THE PLANETS.
1. When it was discovered, by Copernicus and Galileo, that Mercury,
Venus, Mars, Jupiter, Saturn, which had hitherto been regarded only as
"wandering fires, that move in mystic dance," were really, in many
circumstances, bodies resembling the Earth;--that they and the Earth
alike, were opaque globes, revolving about the Sun in orbits nearly
circular, revolving also about their own axes, and some of them
accompanied by their Satellites, as the Earth is by the Moon;--it was
inevitable that the conjecture should arise, that they too had
inhabitants, as the Earth has. Each of these bodies were seemingly
coherent and solid; furnished with an arrangement for producing day and
night, summer and winter; and might therefore, it was naturally
conceived, have inhabitants moving upon its solid surface, and reckoning
their lives and their employment by days, and months, and years. This
was an unavoidable guess. It was far less bold and sweeping than the
guess that there are inhabitants in the region of the Fixed Stars, but
still, like that, it was, for the time at least, only a guess; and like
that, it must depend upon future explorations of these bodies and their
conditions, whether the guess was confirmed or discredited. The
conjecture could not, by any moderately cautious man, be regarded as so
overwhelmingly probable, that it had no need of further proof. Its final
acceptance or rejection must depend on the subsequent progress of
astronomy, and of science in general.
2. We have to consider then how far subsequent discoveries have given
additional value to this conjecture. And, as, in the first place,
important among such discoveries, we must note the addition of several
new planets to our system. It was found, by the elder Herschel, (in
1781,) that, far beyond Saturn, there was another planet, which, for a
time, was called by the name of its sagacious discoverer; but more
recently, in order to conform the nomenclature of the planets to the
mythology with which they had been so long connected, has been termed
_Uranus_. This was a vast extension of the limits of the solar system.
The Earth is, as we have already said, nearly a hundred millions of
miles from the Sun. Jupiter is at more than five times, and Saturn
nearly at ten times this distance: but Uranus, it was found, describes
an orbit of which the radius is about nineteen times as great as that of
the Earth. But this did not terminate the extension of the solar system
which the progress of astronomy revealed. In 1846, a new planet, still
more remote, was discovered: its existence having been divined, before
it was seen, by two mathematicians, Mr. Adams, of Cambridge, and M.
Leverrier, of Paris, from the effects of its force upon Uranus. This new
planet was termed Neptune: its distance from the Sun is about thirty
times the Earth's distance. Besides these discoveries of large planets,
a great number of small planets were detected in the region of the solar
system which lies between the orbits of Mars and Jupiter. This series of
discoveries began on the first day of 1801, when Ceres was detected by
Piazzi at Palermo; and has gone on up to the present time, when
twenty-three of these small bodies have been brought to light; and
probably the group is not yet exhausted.
3. Now if we have to discuss the probability that all these bodies are
inhabited, we may begin with the outermost of them at present known,
namely Neptune. How far is it likely that this globe is occupied by
living creatures which enjoy, like the creatures on the Earth, the light
and heat of the Sun, about which the planet revolves? It is plain, in
the first place, that this light and heat must be very feeble. Since
Neptune is thirty times as far from the sun as the earth is, the
diameter of the sun as seen from Neptune will only be one-thirtieth as
large as it is, seen from the earth. It will, in fact, be reduced to a
mere star. It will be about the diameter under which Jupiter appears
when he is nearest to us. Of course its brightness will be much greater
than that of Jupiter; nearly as much indeed, as the sun is brighter than
the moon, both being nearly of the same size: but still, with our
full-moonlight reduced to the amount of illumination which we receive
from _a full Jupiter_, and our sun-light reduced in nearly the same
proportion, we should have but a dark, and also a cold world. In fact,
the light and the heat which reach Neptune, so far as they depend on the
distance of the sun, will each be about nine hundred times smaller than
they are on the earth. Now are we to conceive animals, with their vital
powers unfolded, and their vital enjoyments cherished, by this amount of
light and heat? Of course, we cannot say, with certainty, that any
feebleness of light and heat are inconsistent with the existence of
animal life: and if we had good reason to believe that Neptune is
inhabited by animals, we might try to conceive in what manner their
vital scheme is accommodated to this scanty supply of heat and light. If
it were certain that they were there, we might inquire how they could
live there, and what manner of creatures they could be. If there were
any general grounds for assuming inhabitants, we might consider what
modifications of life their particular conditions would require.
4. But is there any such general ground!? Such a ground we should have,
if we could venture to assume that _all_ the bodies of the Solar System
are inhabited;--if we could proceed upon such a principle, we might
reject or postpone the difficulties of particular cases.
5. But is such an assumption true? Is such a principle well founded? The
best chance which we have of learning whether it is so, is to endeavor
to ascertain the fact, in the body which is nearest to us; and thus, the
best placed for our closer scrutiny. This is, of course, the Moon; and
with regard to the Moon, we have, again, this advantage in beginning the
inquiry with her:--that she, at least, is in circumstances, as to light
and heat, so far as the Sun's distance affects them, which we know to be
quite consistent with animal and vegetable life. For her distance from
the Sun is not appreciably different from that of the Earth; her
revolutions round the earth do not make nearly so great a difference, in
her distance from the sun, as does the earth's different distances from
the sun in summer and in winter: the fact also being, that the earth is
considerably nearer to the sun in the winter of this our northern
hemisphere, than in the summer. The moon's distance from the sun then,
adapts her for habitation: is she inhabited?
