FORMATION OF PLANETS
In our first two chapters we saw what sign-posts in the sky there are
pointing to the course evolution of a solar system probably follows,
and secondly, what evidence there is that our system took this road.
We now come to a question not so easy to precise,—the actual details
of the journey. It is always difficult to descend from a glittering
panoramic survey to particular path-finding. The obstacles loom so much
larger on a near approach.
Most men shy at decisions and shun self-committal to any positive
course, but when it comes to constructing a cosmogony, few at all
qualified hesitate to frame one if the old does not suit. The safety
in so doing lies in the fact that nothing in particular happens if
it refuses to work. Its absurdity is promptly shown up, it is true,
by some one else. For there is almost as good a trade in exposing
cosmogonies as in constructing them. But no special opprobrium attaches
to failure, because everybody has failed, from Laplace down, or up,
as you are pleased to consider it. Besides it is really not so easy
to do, as one is tempted to believe before his book is published.
Then only does the difficulty dawn, with a speed and clarity inversely
proportional to the previous relation of the critic to the author. For
the author himself is apt to be blind. With the fatal fondness of a
parent for his offspring it is rare for the defects to be so glaringly
apparent to their perpetrator. At the worst he considers them venial
faults which can be glossed away.
Attacking the subject in this judicial spirit, the reader can hardly
expect me to satisfy him with a cosmogony entirely home-made, but
at best to pursue a happy middle course between creator and critic,
advocating only such portions as happen to be my own, while sternly
exposing the mistakes of others.
In undertaking the hazardous climb toward the origin of things two
qualities are necessary in the explorer: a quick eye for possibilities
and a steady head in testing them. Without the discernment to perceive
relations no ascent to first principles is possible; and without the
support of quantitative criterion, one is in danger of becoming giddy
from one’s own imagination. Congruities must first hint at a path;
physical laws then determine its feasibility.
An eye for congruities is the first essential. For congruity alone
accuses an underlying law. It is the analogic that with logic leads to
great generalizations. Certain concords of the sort in the motions of
the planets were what suggested to Laplace his system of the world.
With the uncommon sense of a mathematician he perceived that such
accordances were not necessitated by the law of gravitation, and on the
other hand, could not be due to chance. The laws of probability showed
millions to one against it. One of these happy harmonies was that all
the large planets revolved about the Sun in substantially the same
plane; another that they all travelled in the same sense (direction).
Had they been unrelated bodies at the start, such agreement in motion
was mathematically impossible. Their present consensus implied a common
origin for all. In other words, the solar system must have grown to be
what it is, not started so.
This basic fact we may consider certain. But from it we would fain
go on to find out how it evolved. To do so the same process must be
followed. Considering, then, our solar system from this point of view,
one cannot but be struck by some further congruities it presents. These
are not quite those that inspired Laplace, because of discoveries
since, and demand in consequence a theory different from his.
The out about constructing a theory is that fresh facts will come
along and knock for admission after the door is shut. They prove
irreconcilables because they were not consulted in advance. The
consequence is that since Laplace’s time new relations have come to
light, and some supposed concords have had to be given up; so that were
he alive to-day he would himself have formulated some other scheme.
Two, however, are still as true: that the planets all revolve in the
same plane and in the same sense, and that sense that of the Sun’s
rotation. But so general a congruity as this points only to an original
common moment of momentum and is equally explicable however that motion
was brought about. It seems quite compatible with an original shock. To
say that it was caused by a disruption is simply to go one step farther
back than Laplace. If, then, such a catastrophe did occur as the
meteorites aver, we may perhaps draw some interesting inferences about
it from the present state of the system. In a very close approach such
as we must suppose for the disruption, one within Roches’ limit of 2.5
diameters, the stranger, supposing him of equal size, would sweep from
one side of the former Sun to the other in about two hours, and the
brunt of the disrupting pull occur within that time. That the former
Sun was rotating slowly seems established by the time, twenty-eight
days, it now takes to go round. In which case the orbits of the
masses which were to form the planets would all lie in about the same
plane,—the plane of the tramp’s approach. If there were exceptions,
they should be found in the innermost. For such should partake most
largely of the Sun’s own original rotation and travel therefore most
nearly in its plane. And as a fact Mercury, the Benjamin, does differ
from the others by revolving in a plane inclined some 7° to their mean,
agreeing in this with the Sun’s own rotation, with whose plane it was
probably originally coincident (digression from it now being due to
secular retrogression of the planets’ nodes) [see NOTE 4].
