THE MAJOR PLANETS
It is a striking change to pass from Ceres, the giant of the minor
planets, to Jupiter, the giant of the major planets. Instead of a world
that the Earth exceeds in volume 5000 times, we are confronted by one that
exceeds the Earth 1400 times. Ceres, when viewed through a large
telescope, is just able to present a perceptible disc; Jupiter offers the
largest shown by any heavenly body after the Sun and Moon.
And that disc is one that never fails to charm the attentive student, for
it abounds in colour, movement and change. The late Prof. James Keeler, an
observer of the first rank, having the advantage of observing the planet
from the summit of Mt. Hamilton and with the great 36-inch telescope of
the Lick Observatory, thus describes the aspect of the planet in 1889.
"Seen with this instrument on a fine night, the disc of Jupiter was a
most beautiful object, covered with a wealth of detail which could not
possibly be accurately represented in a drawing.... Scarcely any
portion of Jupiter, except the Red Spot and the extreme polar regions,
was of a uniform tint, the surface being mottled with flocculent and
more or less irregular cloud masses.... The equatorial zone, occupying
the space between the red belts, was marked in the centre by a
salmon-coloured stripe, which was occasionally interrupted by an
extension of the white clouds on the sides of the zone. The edges were
brilliant white, and were formed of rounded cloud-like masses, which
at certain places extended into the red belts as long streamers....
Near their junction with the equatorial zone, the streamers were white
and definite in outline, but they became redder in tint toward their
outer extremities, and more diffuse, until they were lost in the
general red colour of the background. When the seeing was good they
were seen to be formed of irregular rounded or feathery clouds, fading
toward the outer ends, until the structure could no longer be
distinguished.... The portions of the equatorial zone surrounding the
roots of well-marked streamers were somewhat brighter than at other
places, and it is a curious circumstance that they were almost
invariably suffused with a pale olive-green colour, which seemed to be
associated with great disturbance, and which was rarely seen
elsewhere.... The red belts presented on all occasions the appearance
of a passive medium, in which the phenomena of the streamers and other
forms ... were manifested. The phenomena would be exactly reproduced
by streamers of cloudy white matter floating in a semi-transparent
reddish fluid, sometimes submerged and sometimes rising to the
surface.... The dark spots frequently seen on the red belts usually
occupied spaces left by sharp turns in the streamers, and they were of
the same colour as the belts, but deeper in tint, as if the fluid
medium could be seen to a greater depth."[15]
In other words, Jupiter is a striped or banded planet, the bands lying
along the direction of turning. These bands are coloured in varying tints,
and the planet rotates very rapidly, for the details in the bands pass
quickly from one limb to the other. And not only is the speed of rotation
of the whole very rapid--Jupiter turns about its axis in a little less
than ten hours, so that a particle at its equator moves through 466 miles
in each minute--but the various items that form the bands rotate in
different times. They may also alter their form and their colour. Jupiter
seems, then, to be a planet with a great and rapidly changing atmosphere
that extends above a shoreless sea formed of some liquified substance or
substances--the whole in a state of flux.
But if we turn back to the Table, we see that Jupiter at its mean distance
from the Sun is 5.2 times that of the Earth; that is to say, it receives
only 1/27th of the light and heat that we receive. But in Chapter VIII, we
learnt from Mars that as this receives only 3/7ths of the Earth's light
and heat, its mean temperature would sink to -30 deg.C.; the Earth's being
16 deg.C. Mars is therefore almost always a frozen planet; frozen except on
its mere surface when this is exposed to the full rays of the Sun. No sea
there would ever be melted to a depth of more than a few inches, even at
noonday in midsummer. And yet Mars has at least ten times the advantages
of Jupiter. Jupiter, then, must be a frozen planet through and through; no
liquid of any sort can exist on its surface; no vapour of any substance
can exist in its atmosphere. It must be icebound even at its summer
noonday.
Yet, from the description given by Prof. Keeler, it is manifestly not so;
and another item in the Table emphasizes that it cannot be so. The density
of the Sun is 1.4 that of water, Jupiter's is 1.33, showing that but a
very small proportion (if any) of its bulk can be solid; the rest must be
vaporous, or at least fluid. How then can we reconcile these
inconsistencies?