6. The answer to this question, so far as we can answer it, may involve
something more than those mere astronomical conditions, her distance
from the sun, and the nature of her motions. But still, if we are
compelled to answer it in the negative;--if it appear, by strong
evidence, that the Moon is not inhabited; then is there an end of the
general principle, that, _all_ the bodies of the solar system are
inhabited, and that we must begin our speculations about each, with this
assumption. If the Moon be not inhabited, then, it would seem, the
belief that each special body in the system is inhabited, must depend
upon reasons specially belonging to that body; and cannot be taken for
granted without such reasons. Of the two bodies of the solar system
which alone we can examine closely, so as to know anything about them,
the Earth and the Moon, if the one be inhabited, and the other blank of
inhabitants, we have no right to assume at once, that any other body in
the solar system belongs to the former of these classes rather than to
the latter. If, even under terrestrial conditions of light and heat, we
have a total absence of the phenomenon of life, known to us only as a
terrestrial phenomenon; we are surely not entitled to assume that when
these conditions fail, we have still the phenomenon, life. We are not
entitled to _assume_ it; however it may be capable of being afterwards
proved, in any special case, by special reasons; a question afterwards
to be discussed.
7. Is, then, the Moon inhabited? From the moon's proximity to us, (she
is distant only thirty diameters of the earth, less than ten times the
earth's circumference; a railroad carriage, at its ordinary rate of
travelling, would reach her in a month,) she can be examined by the
astronomer with peculiar advantages. The present powers of the telescope
enable him to examine her mountains as distinctly as he could the Alps
at a few hundred miles distance, with the naked eye; with the additional
advantage that her mountains are much more brilliantly illuminated by
the Sun, and much more favorably placed for examination, than the Alps
are. He can map and model the inequalities of her surface, as faithfully
and exactly as he can those of the surface of Switzerland. He can trace
the streams that seem to have flowed from eruptive orifices over her
plains, as he can the streams of lava from the craters of Etna or Hecla.
8. Now, this minute examination of the Moon's surface being possible,
and having been made, by many careful and skilful astronomers, what is
the conviction which has been conveyed to their minds, with regard to
the fact of her being the seat of vegetable or animal life? Without
exception, it would seem, they have all been led to the belief, that the
Moon is not inhabited; that she is, so far as life and organization are
concerned, waste and barren, like the streams of lava or of volcanic
ashes on the earth, before any vestige of vegetation has been impressed
upon them: or like the sands of Africa, where no blade of grass finds
root. It is held, by such observers, that they can discern and examine
portions of the moon's surface as small as a square mile;[1] yet, in
their examination, they have never perceived any alteration, such as the
cycle of vegetable changes through the revolutions of seasons would
produce. Sir William Herschel did not doubt that if a change had taken
place on the visible part of the Moon, as great as the growth or the
destruction of a great city, as great, for instance, as the destruction
of London by the great fire of 1666, it would have been perceptible to
his powers of observation. Yet nothing of the kind has ever been
observed. If there were lunar astronomers, as well provided as
terrestrial ones are, with artificial helps of vision, they would
undoubtedly be able to perceive the differences which the progress of
generations brings about on the surface of our globe; the clearing of
the forests of Germany or North America; the embankment of Holland; the
change of the modes of culture which alter the color of the ground in
Europe; the establishment of great nests of manufactures which shroud
portions of the land in smoke, as those which have their centres at
Birmingham or at Manchester. However obscurely they might discern the
nature of those changes, they would still see that change was going on.
And so should we, if the like changes were going on upon the face of the
Moon. Yet no such changes have ever been noticed. Nor even have such
changes been remarked, as might occur in a mere brute mass without
life;--the formation of new streams of lava, new craters, new crevices,
new elevations. The Moon exhibits strong evidences, which strike all
telescopic observers, of an action resembling, in many respects,
volcanic action, by which its present surface has been formed.[2] But,
if it have been produced by such internal fires, the fires seem to be
extinguished; the volcanoes to be burned out. It is a mere cinder; a
collection of sheets of rigid slag, and inactive craters. And if the
Moon and the Earth were both, at first in a condition in which igneous
eruptions from their interior produced the ridges and cones which
roughen their surfaces; the Earth has had this state succeeded by a
series of states of life in innumerable forms, till at last it has
become the dwelling-place of man; while the Moon, smaller in dimensions,
has at an earlier period completely cooled down, as to its exterior at
least, without ever being judged fit or worthy by its Creator of being
the seat of life; and remains, hung in the sky, as an object on which
man may gaze, and perhaps, from which he may learn something of the
constitution of the universe; and among other lessons this; that he must
not take for granted, that all the other globes of the solar system are
tenanted, like that on which he has his appointed place.