From the relations which advance has left unchanged we pass to those
phenomena which seemed to present congruities in Laplace’s day,
but which have since proved void owing to subsequent detection of
exceptions. Time prevents my making the catalogue complete, but the
reader shall be shown enough to satisfy him of the problem’s complexity
and to whet his desire for further research—on the part, preferably, of
others.
[Illustration: CHART SHOWING INCREASING TILTS OF THE MAJOR PLANETS.]
First comes, then, the rotations of the planets upon their axes, which
Laplace supposed to be all in the same direction, counter to the hands
of a clock; for the heavens mark time oppositely from us. All those
within and including Saturn, the only ones he knew, turn, indeed, in
the same sense that they travel round the Sun. But Uranus departs from
that direction by a right angle, wallowing rather than spinning in his
orbit; while Neptune goes still farther in idiosyncratic departure
and actually turns in the opposite direction. Here, then, Laplace’s
congruity breaks down, but in its place a little attention will show
that a new one has arisen. For Saturn’s tilt is 27° and Jupiter’s 3°,
so that with the major planets there is revealed a systematic righting
of the planetary axes from inversion through perpendicularity to
directness as one proceeds inward toward the Sun.
Another congruity supposed to exist a century ago was the exemplary
agreement of all the satellites to follow in their planetary circuits
the pattern set them by their primaries round the Sun. But as man has
penetrated farther into space and photographic plates have come to be
employed, satellites have been revealed which depart from this orderly
arrangement. This is the case with the ninth, the outermost, satellite
of Saturn and with the eighth, the outermost, of Jupiter. But, as
before, the breaking down of one congruity seems but the establishing
of another. It appears that only the most distant satellites are
permitted such unconformity of demeanor. For departure from the
supposed orthodoxy occurs in both instances where the distance is most,
and does not occur in the case of all the other satellites found since
Laplace’s day, eleven in number, nearer their planets.
A third congruity formerly believed in has suffered a like fate; to
wit, that satellites always moved in or near the equatorial plane of
their primary. All those first discovered did; the four large ones of
Jupiter, the main ones of Saturn, and probably those of Uranus and
Neptune. Even the satellites of Mars conformed. Iapetus alone seemed to
make exception, and that by a glossable amount. But this orderliness,
too, has been disposed of, only, like the others, to experience a
resurrection in a different form.
[Illustration]
On examining more precisely the inclinations of these orbits some years
ago, an interesting relation between them and the distances of the
satellites from their primaries forced itself on my notice. The tilt
increased as the distance grew. The only exceptions were very tiny
bodies occupying a sort of asteroidal relation to the rest.
A diagram will make this clear. The kernel of it dates from the
lectures then delivered before the Massachusetts Institute of
Technology in 1901. The interesting thing now about it is that the
congruity there pointed out has been conformed to by every satellite
discovered since,—the sixth, seventh, and eighth of Jupiter and the
ninth and tenth of Saturn. It is evident that we already know enough of
the geniture of our system to prophesy something about it and have the
prophecy come true.
Closely connected with the previous relation is a fourth concordance
clearly of mechanical origin, the relation of the orbital
eccentricities of the satellites to their distances from their
respective planets. The satellites pursue more and more eccentric
orbits according as they stand removed from planetary proximity.
A fifth congruity is no less striking. All the satellites of all the
planets that we can observe well enough to judge of turn the same face
always to their lords. That the Moon does so to the Earth is a fact of
everyday knowledge, and the telescope hints that the same respectful
regard is paid by Jupiter’s and Saturn’s retinues to them. What is
still more remarkable, Mercury and Venus turn out to observe the like
vassal etiquette with reference to the Sun. And it will be noticed that
they stand to him the nearest of his court. Here, then, is a law of
proximity which points conclusively to some well-established force.
Last is a remarkable congruity which study disclosed to me likewise
some years ago, and which has received corroboration in discoveries
since. This congruity is the peculiar arrangement of the masses in the
solar system.
