes than the other belts. But the Jovian surface is not always so
tranquil, great changes being observed during the terrific storms which
sometimes occur on this mighty planet, when all becomes disorder and
confusion on its usually calm surface; and nothing on the Earth can give
us a conception of the velocity with which some of its clouds and spots
are animated. New belts quickly form, while old ones disappear. The
usual parallelism of the belts no more exists. Huge, white, cumulus-like
masses advance and spread out, the rosy equatorial belt enlarges
sometimes to two or three times its usual size, and occupies two-thirds,
or more, of the disk, the rosy tint spreading out in a very short time.
At times very dark bands extending across the disk are transformed into
knots or dark spots, which encircle the planet with a belt, as it were,
of jet black beads. Sometimes, also, a secondary but narrower rosy belt
forms either in the northern or the southern hemisphere, and remains
visible for a few days or for years at a time.
On May 25, 1876, I witnessed one of the grandest commotions which can be
conceived as taking place in an atmosphere. All the southern hemisphere
of Jupiter, from equator to pole, was in rapid motion, the belts and
spots being transported entirely across the disk, from the eastern to
the western limb, in one hour's time, during which the equatorial belt
swelled to twice its original breadth, towards the south.
Now, when one stops for a moment to think what is signified by that
motion of the dark spots across the little telescopic disk of Jupiter in
an hour's time, he may arrive at some conception of the magnitude of the
Jovian storms, compared with those of our globe. The circumference of
Jupiter's equator, as stated above, is 276,460 miles; half this number,
or 138,230 miles, represents the length of the equatorial line seen from
the Earth. Now, after taking into account the rotation of the planet,
which somewhat diminishes the apparent motion, we arrive at the
astonishing result that the spots and markings were carried along by
this Jovian storm, at the enormous rate of 110,584 miles an hour, or
over 30.7 miles a second. On our globe, a hurricane or tornado, which
blows at the rate of 100 miles an hour, sweeps everything before it.
What, then, must be expected from a velocity over 1,105 times as great?
Enormous as this motion may appear, its occurrence cannot be doubted,
since it is disclosed by direct observation.
The surface of Jupiter, it would seem, has its periods of calm and
activity like that of the Sun, although it is not yet known, as it is
for the latter, that they recur with approximate regularity.
My observations of this planet, which embrace a period of ten years,
seem to point in that direction, for they show, at least, that Jupiter
has its years of calm and its years of disturbances. The year 1876 was a
year of extraordinary disturbance on Jupiter. Changes in the markings
were going on all the time, and no one form could be recognized the next
day, or even sometimes the next hour, as shown above. The cloudy
envelope of the planet was in constant motion, the equatorial belt,
especially, showing the signs of greatest disturbance, being, for the
most part, two or three times as wide as in other years. After 1876 the
calm was very great on the planet, only a slight change now and then
being noticeable, the same forms being recognized day after day, month
after month, and even year after year. In one case the same marking has
been observed for seventeen consecutive months, and in another for
twenty-eight months. This state of quietude lasted until October, 1880,
when considerable commotion occurred on the northern hemisphere, where
large round black spots, somewhat resembling the Sun-spots, formed in
the cloudy atmosphere, and finally changed, towards the end of December,
into a narrow pink belt, which still exists.
[PLATE IX.--THE PLANET JUPITER.
Observed November 1, 1880, at 9h. 30m. P.M.]