9. It is true, that in coming to this conclusion, the astronomers of
whom I speak, have been governed by other reasons, besides those which I
have mentioned, the absence of any changes, either rapid or slow,
discoverable in the Moon's face. They have seen reason to believe that
water and air, elements so essential to terrestrial life, do not exist
in the Moon. The dark spaces on her disk, which were called _seas_ by
those who first depicted them, have an appearance inconsistent with
their being oceans of water. They are not level and smooth, as water
would be; nor uniform in their color, but marked with permanent streaks
and shades, implying a rigid form. And the absence of an atmosphere of
transparent vapor and air, surrounding the moon, as our atmosphere
surrounds the earth, is still more clearly proved, by the absence of all
the optical effects of such an atmosphere, when stars pass behind the
moon's disk, and by the phenomena which are seen in solar eclipses, when
her solid mass is masked by the Sun.[3] This absence of moisture and air
in the Moon, of course, entirely confirms our previous conclusion, of
the absence of vegetable and animal life; and leaves us, as we have
said, to examine the question for the other bodies, on their special
grounds, without any previous presumption that such life exists.
Undoubtedly the aspect of the case will be different in one feature,
when we see reason to believe that other bodies have an atmosphere; and
if there be in any planet sufficient light and heat, and clouds and
winds, and a due adjustment of the power of gravity, and the strength of
the materials of which organized frames consist, there may be, so far as
we can judge, life of some kind or other. But yet, even in those cases,
we should be led to judge also, by analogy, that the life which they
sustain is more different from the terrestrial life of the present
period of the earth, than that is from the terrestrial life of any
former geological period, in proportion as the conditions of light and
heat, and attraction and density, are more different on any other
planet, than they can have been on the earth, at any period of its
history.
10. Let us then consider the state of these elements of being in the
other planets. I have mentioned, among them, the force of gravity, and
the density of materials; because these are important elements in the
question. It may seem strange, that we are able, not only to measure the
planets, but to weigh them; yet so it is. The wonderful discovery of
universal gravitation, so firmly established, as the law which embraces
every particle of matter in the solar system, enables us to do this,
with the most perfect confidence. The revolutions of the satellites
round their primary planets, give us a measure of the force by which the
planets retain them in their orbits; and in this way, a measure of the
quantity of matter of which each planet consists. And other effects of
the same universal law, enable us to measure, though less easily and
less exactly, the masses, even of those planets which have no
satellites. And thus we can, as it were, put the Earth, and Jupiter or
Saturn, in the balance against each other; and tell the proportionate
number of pounds which they would weigh, if so poised. And again, by
another kind of experiment, we can, as we have said, weigh the earth
against a known mountain; or even against a small sphere of lead duly
adjusted for the purpose. And this has been done; and the results are
extremely curious; and very important in our speculations relative to
the constitution of the universe.
11. And in the first place, we may remark that the Earth is really much
less heavy than we should expect, from what we know of the materials of
which it consists. For, measuring the density, or specific gravity, of
materials, (that is their comparative weight in the same bulk,) by their
proportion to water, which is the usual way, the density of iron is 8,
that of lead 11, that of gold 19: the ordinary rocks at the Earth's
surface have a density of 3 or 4. Moreover, all the substances with
which we are acquainted, contract into a smaller space, and have their
density increased, by being subjected to pressure. Air does this, in an
obvious manner; and hence it is, that the lower parts of our atmosphere
are denser than the upper parts; being pressed by a greater
superincumbent weight, the weight of the superior parts of the
atmosphere itself. Air is thus obviously and eminently elastic. But all
substances, though less obviously and eminently, are still, really, and
in some degree, elastic. They all contract by compression. Water for
instance, if pressed by a column of water 100000 feet high, would be
reduced to a bulk one-tenth less than before. In the same manner iron,
compressed by a column of iron 90000 feet high, loses one-tenth of its
bulk, and of course gains so much in density. And the like takes place,
in different amounts, with all material whatever. This is the rate at
which compression produces its effect of increasing the density, in
bodies which are in the condition of those which lie around us. But if
this law were to go on at the same rate, when the compression is
greatly increased, the density of bodies deep down towards the centre of
the Earth must be immense. The Earth's radius is above 20 million feet.
At a million feet depth we should have matter subjected to the pressure
of a column of a million feet of superincumbent matter, heavier than
water; and hence we should have a compression of water 10 times as great
as we have mentioned; and, therefore, the bulk of the water would be
reduced almost to nothing, its density increased almost indefinitely:
and the same would be the case with other materials, as metals and
stones. If, therefore, this law of compression were to hold for these
great pressures, all materials whatever, contained in the depths of the
Earth's mass, must be immensely denser, and immensely specifically
heavier, than they are at the surface. And thus, the Earth consisting of
these far denser materials towards the centre, but, nearer the surface,
of lighter materials, such as rock, and metals, in their ordinary state,
must, we should expect, be, on the whole, much heavier than if it
consisted of the heaviest ordinary materials; heavier than iron, or than
lead; hundreds of times perhaps heavier than stone.
12. This, however, is not found to be so. The expectation of the great
density of the Earth, which we might have derived from the known laws of
condensation of terrestrial substances, is not confirmed. The mass of
the Earth being weighed, by means of such processes as we have already
referred to, is found to be only five times heavier than so much water:
less heavy than if it were made of iron: less than twice as heavy as if
it were made of ordinary rock. This, of course, shows us that the
condensation of the interior parts of the Earth's mass, is by no means
so great as we should have expected it to be, from what we know of the
laws of condensation here; and from considering the enormous pressure of
superincumbent materials to which those interior parts are subjected.