Consider first the way in which the several planets, as respects size,
stand ordered in distance from the sun. Nearest to him is Mercury, the
smallest of all the principal ones. Venus and the Earth follow, each
larger than the last; then comes Mars, of distinctly less bulk, and
so to the asteroids, of almost none. After this the mass rises again
to its maximum in Jupiter, and then subsequently falls through Saturn
to Uranus and Neptune. Here we mark a more or less regular gradation
between mass and position, a curve in which there are two ups and
downs, the outer swell being much the larger, though the inner, too, is
sufficiently pronounced.
Now turn to Saturn and his family, which is the most numerous of the
secondary systems and that having the greatest span. Under Saturn’s
wing, as it were, is the ring, itself a congeries of tiny satellites.
Then comes Mimas, the smallest of the principal ones; then Enceladus,
a little larger; then Tethys, the biggest of the three. Next stands
Dione, smaller than Tethys. Then the mass increases with Rhea, reaching
its culmination in Titan, after which it declines once more. Strangely
reproductive this of the curve we marked in the arrangement of the
planets themselves, even to the little inner rise and fall.
[Illustration: MASSES OF PLANETS AND SATELLITES.]
Striking as such analogous ordering is, it is not all. For, scanning
the Jovian system, we find the main curve here again; Ganymede, the
Jupiter or Titan of the system, standing in the same medial position
as they. Lastly, taking up Uranus and his family of satellites, the
same order is observable there. Titania, the largest, is posted in the
centre.
Thus the order in which the little and the big are placed with
reference to their controlling orb is the same in the solar system
and in that of every one of its satellite families. Method here is
unmistakable. Nor is it easy to explain unless the cause in all was
like. That the rule in the placing of the planets should be faithfully
observed by them in the ordering of their own domestic retinues, is
not the least strange feature of the arrangement. It argues a common
principle for both. Not less significant is the secondary hump in
their distribution, denoting recrudescence farther in of the primary
procedure shown without.
One point to be particularly noticed in these latter-day congruities
is that they are not simply general concords like the older ones—the
fact that the planets move in one plane or in the same sense in that
plane—but detailed placings, ordered according to the distances of
the planets from the Sun or of the satellites from the planets. They
are thus not simply of the combinative but of the permutative order
of probabilities, a much higher one; in other words, the chance that
they can be due to chance is multiplicately small. Thus just as these
analogies are by so much more remarkable, so are they by so much more
cogent. They tell us not only of an evolution, but they speak of the
very manner of its work. They do not simply generalize, they specify
the mode of action. The difficulty is to understand their language. It
is a case of celestial hieroglyphics to which we lack the key.
In attempting now to discover how all this came about we notice first
that the system could not have originated in the beautifully simple way
suggested by Laplace, because of several impossibilities in the path.
If rings were shed, as he supposed, from a symmetric contracting mass,
they should have resulted in something even more symmetric than we
observe to-day. In the next place they could not, it would appear, even
if formed, have collected into planets.
Nor could there have been an original “fire-mist” with which as a
stock in trade Laplace thriftily endowed his nebula to start with—the
necessity for which has been likened to our supposed descent from
monkeys; but which in truth is as misty a conception of the facts in
the one case as it is a monkeying with them in the other. Darwin’s
theory distinctly avers that we were _not_ descended from monkeys;
and Laplace’s fire-mist under modern examination evaporates away.
It is an interesting outcome of modern analysis that the very fact
which suggested the annular genesis of planets to Laplace, the rings
of Saturn, should now probably be deemed a striking instance of the
reverse. Far from its being an exemplar in the heavens of the pristine
state of the solar system, we may now see in it a shining pattern of
how the devolution of bodies comes about. For instead of typifying an
unfortunate set of particles which untoward circumstance has prevented
from coalescing into a single orb, it almost certainly represents the
distraught state to which a once more compact congeries of them has
been brought by planetary interference. For to just such fate must
the stresses in it caused by Saturn have eventually led. Disruption
inevitable to such a group the observation of comets demonstrates is
daily taking place. When a comet passes round the Sun or near a planet,
the partitive pulls of the body tend to dismember it, and the same is
_a fortiori_ true of matter circulating round a planet as relatively
near as the meteoric particles that constitute Saturn’s rings. Starting
as a congeries, it was pulled out more and more into a ring until it
became practically even throughout. And the very action that produced
it tends to keep it as surprisingly regular as we note to-day.
