JUPITER, THE GREATEST OF KNOWN WORLDS
When we are thinking of worlds, and trying to exalt the imagination with
them, it is well to turn to Jupiter, for there is a planet worth
pondering upon! A world thirteen hundred times as voluminous as the
earth is a phenomenon calculated to make us feel somewhat as the
inhabitant of a rural village does when his amazed vision ranges across
the million roofs of a metropolis. Jupiter is the first of the outer and
greater planets, the major, or Jovian, group. His mean diameter is
86,500 miles, and his average girth more than 270,000 miles. An
inhabitant of Jupiter, in making a trip around his planet, along any
great circle of the sphere, would have to travel more than 30,000 miles
farther than the distance between the earth and the moon. The polar
compression of Jupiter, owing to his rapid rotation, amounts in the
aggregate to more than 5,000 miles, the equatorial diameter being 88,200
miles and the polar diameter 83,000 miles.
Jupiter's mean distance from the sun is 483,000,000 miles, and the
eccentricity of his orbit is sufficient to make this distance variable
to the extent of 21,000,000 miles; but, in view of his great average
distance, the consequent variation in the amount of solar light and heat
received by the planet is not of serious importance.
When he is in opposition to the sun as seen from the earth Jupiter's
mean distance from us is about 390,000,000 miles. His year, or period of
revolution about the sun, is somewhat less than twelve of our years
(11.86 years). His axis is very nearly upright to the plane of his
orbit, so that, as upon Venus, there is practically no variation of
seasons. Gigantic though he is in dimensions, Jupiter is the swiftest of
all the planets in axial rotation. While the earth requires twenty-four
hours to make a complete turn, Jupiter takes less than ten hours (nine
hours fifty-five minutes), and a point on his equator moves, in
consequence of axial rotation, between 27,000 and 28,000 miles in an
hour.
The density of the mighty planet is slight, only about one quarter of
the mean density of the earth and virtually the same as that of the sun.
This fact at once calls attention to a contrast between Jupiter and our
globe that is even more significant than their immense difference in
size. The force of gravity upon Jupiter's surface is more than two and a
half times greater than upon the earth's surface (more accurately 2.65
times), so that a hundred-pound weight removed from the planet on which
we live to Jupiter would there weigh 265 pounds, and an average man,
similarly transported, would be oppressed with a weight of at least 400
pounds. But, as a result of the rapid rotation of the great planet, and
the ellipticity of its figure, the unfortunate visitor could find a
perceptible relief from his troublesome weight by seeking the planet's
equator, where the centrifugal tendency would remove about twenty pounds
from every one hundred as compared with his weight at the poles.
If we could go to the moon, or to Mercury, Venus, or Mars, we may be
certain that upon reaching any of those globes we should find ourselves
upon a solid surface, probably composed of rock not unlike the rocky
crust of the earth; but with Jupiter the case would evidently be very
different. As already remarked, the mean density of that planet is only
one quarter of the earth's density, or only one third greater than the
density of water. Consequently the visitor, in attempting to set foot
upon Jupiter, might find no solid supporting surface, but would be in a
situation as embarrassing as that of Milton's Satan when he undertook to
cross the domain of Chaos:
"Fluttering his pinions vain, plumb down he drops,
Ten thousand fathom deep, and to this hour
Down had been falling had not, by ill chance,
The strong rebuff of some tumultuous cloud.
Instinct with fire and niter, hurried him
As many miles aloft; that fury stayed,
Quenched in a boggy Syrtis, neither sea
Nor good dry land, nigh foundered, as he fares,
Treading the crude consistence, half on foot,
Half flying."
The probability that nothing resembling a solid crust, nor, perhaps,
even a liquid shell, would be found at the visible surface of Jupiter,
is increased by considering that the surface density must be much less
than the mean density of the planet taken as a whole, and since the
latter but little exceeds the density of water, it is likely that at the
surface everything is in a state resembling that of cloud or smoke. Our
imaginary visitor upon reaching Jupiter would, under the influence of
the planet's strong force of gravity, drop out of sight, with the speed
of a shot, swallowed up in the vast atmosphere of probably hot, and
perhaps partially incandescent, gases. When he had sunk--supposing his
identity could be preserved--to a depth of thousands of miles he might
not yet have found any solid part of the planet; and, perchance, there
is no solid nucleus even at the very center.