The laws of condensation, it would seem, do not go on operating for
these enormous pressures, by the same progression as for smaller
pressure. If a mass of a material is compressed into nine-tenths its
bulk by the weight of a column of 100000 feet high, it does not follow
that it will be again compressed into nine-tenths of its condensed bulk,
by another column of 100000 feet high. The compression and condensation
reach, or tend to, a limit; and probably, before they have gone very
far. It may be possible to compress a piece of iron by one-thousandth
part, even by such forces as we can use; and yet it may not be possible
to compress the same piece of iron into one half its bulk, even by the
weight of the whole Earth, if made to bear upon it. This appears to be
probable: and this will explain, how it is, that the materials of the
Earth are not so violently condensed as we should have supposed; and
thus, why, the Earth is so light.
13. We must avoid drawing inferences too boldly, on a subject where our
means of knowledge are so obscure as they are with regard to the
interior of the Earth; but yet, perhaps, we may be allowed to say, that
the result which we have just stated, that the Earth is so light,
suggests to us the belief that the interior consists of the same
materials as the exterior, slightly condensed by pressure.[4] We find no
encouragement to believe that there is a nucleus within, of some
material, different from what we have on the outside; some metal, for
instance, heavier than lead. If the earth were of granite, or of lava,
to the centre, it would, so far as we can judge, have much the same
weight which it now has. Such a central mass, covered with the various
layers of stone, which form the upper crust of the Earth, would
naturally make this globe of at least the weight which it really has.
And therefore, if we were to learn that a planet was much lighter than
this, as to its materials,--much less dense, taking the whole mass
together,--we should be compelled to infer that it was, throughout, or
nearly so, formed of less compact matter than metal and stone; or else,
that it had internal cavities, or some other complex structure, which it
would be absurd to assume, without positive reasons.
14. Now having decided these views from an examination of the Earth, let
us apply them to other planets, as bearing upon the question of their
being inhabited; and in the first place, to Jupiter. We can, as we have
said, easily compare the mass of Jupiter and of the Earth; for both of
them have Satellites. It is ascertained, by this means, that the mass of
weight of Jupiter is about 333 times the weight of the earth; but as his
diameter is also 11 times that of the earth, his bulk is 1331 times that
of the earth: (the _cube_ of 11 is 1331); and, therefore, the density of
Jupiter is to that of the earth, only as 333 to 1331, or about 1 to 4.
Thus the density of Jupiter, taken as a whole, is about a quarter of the
earth's density; less than that of any of the stones which form the
crust of the earth; and not much greater than the density of water.
Indeed, it is tolerably certain, that the density of Jupiter is not
greater than it would be, if his entire globe were composed of water;
making allowance for the compression which the interior parts would
suffer by the pressure of those parts superincumbent. We might,
therefore, offer it as a conjecture not quite arbitrary, that Jupiter is
a mere sphere of water.
15. But is there anything further in the appearance of Jupiter, which
may serve to contradict, or to confirm, this conjecture? There is one
circumstance in Jupiter's form, which is, to say the least, perfectly
consistent with the supposition, that he is a fluid mass; namely, that
he is not an exact sphere, but oblate, like an orange. Such a form is
produced, in a fluid sphere, by a rotation upon its axis. It is
produced, even in a sphere which is (at present at least,) partly solid
and partly fluid; and the oblateness of the earth is accounted for in
this way. But Jupiter, who, while he is much larger than the earth,
revolves much more rapidly, is much more oblate than the earth. His
polar and equatorial diameters are in the proportion of 13 to 14. Now it
is a remarkable circumstance, that this is the amount of oblateness,
which, on mechanical principles, would result from his time of
revolution, if he were entirely fluid, and of the same density
throughout.[5] So far, then, we have some confirmation at least, of his
being composed entirely of some fluid which in its density agrees with
water.
16. But there are other circumstances in the appearances of Jupiter,
which still further confirm this conjecture of his watery constitution.
His belts,--certain bands of darker and lighter color, which run
parallel to his equator, and which, in some degree, change their form,
and breadth, and place, from time to time,--have been conjectured, by
almost all astronomers, to arise from lines of cloud, alternating with
tracts comparatively clear, and having their direction determined by
currents analogous to our trade-winds, but of a much more steady and
decided character, in consequence of the great rotatory velocity.[6]
Now vapors, supplying the materials of such masses of cloud, would
naturally be raised from such a watery sphere as we have supposed, by
the action of the Sun; would form such lines; and would change their
form from slight causes of irregularity, as the belts are seen to do.
The existence of these lines of cloud does of itself show that there is
much water on Jupiter's surface, and is quite consistent with our
conjecture, that his whole mass is water.[7]
17. Perhaps some persons may be disposed to doubt whether, if Jupiter
be, as we suppose, merely or principally a mass of water and of vapor,
we are entitled to extend to him the law of universal gravitation, which
is the basis of our speculations. But this doubt may be easily
dismissed. We know that the waters of the earth are affected by
gravitation; not only towards the earth, as shown by their weight, but
towards those distant bodies, the Sun and the Moon; for this gravitation
produces the tides of the ocean. And our atmosphere also has weight, as
we know; and probably has also solar and lunar tides, though these are
marked by many other causes of diurnal change. We have, then, the same
reason for supposing that air and water, in other parts of the system,
are governed by universal gravitation, and exercise themselves the
attractive force of gravitation, which we have for making the like
suppositions with regard to the most solid bodies. Whatever argument
proves universal gravitation, proves it for all matter alike; and
Newton, in the course of his magnificent generalization of the law, took
care to demonstrate, by experiment, as well as by reasoning, that it
might be so generalized.
