THE EVOLUTION OF THE STARS AND THE FORMATION
OF THE EARTH. IV
BY WILLIAM WALLACE CAMPBELL
DIRECTOR OF THE LICK OBSERVATORY, UNIVERSITY OF CALIFORNIA
THE PLANETESIMAL HYPOTHESIS
THE most elaborate structure yet proposed to explain the origin of the
solar system is the planetesimal hypothesis by Chamberlin and
Moulton. The energy which these investigators have devoted to formulating
and testing this hypothesis, in the light of the principles of
mechanics, has been commensurate with the importance of the subject.
They postulate that the materials now composing the Sun, planets, and
satellites, at one time existed as a spiral nebula, or as a great spiral
swarm of discrete particles, each particle in elliptic motion about the
central nucleus. The authors go further back and endeavor to account
for the origin of the spiral nebula, but this phase of the subject is not
vital to their hypothesis. However, it conduces to clearness in presenting
their hypothesis to begin with the earlier process.
It may happen, once in a while, that two stars will collide. If the
collision is a grazing one, they say, a spiral nebula will be formed.
However, a fairly close approach of two stars will occur in vastly
greater frequency and the effect of this approach will also be to form a
spiral nebula or two such nebulæ. The authors recall that our Sun is
constantly ejecting materials to a considerable height to form the prominences,
and that the attractions of a great star passing fairly close to
our solar system would assist this process of expulsion of matter from
the Sun. A great outbreak or ejection of matter would occur not only
on the side of our Sun turned toward the disturbing body, but on the
opposite side as well, for the same reason that tides in our oceans are
raised on the side opposite the Moon as well as on the side toward the
Moon. As the Sun and disturbing star proceeded in their orbits, the
stream of matter leaving our Sun on the side of the disturbing body
would try to follow the other star; and the stream of matter leaving the
other side of the Sun would shoot out in curves essentially symmetrical
with those in the first stream. As the disturbing star approached and
receded the paths taken by the ejected matter would be successively
along curves such as are represented by the dotted lines in Fig. 28. At
any given moment the ejected matter would lie on the two heavy lines.
The matter would not be moving along the heavy lines, but nearly at
right angles to them, in the directions that the lighter curves are pointing.
As the ejections would not be continuous, but on the contrary
intermittent, because of violent pulsations of the Sun's body, there
would be irregularities in the two spiral streamers. The materials
drawn out of the Sun would revolve around it in elliptic orbits after the
disturbing body had passed beyond the distance of effective disturbance,
as illustrated in Fig. 29. The orbits of the different masses would
have different sizes and different eccentricities. There would also be
a wide distribution of finely-divided material between the main branches
of the spiral. All of the widespread gaseous matter, hot when it left the
Sun, would soon become cold, by expansion and radiation; and only
the massive nuclei would remain gaseous and hot.
I see no reason to question the efficiency of this ingenious explanation
of the origin of a spiral nebula: the close passage of two massive
stars could, in my opinion, produce an effect resembling a spiral nebula,
quite in accordance with Moulton's test calculations upon the subject.
Some of the spirals have possibly been formed in this way (see Fig. 30);
but that the tens of thousands of spirals known to exist in the sky have
actually been produced in this manner is another question, and one
which, in my opinion, is open to grave doubt. But to this point we shall
return later.
There are marked advantages in starting the evolution of the solar
system from a spiral nebula, aside from the fact that spirals are abundant,
and therefore represent a standard product of development. The
material is thinly and very irregularly distributed in a plane passing
through the Sun, and the motions around the Sun are all in the same
direction. The great difficulty in the Laplace hypothesis, as to the
constancy of the moment of momentum, is here eliminated. There are
well-defined condensations of nuclei at quite different distances from
the Sun. According to this hypothesis the principal nuclei are the
beginnings of the future planets. They draw into themselves the
materials with which they come in contact by virtue of the crossings
of the orbits of various sizes and various eccentricities. The growth of
the planets is gradual, for the sweeping up and combining process must
be excessively slow. The satellites are started from those smaller nuclei
which happen to be moving with just the right speeds not to escape entirely
the attractions of the principal nuclei, nor to fall into them.
The planes of the planetary orbits and, in general, the planes of the
satellite orbits should agree quite closely with each other, but they
could differ and should differ from that of the Sun's equator.