No, the planets probably were otherwise generated and may have looked
in their earlier stages as the knots in the spiral nebulæ do to-day.
But this does not mean that we can detail the process [see NOTE 5].
Taking now the congruities for guide, we proceed to see what they
affirm or negative. Laplace, when he ventured on his exposition of
the system of the world, did so “with the mistrust which everything
which is not the direct outcome of observation or calculation must
inspire.” To all who know how even figures can lie this caution will
seem well timed. The best we can do to keep our heads steady is to
lay firm hold at each step on the great underlying principles of
physics. One of these is the conservation of the moment of momentum.
This expression embodies one of the grandest generalizations of cosmic
mechanics. The very phrase is fittingly sonorous, with something of
that religious sublimity which the dear old lady said she found such
a consolation in the biblical word Mesopotamia. Indeed the idea is
grand for its very simplicity. Momentum means the quantity of motion
in a body. It is the speed into the number of particles or the mass.
Moment of momentum denotes the rotatory power of it round an axis. Now
the curious and interesting thing about this quantity is that it can
neither be diminished nor increased. It is an abstraction from which
nothing can be abstracted—but results. It is the one unalterable thing
in a universe of change. What it was in the beginning in a system,
that it forever remains. Because of this unchangeableness we can use
it very effectively for purposes of deduction. One of these is in
connection with that other great principle of physics, the conservation
of energy. By the mutual action of particles on one another, by
contraction, by tidal pulls, and so on, some energy of motion is
constantly being changed into heat and thus dissipated away. Energy of
motion, therefore, is slowly being lost to the system, and the only
stable state for the bodies composing it is when their energy of motion
has decreased to the minimum consistent with the initial moment of
momentum. This principle we shall find very fecund in its application.
It means that our whole system is evolving in a way to lessen its
energy of motion while keeping its quantity of motion unchanged. The
universe always does a thing with the least possible expenditure of
force and gets rid of its superfluous energy by parting with it to
space. Philosophers may wrangle over its being the best possible of
worlds, but it is incontrovertibly mechanically the laziest, which a
pessimistic friend of mine says proves it the best.
Now this generalization finds immediate use in explaining certain
features of the solar system. In looking over the congruities it will
be seen that deviation from the principal plane of the system or
departure from a circular orbit is always associated with smallness
in size. The insignificant bodies are the erratic ones. Now it has
been shown mathematically in several different ways that when small
particles collect into a larger mass, the collisions tend to make
the resultant orbit of the combination both more circular and more
conformant to the general plane than its constituents. But we may see
this more forthrightly by means of the general principle enunciated
above. For in fact both results are direct outcomes of the conservation
of moment of momentum. Given a certain moment of momentum for the
system, the total energy of the bodies is least when they all move in
one plane. This is evident at once because the components of motion
at right angles to the principal plane add nothing to the moment
of momentum of the system. It is also least when the bodies all
revolve in circles about the centre of gravity. The circle has some
interesting properties which almost justify the regard paid to it by
the ancients as the only perfect figure. It encloses the maximum area
for a given periphery, so that according to the old legends, if one
were given as much land as he could enclose with a certain bull’s
hide, he should, after cutting the hide into strips, arrange these
along the circumference of a circle. Now this property of the circle
is intimately connected with the fact that a body revolving in a
circle has the greatest moment of momentum for the least expenditure
of energy. For under the same central force all ellipses of the same
longest diameters—major axes these are technically called—are described
in the same time, and with the same energy, and of all such, the circle
encloses the greatest area, which area measures the moment of momentum
[see NOTE 6].
Given a certain moment of momentum, then the energy is least when the
bodies all move in one plane and all travel in circles in that plane.
As energy is constantly being dissipated while any alteration among
the bodies is going on, to coplanarity and circularity of path all
the bodies must tend, if by collision they be aggregated into larger
masses. As in the present state of our system the small bodies travel
out of the general plane in eccentric ellipses while the big ones
travel in it in approximate circles, the facts indicate that the origin
of the larger masses was due to development by aggregation out of
smaller particles.