18. As bearing upon the question of life in Jupiter, there is another
point which requires to be considered; the force of gravity at his
surface. Though, equal bulk for equal bulk, he is lighter than the
earth, yet his bulk is so great that, as we have seen, he is altogether
much heavier than the earth. This, his greater mass, makes bodies, at
equal distances from the centres, ponderate proportionally more to him
than they would do to the earth. And though his surface is 11 times
further from his centre than the earth's is, and therefore the gravity
at the surface is thereby diminished, yet, even after this deduction,
gravity at the surface of Jupiter is nearly two and a half times that on
the earth.[8] And thus a man transferred to the surface of Jupiter would
feel a stone, carried in his hands, and would feel his own limbs also,
(for his muscular power would not be altered by the transfer,) become
2-1/2 times as heavy, as difficult to raise, as they were before. Under
such circumstances animals of large dimensions would be oppressed with
their own weight. In the smaller creatures on the earth, as in insects,
the muscular power bears a great proportion to the weight, and they
might continue to run and to leap, even if gravity were tripled or
quadrupled. But an elephant could not trot with two or three elephants
placed upon his back. A lion or tiger could not spring, with twice or
thrice his own weight hung about his neck. Such an increase of gravity
would be inconsistent then, with the present constitution and life of
the largest terrestrial animals; and if we are to suppose planets
inhabited, in which gravity is much more energetic than it is upon the
earth, we must suppose classes of animals which are adapted to such a
different mechanical condition.
19. Taking into account then, these circumstances in Jupiter's state;
his (probably) bottomless waters; his light, if any, solid materials;
the strong hand with which gravity presses down such materials as there
are; the small amount of light and heat which reaches him, at 5 times
the earth's distance from the sun; what kind of inhabitants shall we be
led to assign to him? Can they have skeletons where no substance so
dense as bone is found, at least in large masses? It would seem not
probable.[9] And it would seem they must be dwellers in the waters, for
against the existence there of solid land, we have much evidence. They
must, with so little of light and heat, have a low degree of vitality.
They must then, it would seem, be cartilaginous and glutinous masses;
peopling the waters with minute forms: perhaps also with larger
monsters; for the weight of a bulky creature, floating in the fluid,
would be much more easily sustained than on solid ground. If we are
resolved to have such a population, and that they shall live by food, we
must suppose that the waters contain at least so much solid matter as is
requisite for the sustenance of the lowest classes; for the higher
classes of animals will probably find their food in consuming the lower.
I do not know whether the advocates of peopled worlds will think such a
population as this worth contending for: but I think the only doubt can
be, between such a population, and none. If Jupiter be a mere mass of
water, with perhaps a few cinders at the centre, and an envelope of
clouds around it, it seems very possible that he may not be the seat of
life at all. But if life be there, it does not seem in any way likely,
that the living things can be anything higher in the scale of being,
than such boneless, watery, pulpy creatures as I have imagined.
20. Perhaps it may occur to some one to ask, if this planet, which
presents so glorious an aspect to our eyes, be thus the abode only of
such imperfect and embryotic lumps of vitality as I have described; to
what purpose was all that gorgeous array of satellites appended to him,
which would present, to intelligent spectators on his surface, a
spectacle far more splendid than any that our skies offer to us: four
moons, some as great, and others hardly less, than our moon, performing
their regular revolutions in the vault of heaven. To which it will
suffice, at present, to reply, that the use of those moons, under such a
supposition, would be precisely the same, as the use of our moon, during
the myriads of years which elapsed while the earth was tenanted by
corals and madrepores, shell-fish and belemnites, the cartilaginous
fishes of the Old Red Sandstone, or the Saurian monsters of the Lias;
and in short, through all the countless ages which elapsed, before the
last few thousand years: before man was placed upon the earth "to eye
the blue vault and bless the _useful_ light:" to reckon by it his months
and years: to discover by means of it, the structure of the universe,
and perhaps, the special care of his Creator for him alone of all his
creatures. The moons of Jupiter, may in this way be of use, as our own
moon is. Indeed we know that they have been turned to most important
purposes, in astronomy and navigation. And knowing this, we may be
content not to know how, either the satellites of Jupiter, or the
satellite of the Earth, tend to the advantage of the brute inhabitants
of the waters.