The cloudy aspect of Jupiter immediately strikes the telescopic
observer. The huge planet is filled with color, and with the animation
of constant movement, but there is no appearance of markings, like those
on Mars, recalling the look of the earth. There are no white polar caps,
and no shadings that suggest the outlines of continents and oceans. What
every observer, even with the smallest telescope, perceives at once is a
pair of strongly defined dark belts, one on either side of, and both
parallel to, the planet's equator. These belts are dark compared with
the equatorial band between them and with the general surface of the
planet toward the north and the south, but they are not of a gray or
neutral shade. On the contrary, they show decided, and, at times,
brilliant colors, usually of a reddish tone. More delicate tints,
sometimes a fine pink, salmon, or even light green, are occasionally to
be seen about the equatorial zone, and the borders of the belts, while
near the poles the surface is shadowed with bluish gray, imperceptibly
deepening from the lighter hues of the equator.
All this variety of tone and color makes of a telescopic view of Jupiter
a picture that will not quickly fade from the memory; while if an
instrument of considerable power is used, so that the wonderful details
of the belts, with their scalloped edges, their diagonal filaments,
their many divisions, and their curious light and dark spots, are made
plain, the observer is deeply impressed with the strangeness of the
spectacle, and the more so as he reflects upon the enormous real
magnitude of that which is spread before his eye. The whole earth
flattened out would be but a small blotch on that gigantic disk!
Then, the visible rotation of the great Jovian globe, whose effects
become evident to a practised eye after but a few minutes' watching,
heightens the impression. And the presence of the four satellites, whose
motions in their orbits are also evident, through the change in their
positions, during the course of a single not prolonged observation,
adds its influence to the effectiveness of the scene. Indeed, color and
motion are so conspicuous in the immense spectacle presented by Jupiter
that they impart to it a powerful suggestion of life, which the mind
does not readily divest itself of when compelled to face the evidence
that Jupiter is as widely different from the earth, and as diametrically
opposed to lifelike conditions, as we comprehend them, as a planet
possibly could be.
The great belts lie in latitudes about corresponding to those in which
the trade-winds blow upon the earth, and it has often been suggested
that their existence indicates a similarity between the atmospheric
circulation of Jupiter and that of the world in which we live. No doubt
there are times when the earth, seen with a telescope from a distant
planet, would present a belted appearance somewhat resembling that of
Jupiter, but there would almost certainly be no similar display of
colors in the clouds, and the latter would exhibit no such persistence
in general form and position as characterizes those of Jupiter. Our
clouds are formed by the action of the sun, producing evaporation of
water; on Jupiter, whose mean distance from the sun is more than five
times as great as ours, the intensity of the solar rays is reduced to
less than one twenty-fifth part of their intensity on the earth, so that
the evaporation can not be equally active there, and the tendency to
form aerial currents and great systems of winds must be proportionally
slight. In brief, the clouds of Jupiter are probably of an entirely
different origin from that of terrestrial clouds, and rather resemble
the chaotic masses of vapor that enveloped the earth when it was still
in a seminebulous condition, and before its crust had formed.
Although the strongest features of the disk of Jupiter are the great
cloud belts, and the white or colored spots in the equatorial zone, yet
the telescope shows many markings north and south of the belts,
including a number of narrower and fainter belts, and small light or
dark spots. None of them is absolutely fixed in position with
reference to others. In other words, all of the spots, belts, and
markings shift their places to a perceptible extent, the changes being
generally very slow and regular, but occasionally quite rapid. The main
belts never entirely disappear, and never depart very far from their
mean positions with respect to the equator, but the smaller belts toward
the north and south are more or less evanescent. Round or oblong spots,
as distinguished from belts, are still more variable and transient. The
main belts themselves show great internal commotion, frequently
splitting up, through a considerable part of their length, and sometimes
apparently throwing out projections into the lighter equatorial zone,
which occasionally resemble bridges, diagonally spanning the broad space
between the belts.