The next principle is of a different character. Half a century ago
celestial mechanics dealt with bodies chiefly as points. The Earth was
treated as a weighted point, and so was the Sun. This was possible
because a sphere acts upon outside bodies as if all its mass were
collected at its centre, and the Sun and many of the planets are
practically spheres. But when it came to nicer questions of their
present behavior and especially of their past career, it grew necessary
to take their shape into account in their mutual effects. One of the
results was the discovery of the great rôle played in evolution by
tidal action. Inasmuch as the planets are not perfectly rigid bodies,
each is subject to tidal deformation by the other, the outside being
pulled more than the centre on one side and less on the other. Bodily
tides are thus raised in it analogous to the surface tides we see in
the ocean, only vastly greater, and these in turn act as a brake on its
rotation.
Now the retrograde motions occurring in the outermost parts of all
the systems, principal and subsidiary, only and always there: the
retrograde rotations of Neptune and Uranus, the retrograde revolutions
of the ninth satellite of Saturn and of the eighth of Jupiter, point to
something fundamental. For when we consider that it is precisely in its
outer portions that any forces shaping the development of the system
have had less time to produce their effect, we perceive that apparent
abnormality now is really survival of the original normal state, only
to be found at present in what has not been sufficiently forced to
change. It suggests that the pristine motion of the constituents of the
scattered agglomerations which went to form the planets was retrograde,
and that their present direct rotations and the direct revolutions of
most of their satellites have been imposed by some force acting since.
Let us inquire if there be a force competent to this end, and what its
mode of action.
Let us see how tidal action would work. Tidal force would raise bulges,
and these, not being carried round with the planet’s rotation except to
a certain distance, due to viscosity, must necessarily act as brakes
upon the planet’s spin. In consequence of the friction they would thus
exert, energy of motion must be lost. So long, then, as tidal forces
can come into play, the energy of the system is capable of decrease.
According to the last principle we considered, the system cannot be
in stable equilibrium until this superfluous energy is lost or until
tidal forces become inoperative, which cannot be till all the bodies
in the system turn the same face to their respective centres of
attraction.
To see this more clearly, take the case of a retrograde spin of a
planet as compared with a direct one. The energy of the planet’s spin
is the same in both cases, because energy depends on the square of a
quantity; to wit, that of the velocity, and is therefore independent
of sign. Not so the moment of momentum. For this depends on the first
power of the speed, and if positive in the one case, must be negative
in the other. The moment of momentum of the whole system, then, is less
in the former case, since the moment of momentum of the retrograde
rotation must be subtracted from, that of the direct rotation be added
to, that of the rest of the system. For a given initial moment of
momentum with which the system was endowed at the start, there is,
then, superfluous energy in the first state which can be got rid of
through reduction to the second. Nature, according to her principles
of least exertion, avails herself of the chance of dispensing with it,
and a direct rotation results. Sir Robert Ball first suggested this
argument.
Tidal action accomplishes the end. In checking up a body rotating
contrary to the general consensus of spin, its first effect is to
start to turn the axis over. For the body is in dynamical unstable
equilibrium with regard to the rest of the system. The righting would
continue, practically to the exclusion of any diminution at first of
the spin, until the body had turned over in its plane so that the spin
became direct. As the force increases greatly with nearness to the
Sun, the effect would be most marked on the nearer, and most so on the
biggest, bodies. This would account for the otherwise strange gradation
from retrograde to direct in the tilts of the axes of the outer
planets, and also for the present tilts of all the inner ones.
Related to the initial retrograde rotations of the planets, and in a
sense survivals from an earlier state of things, are two of the latest
discoveries of motions in the solar system, the retrograde orbital
movements of the ninth satellite of Saturn and the eighth of Jupiter.
Considered so anomalous as scarcely at first to be believed, it has
been stated that they directly contradict the theory of Laplace.