21. There is another point, connected with this doctrine of the watery
nature of Jupiter, which I may notice, though we have little means of
knowledge on the subject. Jupiter being thus covered with water, is the
water ever converted into ice? The planet is more than 5 times as far
from the sun as the earth is: the heat which he receives is, on that
account, 25 times less than ours. The veil of clouds which covers a
large part of his surface, must diminish the heat still further. What
effect the absence of land produces, on the freezing of the ocean, it is
not easy to say. We cannot, therefore, pronounce with any confidence
whether his waters are ever frozen or not. In the next considerable
planet, Mars, astronomers conceive that they do trace the effects of
frost; but in Mars we have also appearances of land. In Jupiter, we are
left to mere conjecture; whether continents and floating islands of ice
still further chill the fluids of the slimy tribes whom we have been led
to regard as the only possible inhabitants; or whether the watery globe
is converted into a globe of ice; retaining on its surface, of course,
as much fluid as is requisite, under the evaporating power of the sun,
to supply the currents of vapor which form the belts. In this case,
perhaps, we may think it most likely that there are no inhabitants of
these shallow pools in a planet of ice: at any rate, it is not worth
while to provide any new speculations for such a hypothesis.
22. We may turn our consideration from Jupiter to Saturn; for in many
respects the two planets are very similar. But in almost every point,
which is of force against the hypothesis of inhabitants, the case is
much stronger in Saturn than it is in Jupiter. Light and heat, at his
distance, are only one ninetieth of those at the Earth. None but a very
low degree of vitality can be sustained under such sluggish influences.
The density of his mass is hardly greater than that of cork; much less
than that of water: so that, it does not appear what supposition is left
for us, except that a large portion of the globe, which we see as his,
is vapor. That the outer part of the globe is vapor, is proved, in
Saturn as in Jupiter, by the existence of several cloudy streaks or
belts running round him parallel to his equator. Yet his mass, taken
altogether, is considerable, on account of his great size; and gravity
would be greater, at his outer surface, than it is at the earth's. For
such reasons, then, as were urged in the case of Jupiter, we must either
suppose that he has no inhabitants; or that they are aqueous, gelatinous
creatures; too sluggish, almost to be deemed alive, floating on their
ice-cold waters, shrouded forever by their humid skies.
23. Whether they have eyes or no, we cannot tell; but probably if they
had, they would never see the Sun; and therefore we need not commiserate
their lot in not seeing the host of Saturnian satellites; and the Ring,
which to an intelligent Saturnian spectator, would be so splendid a
celestial object. The Ring is a glorious object for man's view, and his
contemplation; and therefore is not altogether without its use. Still
less need we (as some appear to do) regard as a serious misfortune to
the inhabitants of certain regions of the planet, a solar eclipse of
fifteen years' duration, to which they are liable by the interposition
of the Ring between them and the Sun.[10]
24. The cases of Uranus and Neptune are similar to that of Saturn, but
of course stronger, in proportion to their smaller light and heat. For
Uranus, this is only 1-360th, for Neptune, as we have already said,
1-900th of the light and heat at the earth. Moreover, these two new
planets agree with Jupiter and with Saturn, in being of very large size
and of very small density; and also we may remark, one of them, probably
both, in revolving with great rapidity, and in nearly the same period,
namely, about 10 hours: at least, this has been the opinion of
astronomers with regard to Uranus. The arguments against the hypothesis
of these two planets being inhabited, are of course of the same kind as
in the case of Jupiter and Saturn, but much increased in strength; and
the supposition of the probably watery nature and low vitality of their
inhabitants must be commended to the consideration of those who contend
for inhabitants in those remote regions of the solar system.
25. We may now return towards the Sun, and direct our attention to the
planet Mars. Here we have some approximation to the condition of the
Earth, in circumstances, as in position. It is true, his light and heat,
so far as distance from the Sun affects them, are less than half those
at the Earth. His density appears to be nearly equal to that of the
Earth, but his mass is so much smaller, that gravity at his surface is
only one-half of what it is here. Then, as to his physical condition,
so far as we can determine it, astronomers discern in his face[11] the
outlines of continents and seas. The ruddy color by which he is
distinguished, the red and fiery aspect which he presents, arise, they
think, from the color of the land, while the seas appear greenish.
Clouds often seem to intercept the astronomer's view of the globe, which
with its continents and oceans thus revolves under his eye; and that
there is an atmosphere on which such clouds may float, appears to be
further proved, by brilliant white spots at the poles of the planet,
which are conjectured to be snow; for they disappear when they have been
long exposed to the sun, and are greatest when just emerging from the
long night of their polar winter; the snow-line then extending to about
six degrees (reckoned upon the meridian of the planet) from the pole.
Moreover, Mars agrees with the earth, in the period of his rotation;
which is about 24 hours; and in having his axis inclined to his orbit,
so as to produce a cycle of long and short days and nights, a return of
summer and winter, in every revolution of the planet.
26. We have here a number of circumstances which speak far more
persuasively for a similarity of condition, in this planet and the
Earth, than in any of the cases previously discussed. It is true, Mars
is much smaller than the earth, and has not been judged worthy of the
attendance of a satellite, although further from the Sun; but still, he
may have been judged worthy of inhabitants by his Creator. Perhaps we
are not quite certain about the existence of an atmosphere; and without
such an appendage, we can hardly accord him tenants. But if he have
inhabitants, let us consider of what kind they must be conceived to be,
according to any judgment which we can form. The force of his gravity is
so small, that we may allow his animals to be large, without fearing
that they will break down by their own weight. In a planet so dense,
they may very likely have solid skeletons. The ice about his poles will
cumber the seas, cold even for the want of solar heat, as it does in our
arctic and antarctic oceans; and we may easily imagine that these seas
are tenanted, like those, by huge creatures of the nature of whales and
seals, and by other creatures which the existence of these requires and
implies. Or rather, since, as we have said, we must suppose the
population of other planets to be more different from our existing
population, than the population of other ages of our own planet, we may
suppose the population of the seas and of the land of Mars, (if there be
any, and if we are not carrying it too high in the scale of vital
activity,) to differ from any terrestrial animals, in something of the
same way in which the great land and sea saurians, or the iguanodon and
dinotherium, differed from the animals which now live on the earth.