The authors call attention to the fact that the Sun's equator is
inclined
at a small angle, 7°, to the common planes of the planetary system,
and Chamberlin holds this to be one of the strong points in favor
of the planetesimal hypothesis. He reasons thus: the star which passed
close to our Sun and drew out the planetary materials in the form of
spiral streams must have moved in the plane of the spiral; that is, in
the plane of our planetary system. Some of the materials would be
drawn out from our Sun only a very short distance and then fall back
upon the Sun. Great tidal waves would be formed on opposite sides
of the Sun, and these would try to follow the disturbing body. The
effect of these waves and of the materials which fall back would be to
change the Sun's original rotation plane in the direction of the disturbing
body's orbital plane.
Now the chance for a disturbing star's passing around our Sun in a
plane making a large angle, say from 45° to 90°, with the Sun's
equator, is much greater than for a small angle 0° to 45°. The chances are
greatest that the angle will be 90°. Only those disturbing stars which
approach our Sun precisely in the plane of the Sun's equator could
move around the Sun in this plane. All those approaching along any
line parallel to the Sun's equatorial plane, but lying outside of this
plane, and all those whose directions of approach make any angle whatever
with the equatorial plane, would find it impossible to move in that
plane. That the angle under this hypothesis is only 7° is surprising,
though, as we are dealing with but a single case, we can not say, I
think, that this militates either for or against the hypothesis. We are
entitled to say only that unless the approach was so close as to cause
disturbances in our Sun to relatively great depths, the angle referred
to would have only one chance in ten or fifteen or twenty to be as
small as 7°. Any disturbance which succeeded in taking out of the
Sun only 1/7 of 1 per cent. of its mass could scarcely succeed in shifting
the axis of rotation of the remaining 99 6/7 per cent. very much, I think.
If the angle were 30° or 50° or 80°, instead of 7°, the case
for the planetesimal hypothesis would be somewhat stronger.
A remarkable fact concerning the Sun is that the equatorial region
rotates once around in a shorter time than the regions in higher latitudes
require. The rotation period of the Sun's equator is about 24
days; the period at latitude 45° is 28 days; and at 75°, 33 days. The
planetesimal hypothesis attributes this equatorial acceleration to the
falling back into the Sun of the materials which had been lifted out
to a short distance by the disturbing body, and to the forward-rushing
tide raised in the equatorial regions by the disturbing body. This may
well have occurred. However, we must remember that the same phenomenon
exists certainly in Jupiter and Saturn, and quite probably in
Uranus and Neptune; that is, in all the bodies in the system that are
gaseous and free to show the effect. It seems to be the result of a
principle which has operated throughout the solar system, not requiring,
at least not directly requiring, the passage of a disturbing star. I
think the most plausible explanation of this curious phenomenon is
that great quantities of materials originally revolving around the Sun
and around each of the planets have gradually been drawn into these
bodies, by preference into their equatorial areas. Such masses of
matter moving in orbits very close to these bodies must have traveled
with speeds vastly higher than the surface speeds of the bodies. To
illustrate, the rotational velocity of a particle now in the Sun's surface
at the equator is approximately 2 km. per second. A small body revolving
around the Sun close to his surface, rapidly enough to prevent
its falling quickly upon the Sun, must have a velocity of more than
400 km. per second. If, now, this small body encounters some resistance
it will fall into the Sun, and as it is traveling more than 200
times as rapidly as the solar materials into which it drops, it will both
generate heat and accelerate the rotational velocity of the surrounding
materials. In the same way the equatorial accelerations in Jupiter and
Saturn can receive simple explanation. The point is not necessarily in
opposition to the planetesimal hypothesis; but whatever the explanation,
it ought to apply to the planet as well as to the Sun.
If the spiral nebulæ have been formed in accordance with
Chamberlin and Moulton's hypothesis, the secondary nuclei in them must
revolve in a great variety of elliptic orbits. The orbits would intersect,
and in the course of long ages the separate masses would collide and
combine and the number of separate masses would constantly grow
smaller. Moulton has shown that in general the combining of two
masses whose orbits intersect causes the combined mass to move in an
orbit more nearly circular than the average orbit of the separate masses,
and in general in orbit planes more nearly coincident with the general
plane of the system. Accordingly, the major planets should move in
orbits more nearly circular and more nearly in the plane of the system
than do the asteroids; and so they do. If the asteroids should combine
to form one planet the orbit of this planet should be much less eccentric
than the average of all the present asteroid eccentricities, and the
deviation of its orbit plane should be less than the average deviation
of the present planes. We can not doubt that this would be the case.