This is true; in the same sense and no more in which they directly
contradict the contradictor, one of the latest theories. For neither
theory has anything to explain them as the result of law. That they
cannot be the sport of indifferent chance seems evidenced by their
occupying similar external positions in their respective systems. As
the product of a law we must regard them, and to find that law we now
turn. Suppose the planet originally to have been rotating backward, or
in the direction of the hands of a clock. At this time the satellite,
which may never have formed a part of its mass, was travelling backward
too, according to what we have said. Then under the friction of
the tides raised on the planet by the Sun, the planet proceeded to
turn over. It continued to do so until it spun direct. During this
process there was no passage through zero of its moment of momentum
_considered with regard to itself_, and therefore no difficulty on that
score of supposing that it successively generated satellites at all
degrees of inclination. That its children are of the nature of adopted
waifs, Babinet’s criterion (1861) would seem to imply. But it must
be remembered that the Sun has been slowing up the planet’s rotation
now for æons. As it turned over, its tidal bulges tended to carry
over with it such satellites as it already had. This effect was much
greater on the nearer ones, both because they were nearer and because
they were much larger than the outer. So that the nearer kept with the
planet, the others lagged proportionately behind. This suggests itself
to account for the facts, but the subject involves so much that is
uncertain that I submit the hypothesis with the distrust which Laplace
has so eminently bespoken. I advance in its favor only the three
striking facts: that a steady progression in their tilts of rotation
is observable from Neptune to Jupiter and a substantially accordant
one from Mars to Mercury; secondly, that the satellites turn their
faces to their primaries, as likewise do Mercury and Venus to the Sun;
and, thirdly, that the orbits of the satellites of all the planets are
themselves tilted in accordance with what it would require [see NOTE 7].
After the axial spins have been made over to the same sense, the second
consequence of tidal action in the case of two bodies revolving about
their common centre of gravity is to slow down both spins until first
the smaller and then the larger turn the same face to each other and
remain thus constant ever after. Now such is precisely the pass to
which we observe the satellites of the planets have come. All that we
can be sure of now turn the same face always to their primary. The Moon
was the first to betray her attitude, because the one we can best note.
On scrutiny, however, Jupiter’s satellites, so far as we can make out,
do the like; and Saturn’s, too. And a very proper attitude it is, this
regard paid to compelling attraction. Thus one of the congruities we
noticed stands accounted for. The satellites could hardly have been at
first so observant; time has brought about this unfailing recognition
of their lords.
Of the peculiar massing of the bodies in the family of the Sun, and
the still stranger copying of it in their own domestic circles, little
can as yet be said in interpretation. That the planetary families and
their ancestral group should agree is not the least strange part of
the affair. It shows that none of them was fortuitous, but that at
the formation of all some common principle presided, apportioning the
aggregations to their proper place. But it is such fine print of the
system’s history as at present to preclude discernment.
So much for the details we may deduce of the method of our birth. We
perceive unmistakably that our solar system grew to be what it is,
and that it developed by agglomeration of its previously shattered
fragments into the planets we behold to-day, but exactly how the
process progressed we are as yet unable to precise. We are, however,
as what I have mentioned and tabled show, every day accumulating data
which will enable an eventual determination probably to be reached.
From the fact of agglomeration, the essence of the affair, we turn to
the traces it has left upon its several offspring.
Just as the continued existence to-day of meteorites _in statu quo_
informs us of a previous body from which our nebula sprang; so a
physical characteristic of our own earth at the present time shows it
to have evolved from that nebula—even though we cannot make out all the
steps. Of its having done so, we are far more sure than of how it did.
That primitive man perceived that somewhere below him was a fiery
region which was not an agreeable abode, is plain from his consigning
to such Tophet those whose religious tenets did not square with
his own. That his conception of it was not strictly scientific is
evidenced by his not realizing that to bury his enemies was the way
to make them take the first step of the journey thither. Indeed, the
vindictive venting of his notions clearly indicates their source as
volcanic, rather than bred of a general disapproval of a downward
descent either in silicates or sin.
It was not till man began to bore into the Earth for metallic or
potable purposes that he brought to light the generic fact that it
was everywhere hotter as one went down. And this not only in a very
regular, but in a most speedy, manner. The temperature increased in a
really surprising way 1° F. for every sixty-five feet of descent. As
the rise continued unabated to the limit of his borings, becoming very
unpleasant at its end, it was clear that at a depth of thirty-five
miles even so refractory a substance as platinum must melt, and
practically all the Earth except a thin crust be molten or even gaseous.