27. That we need not discuss the question, whether there are intelligent
beings living on the surface of Mars, perhaps the reader will allow,
till we have some better evidence that there are living things there at
all; if he calls to mind the immense proportion which, on the earth, far
better fitted for the habitation of the only intelligent creature which
we know or can conceive, the duration of unintelligent life has borne to
that of intelligent. Here, on this Earth, a few thousand years ago,
began the life of a creature who can speculate about the past and the
future, the near and the absent, the Universe and its Maker, duty and
immortality. This began a few thousand years ago, after ages and myriads
of ages, after immense varieties of lives and generations, of corals and
mollusks, saurians, iguanodons, and dinotheriums. No doubt the Creator
might place an intelligent creature upon a planet, without all this
preparation, all this preliminary life. He has not chosen to do so on
the earth, as we know; and that is by much the best evidence attainable
by us, of what His purposes are. It is also possible that He should, on
another planet, have established creatures of the nature of corals and
mollusks, saurians and iguanodons, without having yet arrived at the
period of intelligent creatures: especially if that other planet have
longer years, a colder climate, a smaller mass, and perhaps no
atmosphere. It is also possible that He should have put that smaller
planet near the Earth, resembling it in some respects, as the Moon does,
but without any inhabitants, as she has none; and that Mars may be such
a planet. The probability against such a belief can hardly be considered
as strong, if the arguments already offered be regarded as effective
against the opinion of inhabitants in the other planets, and in the
Moon.
28. The numerous tribe of small bodies, which revolve between Jupiter
and Mars, do not admit of much of the kind of reasoning, which we have
applied to the larger planets. They have, with perhaps one exception
(Vesta) no disk of visible magnitude; they are mere dots, and we do not
even know that their form is spherical. The near coincidence of their
orbits has suggested, to astronomers, the conjecture that they have
resulted from the explosion of a larger body, and from its fracture into
fragments. Perhaps the general phenomena of the universe suggest rather
the notion of a collapse of portions of sidereal matter, than of a
sudden disruption and dispersion of any portion of it; and these small
bodies may be the results of some imperfectly effected concentration of
the elements of our system; which, if it had gone on more completely and
regularly, might have produced another planet, like Mars or Venus.
Perhaps they are only the larger masses, among a great number of smaller
ones, resulting from such a process: and it is very conceivable, that
the meteoric stones which, from time to time, have fallen upon the
earth's surface, are other results of the like process:--bits of planets
which have failed in the making, and lost their way, till arrested by
the resistance of the earth's atmosphere. A remarkable circumstance in
these bodies is, that though thus coming apparently from some remote
part of the system, they contain no elements but such as had already
been found to exist in the mass of the earth; although some substances,
as nickel and chrome, which are somewhat rare in the earth's materials,
are common parts of the composition of meteoric stones. Also they are of
crystalline structure, and exhibit some peculiarities in their
crystallization. Such as these strange visitors are, they seem to show
that the other parts of the solar system contain the same elementary
substances, and are subject to the same laws of chemical synthesis and
crystalline force, which obtain in the terrestrial region. The smallness
of these specimens is a necessary condition of their reaching us; for if
they had been more massive, they would have followed out the path of
their orbits round the sun, however eccentric these might be. The great
eccentricity of the smaller planets, their great deviation from the
zodiacal path, which is the highway of the large planets, their great
number, probably by no means yet exhausted by the discoveries of
astronomers; all fall in with the supposition that there are, in the
solar system, a vast multitude of such abnormal planetoidal lumps. As I
have said, we do not even know that they are approximately spherical;
and if they are of the nature of meteoric stones, they are mere crude
and irregularly crystallized masses of metal and earth. It will
therefore, probably, be deemed unnecessary to give other reasons why
these planetoids are not inhabited. But if it be granted that they are
not, we have here, in addition to the moon, a large array of examples,
to prove how baseless is the assumption, that all the bodies of the
solar system are the seats of life.