It is in the dimensions of Jupiter that we find the answer. The mass of
the planet is 317 times that of the Earth; it is indeed nearly three times
as great as that of all the other planets put together. But the
aggregation of so vast an amount of material is of itself a source of
heat; the chief source at the present time of the enormous output of heat
from the Sun is ascribed to its gradual contraction; the slow falling of
its substance, if we may so express it, a little nearer to its centre. The
great mass of Jupiter points to its inherent store of heat being much
greater than that of any other planet. And of two bodies equally hot, the
larger must cool more slowly than the smaller. If, therefore, all the
members of the solar system had at one and the same moment possessed the
same surface temperature, that equality would have ceased directly they
began to radiate their heat into space; the temperature of the smaller
bodies falling more rapidly than those of the larger. This is another
example of the principle that has already been noted, that the properties
of a small world are not those of a large world divided by a constant
factor. It is not possible to conceive a model of the solar system in
which all the significant factors should be true to the same scale. If the
diameters and distances were all made on a one-tenth scale, the surfaces
would be one-hundredth of reality, the volumes one-thousandth.
But a radiating body radiates from its surface, while the store of heat
from which that radiation is kept up is supplied by its volume. It
follows, therefore, that a large and heavy world must differ from a small
light world, not merely in scale, but also in kind.
The surface of a world is all that we see of it; it is, therefore, very
commonly all that we consider. But unseen, and hence often unconsidered,
beneath the surface lies its substance or mass, and it is this that
determines the state and condition of the surface; it is the underlying
power. Two men may be contending in a financial struggle; to the eye they
may look alike, equally prosperous; both may have the same amount of money
actually in their pockets; but the one has nothing else, the other has a
great banking account and vast investments, and is, in fact, a
millionaire; and it is his unseen power and resources that will make
themselves felt.
Jupiter therefore introduces us to a new factor in world-condition; not
all its heat is derived from the Sun; much is inherent to it. And though
it is not possible at present to say that the mass of Jupiter being so
much its inherent heat must be this or that quantity as a function of that
mass, yet in general, and neglecting other considerations, we can say that
of two worlds the one with the greater mass will be that with the higher
inherent temperature. This factor of inherent temperature was one that did
not require to be noticed in dealing with the Moon, or Venus, or Mars, for
these and all the planets yet noticed are less in size, surface, volume,
and mass than the Earth, and hence possess less inherent heat. It is only
now that the greater planets are being considered that the question of a
source of heat, other than the Sun, can arise.
But the evidence of such heat on Jupiter is not to be disputed. The albedo
or reflective index of Jupiter has been put by the late Prof. G. Bond, of
Harvard College Observatory, as higher than unity; in other words, that it
emits more light than it receives. This is now generally regarded as an
excessive estimate, but the albedo of the disc as a whole cannot be put
lower than 0.72, or about that of white paper. But many of the "belts" or
dark regions are of a dull copper tint, and the polar caps are dusky, so
that Bond's estimate must be realized for the most brilliant "zones," as
the brighter regions are called; certainly for the whitest of the white
spots.
No direct evidence of inherent luminosity has been obtained, for the
satellites disappear entirely in eclipse. But though their shadows in
transit appear very dark, it is clear that they are not absolutely black,
since sometimes such a shadow is not distinguishable in darkness from the
satellite that casts it; a delicate proof that the background on which it
falls has some intrinsic luminosity.
Unless there is the counteracting effect of a high temperature, the
atmosphere of Jupiter would have a pressure at the surface of 104 lb. to
the square inch, and the level of half pressure be attained at a mile and
a quarter; the reverse condition to that on Mars would obtain, and the
atmosphere of Jupiter would be much denser and much shallower than that of
the Earth. Denser it probably is; shallower it cannot be, for the great
white spots, each often five or six thousand miles in diameter, that range
themselves at times along the equatorial regions till they look like the
portholes of a ship, evidently rise from depths great even as compared
with their size. But it is only by intense heat that the effect of the
great mass of Jupiter in constricting its atmosphere within shallow depths
can be overcome.
Again, the extraordinary lightness of the planet, so little above the
density of water, points in the same direction. So, not less unmistakably,
do the magnitude and rapidity of the atmospheric movements. The clouds and
storms of our own atmosphere are worked by solar heat; solar heat it is
that draws up the vapours and provides the chief part of the energy
manifested in the speed and strength of the air-current. But solar heat
can only give 1/27th the amount of that energy at the distance of Jupiter,
so that, if they were entirely dependent on solar radiation, the winds of
Jupiter should be very feeble.