Mercury and Mars, the smallest planets, should have, according to this
principle, the largest eccentricities and orbital inclinations of any of
the major planets. This is true of the eccentricities, but Mars's orbit
plane, contrarily, has a small inclination. Venus and the Earth, next
in size, should have the next largest inclinations and eccentricities, but
they do not; Venus's eccentricity is the smallest of all. The Earth's
orbital inclination and eccentricity are both small. Jupiter and Saturn,
Uranus and Neptune, should have the smallest orbital inclinations; their
average inclination is about the same as for Venus and the Earth.
They should likewise have the smallest eccentricities. Neptune, the
smallest of the four, has an orbit nearly circular; Jupiter, Saturn and
Uranus have eccentricities more than 4 times those of Venus and the
Earth. Considering the four large planets as one group and the four
small planets as another group, we find that the inclinations of the
orbits of the two groups, per unit mass, are about equal; but the average
eccentricity of the orbits of the large planets, per unit mass, is
21 times that of the orbits of the small
planets.
[1] The evidence, except
as to the asteroids and Mercury, is not favorable to the planetesimal
hypothesis, unless we make special assumptions as to the distribution of
materials in the spiral nebulæ.
The fact that the disturbing body drew 225 times as much matter
a great distance to form the four large planets as it drew out a short
distance to form the four small planets and the asteroids seems difficult of
explanation on the planetesimal hypothesis. However, this distribution of
matter is at present a difficulty in any of the hypotheses. The planetesimal
hypothesis explains well all west to east rotations of the planets
on their axes, but to make Uranus rotate nearly at right angles to the
plane of the system, and Neptune in a plane inclined 135° to the plane
of the system, is a difficulty in any of the hypotheses, unless special
assumptions are made to fit each case.
The authors succeed well, I think, in showing that the satellites
should prefer to revolve around their planets in the direction of the
planetary revolution and rotation, especially for close satellites, and,
on the basis of special assumptions, in the reverse direction for satellites
at a greater distance. They show that the chances favor small
eccentricities for satellites revolving about their planets in the west to
east, or direct sense, and large eccentricities for satellites moving in
retrograde directions. The inner satellite of Mars and the rings of
Saturn make no special difficulty under the planetesimal hypothesis.
The evidence of the comets, as bona fide members of the solar system
which approach the Sun almost, and perhaps quite, indifferently
from all directions, is that the volume of space occupied by the parent
structure of the system was of enormous dimensions, both at right
angles to the present principal plane of the system and in that plane.
We are accustomed to think of the spiral nebulæ as thin relatively to
their major diameters. To this extent the planetesimal hypothesis
does not furnish a good explanation of the origin of comets, unless we
assume that a small amount of matter was widely scattered in all directions
around the parent spiral; and this conception leads to some apparent
difficulties. The origin of the comets is difficult to explain
under any of the hypotheses.
RÉSUMÉ OF HYPOTHESES
Kant's hypothesis had the great defect of trying to prove too much.
It started from matter at rest, and came to grief in trying to give a
motion of rotation to the entire mass through the operation of internal
forces alone—an impossibility. Kant's idea of nuclei or centers of
gravitational attraction, scattered here and there throughout the chaotic
mass, which grew into the planets and their satellites, is very valuable.
Laplace's hypothesis had the great advantage of starting with an
extended mass already in rotation, but it violated fatally the law of
constancy of moment of momentum. We should expect this hypothesis
to create a solar system free from irregularities, very much as if it were
the product of an instrument-maker's precision lathe. The solar system
as it exists is a combination of regularities and many surprising
irregularities.
Chamberlin and Moulton's hypothesis has the advantage of a parent
mass in rotation, practically in a common plane, and with the materials
distributed at distances from the nucleus as nearly in harmony with
the known distribution of matter in the solar system as we care to have
them, except perhaps as to the comets. In effect it retains all the
advantageous qualities of Kant's proposals. It seems to have the flexibility
required in meeting the irregularities that we see in our system.