[Illustration: JUPITER AS SEEN AT THE LICK OBSERVATORY IN 1889. THE
GREAT RED SPOT IS VISIBLE, TOGETHER WITH THE INDENTATION IN THE SOUTH
BELT.]
Perhaps the most puzzling phenomenon that has ever made its appearance
on Jupiter is the celebrated "great red spot," which was first noticed
in 1878, although it has since been shown to be probably identical with
a similar spot seen in 1869, and possibly with one noticed in 1857.
This spot, soon after its discovery in 1878, became a clearly defined
red oval, lying near the southern edge of the south belt in latitude
about 30°. Its length was nearly one third of the diameter of the disk
and its width almost one quarter as great as its length. Translated into
terrestrial measure, it was about 30,000 miles long and 7,000 miles
broad.
In 1879 it seemed to deepen in color until it became a truly wonderful
object, its redness of hue irresistibly suggesting the idea that it was
something hot and glowing. During the following years it underwent
various changes of appearance, now fading almost to invisibility and now
brightening again, but without ever completely vanishing, and it is
still (1901) faintly visible.
Nobody has yet suggested an altogether probable and acceptable theory as
to its nature. Some have said that it might be a part of the red-hot
crust of the planet elevated above the level of the clouds; others that
its appearance might be due to the clearing off of the clouds above a
heated region of the globe beneath, rendering the latter visible through
the opening; others that it was perhaps a mass of smoke and vapor
ejected from a gigantic volcano, or from the vents covering a broad area
of volcanic action; others that it might be a vast incandescent slag
floating upon the molten globe of the planet and visible through, or
above, the enveloping clouds; and others have thought that it could be
nothing but a cloud among clouds, differing, for unknown reasons, in
composition and cohesion from its surroundings. All of these hypotheses
except the last imply the existence, just beneath the visible cloud
shell, of a more or less stable and continuous surface, either solid or
liquid.
When the red spot began to lose distinctness a kind of veil seemed to be
drawn over it, as if light clouds, floating at a superior elevation, had
drifted across it. At times it has been reduced in this manner to a
faint oval ring, the rim remaining visible after the central part has
faded from sight.
One of the most remarkable phenomena connected with the mysterious spot
is a great bend, or scallop, in the southern edge of the south belt
adjacent to the spot. This looks as if it were produced by the spot, or
by the same cause to which the spot owes its existence. If the spot were
an immense mountainous elevation, and the belt a current of liquid, or
of clouds, flowing past its base, one would expect to see some such bend
in the stream. The visual evidence that the belt is driven, or forced,
away from the neighborhood of the spot seems complete. The appearance of
repulsion between them is very striking, and even when the spot fades
nearly to invisibility the curve remains equally distinct, so that in
using a telescope too small to reveal the spot itself one may discover
its location by observing the bow in the south belt. The suggestion of a
resemblance to the flowing of a stream past the foot of an elevated
promontory, or mountain, is strengthened by the fact, which was
observed early in the history of the spot, that markings involved in the
south belt have a quicker rate of rotation about the planet's axis than
that of the red spot, so that such markings, first seen in the rear of
the red spot, gradually overtake and pass it, and eventually leave it
behind, as boats in a river drift past a rock lying in the midst of the
current.
This leads us to another significant fact concerning the peculiar
condition of Jupiter's surface. Not only does the south belt move
perceptibly faster than the red spot, but, generally speaking, the
various markings on the surface of the planet move at different rates
according as they are nearer to or farther from the equator. Between the
equator and latitude 30° or 40° there is a difference of six minutes in
the rotation period--i.e., the equatorial parts turn round the axis so
much faster than the parts north and south of them, that in one rotation
they gain six minutes of time. In other words, the clouds over Jupiter's
equator flow past those in the middle latitudes with a relative
velocity of 270 miles per hour. But there are no sharp lines of
separation between the different velocities; on the contrary, the
swiftness of rotation gradually diminishes from the equator toward the
poles, as it manifestly could not do if the visible surface of Jupiter
were solid.