Further, the difference of presentment due to the difference of latitude
is a fruitful cause of inequalities of temperature and pressure in the
terrestrial atmosphere. But as a degree of latitude on Jupiter is eleven
times as wide as on the Earth, such inequalities connected with a given
difference in latitude are spread over eleven times the distance that they
would be on the Earth, and are, therefore, so much the less pronounced.
Yet, across a gulf of 400 millions of miles we can clearly discern the
bright zones of Jupiter now narrowing down and constricting the red belts,
now thrust apart by them, and can detect changes taking place in an hour
of time over areas equal to that of a terrestrial hemisphere.
A notable peculiarity of Jupiter is found in the proper motions of its
spots. Many of the white spots are exceedingly swift, giving a rotation
period of 9h. 50m. while the equatorial belt in general gives a period 5m.
longer; so that in 119 rotations (nearly 49 days) a white spot will have
passed entirely round the belt, gaining upon it at a rate of nearly 240
miles an hour.
The most famous of all the markings in Jupiter is the Great Red Spot,
which became conspicuous in 1878, since when the spot itself, or at least
the nest in which it lay, has always been visible. It has been identified
with a great red spot observed by Hooke and Cassini in 1664-6, that
appeared and vanished again eight times between 1665 and 1708. It
therefore has had a history practically as long as our telescopic
knowledge of the planet, and may be looked upon as in some sort a
permanent feature. Yet that it is not in the nature of a portion of a
solid crust is clear. It occupies on Jupiter much the position and
relative area of Australia on the Earth, but whereas Australia of
necessity rotates in one piece with all the other continents, the Great
Red Spot has a rotation period which is neither that of the equatorial
belt, nor of the quickly moving white spots, and is not itself stable. An
"Australia on the loose" is impossible, even unthinkable here, but the
Great Red Spot, for all its long duration, is mobile and inconstant, and
is therefore no portion of a solid permanent crust.
The giant planet Jupiter, therefore, offers us an example of what we may
call a "semi-sun"; a world still bubbling with tremendous energies of its
own, still pulsing with its own inherent heat, still without a solid
crust; probably without a solid nucleus, liquid or vaporous throughout.
Whatever the future may hold for such an orb, it is clearly no world for
habitation at present. Full of colour, and movement, and change as it is,
it lacks the Earth's "gloom of iron substance," which is necessary, no
less than its veiling by the plant, as a stage for "the passion and
perishing of mankind."
But if Jupiter be a semi-Sun, still a source of heat, perhaps even of
light, can it yield the means of life to its satellites? For Jupiter is
sun-like, not merely in its own condition, but also in that it is the
centre and ruler of a system of its own. We know already of eight
satellites revolving round it.
Of these eight, only four--the four discovered by Galileo, in the first
days of his possession of a telescope--need be considered; the other four
are of the same order of size as the asteroids, and are indeed much
smaller than Ceres.
But the Galilean satellites are of a higher rank. Europa, the smallest, is
in size a twin to the Moon; Callisto, the outermost, is almost exactly the
size of Mercury; Io, the innermost, is midway between the two in its
dimensions. But Ganymede, the largest, is almost comparable with Mars, its
diameter being 0.45 that of the Earth instead of the 0.53 of Mars.
But the Moon, Mercury, and Mars have all been shown, on the ground of
their small size, to be worlds unfit for habitation; the satellites of
Jupiter are, therefore, all rejected on the same score. Nor can the
greater nearness of their immediate primary compensate for their
remoteness from the Sun. It is true that Jupiter presents to Ganymede a
disc with more than 200 times the apparent area that the Sun presents to
the Earth, but to make up for the falling-off of the solar radiation, each
unit of this area should radiate about 1/250th as much heat as each unit
of the Sun's surface. In other words, the absolute surface temperature of
Jupiter should be 1/4th that of the Sun, or about 1550 deg. C., and this is
higher than can be admitted. The Sun and Jupiter together cannot put
Ganymede in as favourable a position as Mars, much less as favourable as
the Earth.
The case of Jupiter carries with it those of Saturn, Uranus, and Neptune.
All three, from their high albedoes and low densities, are still in a
vaporous condition; still in some sort, semi-Suns; sources of a certain
amount of heat, and not recipients merely. The days are yet far distant
when a solid crust can form on any one of them, and the water condense
from the steamy atmosphere to form oceans, seas, and rivers. Not till
then, if at all, when water as a liquid, water that flows, is present, can
life begin to appear and enter on its long course of change.