Now heat, like money, is easy to dissipate but hard to acquire, as
primitive man was the first to realize. It does not come without
cause. Being a mode of motion, other motion must have preceded it from
which it sprang. So much the doctrine of the conservation of energy
teaches us, a doctrine considered now to have been the great scientific
heirloom of the nineteenth century to the twentieth, yet which in its
day caused the death of its first discoverer, Mayer, of a broken heart
from non-recognition; its second, Helmholtz, was refused publication by
the leading Berlin physical magazine of the time. So quick is man to
delay his own advance.
The only conceivable motion for thus heating the Earth as a whole was
the falling together of its parts. The present heat of the Earth,
then, accuses the concourse of particles in the past to its formation,
or in other words proves that the Earth was evolved out of material
originally more sparcely strewn. It does so not only in a generic but
in a most particular manner, for the heat is distributed just where it
would be by such a process. It is greater to-day within, increasingly,
because when the globe began to cool, the surface necessarily cooled
first and established a regular gradient of heat from core to cuticle.
It is possible to test this qualitative inference quantitatively and
see if the falling together of the meteorites was equal to the task.
Knowing the mechanical equivalent of heat, what we do is to calculate
the quantity of motion involved and then evaluate it in heat. As we
are unaware of the exact law of density of the Earth, and are ignorant
of how much was radiated away in the process, the problem is a little
like estimating the fortune of a man when we do not know the stocks in
which he has invested, and ignore how much he has spent the while. We
only know what he would have been worth had he followed our advice in
the matter of investments and lived as frugally as we recommended. For
here, too, we are obliged to make certain assumptions. Nevertheless
the figure obtained in the case of the planets’ stores of heat is so
enormous as to leave a most ample margin for dissipation. Had the Earth
contracted from a fairly generous expansion to its present state under
the probable law of density suggested by Laplace in another connection,
the heat developed would have been enough to raise the whole globe to
160,000° F. if of iron, 90,000° F. if of stone. As 10,000° F. would
have sufficed for the Earth to have kept up its past, to say nothing of
its present, state, we are justified of our deduction.
Nor is the Earth the only body in the system which thus argues itself
evolved by the falling together of its present constituents. In the
larger planets Jupiter and Saturn we seem to see the heat, far as we
are away. For the cherry hue they disclose between their brighter belts
proves to come from greater absorption there of the green and blue rays
of the spectrum, indicating a greater depth of atmosphere traversed.
Thus these parts lie at a lower level, and their ruddy hue is just what
they should show were they still glowing with a dull red heat.
[Illustration: SPECTROGRAM OF JUPITER, MOON COMPARISON.
LOWELL OBSERVATORY. V. M. SLIPHER.]
Heat is not only the end of the beginning, it is the beginning of the
end as well. It is both the result of the evolving of definite bodies
out of the agglomeration of matter-strewn space, and the cause of the
higher evolution of those globes themselves. For the acquisition of
heat is the necessary preface to all that follows. Heat is a body’s
evolutionary capital whose wise expenditure through cooling down makes
all further advance to higher products possible. A body too small
to have acquired it must remain forever lifeless, as dead as the
meteorites themselves that enter our air as mere inert bits of stone or
iron.
Curiously enough, heat both must have been and then must have been
lost. Like the loss of fortune or of friends sometimes in the ennobling
of character, it is through its passing away that its effects are
realized. For in cooling down from a once heated condition, that train
of events occurs which we most commonly particularize as evolution.
So far in our survey the march of advance has been through masses of
matter, a molar evolution; from this point on it passes into its minute
constituents and becomes a molecular one. The one is the necessary
prelude to the other. Up to this great turning-point in the history of
each member of a solar system we have been busied with the acquisition
of heat, though we may not have been aware of it the while. All the
motions we have studied tended to that end. During these three
chapters, I, II, V, we have been gradually rising in our point of view
until we stand at the temperature pinnacle of the whole process. In the
next three we are to descend upon the other side. The slope we have
come up was of necessity barren; the one we are to go down brings us to
verdure and the haunts of men. Coming from the causes above, we reach
at each step effects more and more related to ourselves which those
causes will help us to explain.