In this respect Jupiter resembles the sun, whose surface also has
different rates of rotation diminishing from the equator. Measured by
the motion of spots on or near the equator, Jupiter's rotation period is
about nine hours fifty minutes; measured by the motion of spots in the
middle latitudes, it is about nine hours fifty-six minutes. The red spot
completes a rotation in a little less than nine hours and fifty-six
minutes, but its period can not be positively given for the singular
reason that it is variable. The variation amounts to only a few seconds
in the course of several years, but it is nevertheless certain. The
phenomenon of variable motion is not, however, peculiar to the red spot.
Mr. W.F. Denning, who has studied Jupiter for a quarter of a century,
says:
"It is well known that in different latitudes of Jupiter there are
currents, forming the belts and zones, moving at various rates of speed.
In many instances the velocity changes from year to year. And it is a
singular circumstance that in the same current a uniform motion is not
maintained in all parts of the circumference. Certain spots move faster
than others, so that if we would obtain a fair value for the rotation
period of any current it is not sufficient to derive it from one marking
alone; we must follow a number of objects distributed in different
longitudes along the current and deduce a mean from the whole."[10]
[Footnote 10: The Observatory, No. 286, December, 1899.]
Nor is this all. Observation indicates that if we could look at a
vertical section of Jupiter's atmosphere we should behold an equally
remarkable contrast and conflict of motions. There is evidence that some
of the visible spots, or clouds, lie at a greater elevation than others,
and it has been observed that the deeper ones move more rapidly. This
fact has led some observers to conclude that the deep-lying spots may be
a part of the actual surface of the planet. But if we could think that
there is any solid nucleus, or core, in the body of Jupiter, it would
seem, on account of the slight mean density of the planet, that it can
not lie so near the visible surface, but must be at a depth of
thousands, perhaps tens of thousands, of miles. Since the telescope is
unable to penetrate the cloudy envelope we can only guess at the actual
constitution of the interior of Jupiter's globe. In a spirit of mere
speculative curiosity it has been suggested that deep under the clouds
of the great planet there may be a comparatively small solid globe, even
a habitable world, closed round by a firmament all its own, whose vault,
raised 30,000 or 40,000 miles above the surface of the imprisoned
planet, appears only an unbroken dome, too distant to reveal its real
nature to watchers below, except, perhaps, under telescopic scrutiny;
enclosing, as in a shell, a transparent atmosphere, and deriving its
illumination partly from the sunlight that may filter through, but
mainly from some luminous source within.
But is not Jupiter almost equally fascinating to the imagination, if we
dismiss all attempts to picture a humanly impossible world shut up
within it, and turn rather to consider what its future may be, guided by
the not unreasonable hypothesis that, because of its immense size and
mass, it is still in a chaotic condition? Mention has been made of the
resemblance of Jupiter to the sun by virtue of their similar manner of
rotation. This is not the only reason for looking upon Jupiter as being,
in some respects, almost as much a solar as a planetary body. Its
exceptional brightness rather favors the view that a small part of the
light by which it shines comes from its own incandescence. In size and
mass it is half-way between the earth and the sun. Jupiter is eleven
times greater than the earth in diameter and thirteen hundred times
greater in volume; the sun is ten times greater than Jupiter in diameter
and a thousand times greater in volume. The mean density of Jupiter, as
we have seen, is almost exactly the same as the sun's.
Now, the history of the solar system, according to the nebular
hypothesis, is a history of cooling and condensation. The sun, a
thousand times larger than Jupiter, has not yet sufficiently cooled and
contracted to become incrusted, except with a shell of incandescent
metallic clouds; Jupiter, a thousand times smaller than the sun, has
cooled and contracted until it is but slightly, if at all, incandescent
at its surface, while its thickening shell, although still composed of
vapor and smoke, and still probably hot, has grown so dense that it
entirely cuts off the luminous radiation from within; the earth, to
carry the comparison one step further, being more than a thousand times
smaller than Jupiter, has progressed so far in the process of cooling
that its original shell of vapor has given place to one of solid rock.
A sudden outburst of light from Jupiter, such as occurs occasionally in
a star that is losing its radiance through the condensation of
absorbing vapors around it, would furnish strong corroboration of the
theory that Jupiter is really an extinguished sun which is now on the
way to become a planet in the terrestrial sense.