29. We have thus performed our journey from the extremest verge of the
Universe, so far as we have any knowledge of it, to the orbit of our own
planet; and have found, till we came into our own most immediate
vicinity, strong reasons for rejecting the assumption of inhabited
worlds like our own; and indeed, of the habitation of worlds in any
sense. And even if Mars, in his present condition, may be some image of
the Earth, in some of its remote geological periods, it is at least
equally possible that he may be an image of the Earth, in the still
remoter geological period before life began. Of peculiar fitnesses which
make the earth suited to the sustentation of life, as we know that it
is, we shall speak hereafter; and at present pass on to the other
planets, Venus and Mercury. But of these, there is, in our point of
view, very little to say. Venus, which, when nearest to us, fills a
larger angle than any other celestial body, except the Sun and the Moon,
might be expected to be the one of which we know most. Yet she is really
one of the most difficult to scrutinize with our telescopes. Astronomers
cannot discover in her, as in Mars, any traces of continents and seas,
mountains and valleys; at least with any certainty.[12] Her illuminated
part shines with an intense lustre which dazzles the sight;[13] yet she
is of herself perfectly dark; and it was the discovery, that she
presented the phases of the Moon, made by the telescope of Galileo,
which gave the first impulse to planetary research. She is almost as
large as the earth; almost as heavy. The light and heat which she
receives from the Sun must be about double those which come to the
earth. We discern no traces of a gaseous or watery atmosphere
surrounding her. Perhaps if we could see her better, we might find that
she had a surface like the moon; or perhaps, in the nearer neighborhood
of the sun, she may have cooled more slowly and quietly, like a glass
which is annealed in the fire; and hence, may have a smooth surface,
instead of the furrowed and pimpled visage which the Moon presents to
us. With this ignorance of her conditions, it is hard to say what kind
of animals we could place in her, if we were disposed to people her
surface; except perhaps the microscopic creatures, with siliceous
coverings, which, as modern explorers assert, are almost indestructible
by heat. To believe that she has a surface like the earth, and tribes of
animals, like terrestrial animals, and like man, is an exercise of
imagination, which not only is quite gratuitous, but contrary to all the
information which the telescope gives us; and with this remark, we may
dismiss the hypothesis.
30. Of Mercury we know still less. He receives seven times as much light
and heat as the Earth; is much smaller than the earth, but perhaps more
dense; and has not, so far as we can tell, any of the conditions which
make animal existence conceivable. If it is so difficult to find
suitable inhabitants for Venus, the difficulty for Mercury is immensely
greater.
31. So far then, we have traversed the Solar System, and have found even
here, the strongest grounds that there can be no animal existence, like
that which alone we can conceive as animal existence, except in the
planet next beyond the earth, Mars; and there, not without great
modifications. But we may make some further remarks on the condition of
the several planets, with regard to what appears to us to be the
necessary elements of animal life.
FOOTNOTES:
[1] More recently, at the meeting of the British Association in
September, 1853, Professor Phillips has declared, that astronomers can
discern the shape of a spot on the Moon's surface, which is a few
hundred feet in breadth.
[2] A person visiting the Eifel, a region of extinct volcanoes, west of
the Rhine, can hardly fail to be struck with the resemblance of the
craters there, to those seen in the moon through a telescope.
[3] Bessel has discussed and refuted (it was hardly necessary) the
conjecture of some persons (he describes them as "the feeling hearts who
would find sympathy even in the Moon") that there may be in the Moon's
valleys air enough to support life, though it does not rise above the
hills.--_Populaere Vorlesungen_, p. 78.
[4] The doctrine that the interior nucleus of the Earth is fluid,
whether accepted or rejected, does not materially affect this argument.
It appears, that in some cases, at least, the melting of substances is
prevented, by their being subjected to extreme pressure; but the
density, the element from which we reason, is measured by methods quite
independent of such questions.
[5] Herschel, 512. Bessel, however, holds that the oblateness of Jupiter
proves that his interior is somewhat denser than his exterior. _Pop.
Vorles._ p. 91.
[6] Herschel, 513.
[7] A difficulty may be raised, founded on what we may suppose to be the
fact, as to the extreme cold of those regions of the Solar System. It
may be supposed that water under such a temperature could exist in no
other form than ice. And that the cold must there be intense, according
to our notion, there is strong reason to believe. Even in the outer
regions of our atmosphere, the cold is probably very many degrees below
freezing, and in the blank and airless void beyond, it may be colder
still. It has been calculated by physical philosophers, on grounds which
seem to be solid, that the cold of the space beyond our atmosphere is
100 deg. below zero. The space near to Jupiter, if an absolute vacuum, in
which there is no matter to receive and retain heat emitted from the
Sun, may, perhaps, be no colder than it is nearer the Sun. And as to the
effect the great cold would produce on Jupiter's watery material, we may
remark, that if there be a free surface, there will be vapor produced by
the Sun's heat; and if there be air, there will be clouds. We may add,
that so far as we have reason to believe, below the freezing point, no
accession of cold produces any material change in ice. Even in the
expeditions of our Arctic navigators, a cold of 40 deg. below zero was
experienced, and ice was still but ice, and there were vapors and clouds
as in our climate. It is quite an arbitrary assumption, to suppose that
any cold which may exist in Jupiter would prevent the state of things
which we suppose.
[8] Herschel, 508.
[9] It may be thought fanciful to suppose that because there is little
or no solid matter (of any kind known to us) in Jupiter, his animals are
not likely to have solid skeletons. The analogy is not very strong; but
also, the weight assigned to it in the argument is small. _Valeat
quantum valere debet._
[10] Herschel, 522.
[11] Herschel, 510.
[12] According to Bessel, Schroeter _once_ saw one bright point on the
dark ground, near the boundary of light in Venus. This was taken as
proving a mountain, estimated at 60,000 feet high. _Pop. Vorles._ p. 86.
[13] Herschel, 509.