Not very long ago, as time is reckoned in astronomy, our sun, viewed
from the distance of the nearer fixed stars, may have appeared as a
binary star, the brighter component of the pair being the sun itself and
the fainter one the body now called the planet Jupiter. Supposing the
latter to have had the same intrinsic brilliance, surface for surface,
as the sun, it would have radiated one hundred times less light than the
sun. A difference of one hundredfold between the light of two stars
means that they are six magnitudes apart; or, in other words, from a
point in space where the sun appeared as bright as what we call a
first-magnitude star, its companion, Jupiter, would have shone as a
sixth-magnitude star. Many stars have companions proportionally much
fainter than that. The companion of Sirius, for instance, is at least
ten thousand times less bright than its great comrade.
Looking at Jupiter in this way, it interests us not as the probable
abode of intelligent life, but as a world in the making, a world,
moreover, which, when it is completed--if it ever shall be after the
terrestrial pattern--will dwarf our globe into insignificance. That
stupendous miracle of world-making which is dimly painted in the grand
figures employed by the writers of Genesis, and the composers of other
cosmogonic legends, is here actually going on before our eyes. The
telescope shows us in the cloudy face of Jupiter the moving of the
spirit upon the face of the great deep. What the final result will be we
can not tell, but clearly the end of the grand processes there in
operation has not yet been reached.
The interesting suggestion was made and urged by Mr. Proctor that if
Jupiter itself is in no condition at present to bear life, its
satellites may be, in that respect, more happily circumstanced. It can
not be said that very much has been learned about the satellites of
Jupiter since Proctor's day, and his suggestion is no less and no more
probable now than it was when first offered.
There has been cumulative evidence that Jupiter's satellites obey the
same law that governs the rotation of our moon, viz., that which compels
them always to keep the same face turned toward their primary, and this
would clearly affect, although it might not preclude, their
habitability. With the exception of the minute fifth satellite
discovered by Barnard in 1892, they are all of sufficient size to retain
at least some traces of an atmosphere. In fact, one of them is larger
than the planet Mars, and another is of nearly the same size as that
planet, while the smallest of the four principal ones is about equal to
our moon. Under the powerful attraction of Jupiter they travel rapidly,
and viewed from the surface of that planet they would offer a wonderful
spectacle.
They are continually causing solar eclipses and themselves undergoing
eclipse in Jupiter's shadow, and their swiftly changing aspects and
groupings would be watched by an astronomer on Jupiter with undying
interest.
But far more wonderful would be the spectacle presented by Jupiter to
inhabitants dwelling on his moons. From the nearer moon, in particular,
which is situated less than 220,000 miles from Jupiter's surface, the
great planet would be an overwhelming phenomenon in the sky.
Its immense disk, hanging overhead, would cover a circle of the
firmament twenty degrees in diameter, or, in round numbers, forty times
the diameter of the full moon as seen from the earth! It would shed a
great amount of light and heat, and thus would more or less effectively
supply the deficit of solar radiation, for we must remember that Jupiter
and his satellites receive from the sun less than one twenty-fifth as
much light and heat as the earth receives.
The maze of contending motions, the rapid flow and eddying of cloud
belts, the outburst of strange fiery spots, the display of rich, varied,
and constantly changing colors, which astonish and delight the
telescopic observer on the earth, would be exhibited to the naked eye of
an inhabitant of Jupiter's nearest moon far more clearly than the
greatest telescope is able to reveal them to us.
Here, again, the mind is carried back to long past ages in the history
of the planet on which we dwell. It is believed by some that our moon
may have contained inhabitants when the earth was still hot and glowing,
as Jupiter appears to be now, and that, as the earth cooled and became
habitable, the moon gradually parted with its atmosphere and water so
that its living races perished almost coincidently with the beginning of
life on the earth. If we accept this view and apply it to the case of
Jupiter we may conclude that when that enormous globe has cooled and
settled down to a possibly habitable condition, its four attendant moons
will suffer the fate that overtook the earth's satellite, and in their
turn become barren and death-stricken, while the great orb that once
nurtured them with its light and heat receives the Promethean fire and
begins to bloom with life.