CONCERNING THE ORIGIN OF SPIRAL NEBULÆ
I think it is very doubtful whether the spiral nebulæ have in general
been formed by the close approaches of pairs of stars, as the authors
have postulated for the assumed solar spiral.[2]
The distribution of the
spirals seems to me to negative the idea. To witness the close approach
of two stars we must look in the direction where the stars are. To the
best of present-day knowledge the stars are in a spheroid whose longer
axes are coincident with the plane of the Milky Way. If this is so,
the close approach of pairs of stars should occur preeminently in the
Milky Way, and we should find the spirals prevailingly in and near the
Milky Way. This is precisely where we do not find them. In fact,
they seem to abhor the Milky Way. The new stars, which are credibly
explained as the products of collisions of stars with nebulæ, are found
preeminently in the Milky Way and almost negligibly in the regions
outside of the Milky Way. Again, the spirals are believed to be, on
the whole, of enormous size. They are too far away to let us measure
their distances by the usual methods, and they move too slowly on the
surface of the sphere to have let us determine their proper motions.
Slipher's recent work with a spectrograph seems to show that the dozen
spirals observed by him are moving with high speeds of approach and
recession; from 300 km. per second approach in the case of the Andromeda
nebula to 1,100 km. per second recession in the case of several
objects. If the spirals are moving at random their speeds at right
angles to the line of sight must be even greater than their speeds of
approach and recession. Unless they are very distant bodies their proper
motions should be detected by observations extending over only a few
years. My colleague Curtis has this year compared recent photographs
of some 25 spirals with photographs of the same object made by Keeler
fifteen years ago. They reveal no appreciable proper motions, or rotations.
In this same interval Neptune has revolved more than 30°.
Slipher has recently measured the rotational speed of one "spindle''
nebula, believed to be a spiral. He finds it to be enormously rapid; no
motions in the solar system approach it in magnitude. The evidence
is to the effect that the spirals are in general very far
away;
[3] perhaps on or beyond the confines of our
stellar system, but not certainly so.
Accordingly, we are led to believe that the spirals studied thus far have
diameters 20 times or 100 times, or in some cases several thousand times,
the diameter of our solar system. It is difficult to avoid the conclusion
that in general they are immensely more massive than is our solar
system. The spiral which has been assumed as the forerunner of our
system must have been of diminutive size as compared with the larger
and brighter spirals which we see to-day.
We are sadly in need of information concerning the constitution of
the spiral nebulæ. Their spectra appear to be prevailingly of the solar
type, except that a very small proportion contain some bright lines in
addition to the continuous spectrum. So far as their spectra are concerned,
they may be great clusters of stars, or they may consist each of
a central star sending its light out upon surrounding dark materials
and thus rendering these materials visible to us. The first alternative
is unsatisfactory, for all parts of spirals have hazy borders, as if the
structure is nebulous or consists of irregular groups of small masses;
and the second alternative is unsatisfactory, for in many spirals the
most outlying masses seem to be as bright as masses of the same areas
situated only one half as far from the center, whereas in general the
inner area should be at least four times as bright as the outer area.
All astronomers are ready to confess that we do not know much about
the conditions existing in spiral nebulæ.
THE EARTH-MOON SYSTEM
Our Earth and Moon form a unique combination in that they are
more nearly of the same size than are any other planet and its satellites
in our system. It required a 26-inch telescope on the Earth to discover
the tiny moons of Mars; but an astronomer on Mars does not need any
telescope to see the Earth and Moon as a double planet—the only
double planet in the solar system.
According to the Kantian school of hypotheses the Earth and Moon
owe their unique character to the accident that two centers of
condensation—two nuclei—not very unequal in mass, were formed close
to each other and were endowed with or acquired motions such that
they revolved around each other. They drew in the surrounding materials;
one of the two bodies got somewhat the advantage of the other
in gravitational attraction; it succeeded in building itself up more
than the other nucleus did; and the Earth and the Moon were the result.
According to the Laplacean hypothesis, on the contrary, the Earth
and Moon were originally one body, gaseous and in rotation. This
ball of gas radiated heat, diminished in size, rotated more and more
rapidly, and finally abandoned a ring of nebulosity, which later broke
up and eventually condensed into one mass called the Moon. The central
mass composed the Earth. It is a curious fact that Venus, which
is only a shade smaller than the Earth, should not have divided into
two bodies comparable with the Earth and Moon. Have the tides on
Venus produced by the Sun always been strong enough to keep the rotation
and revolution periods equal, as they are thought to be now,
and thus to have given no opportunity for a rapidly rotating Venus
to divide into two masses?
A third hypothesis of the Moon's origin is due principally to Darwin.
He and Poincaré have shown that a great rotating mass of fluid
matter, such as the Earth-Moon could be assumed to have been, by
cooling, contracting and increasing rotation speed, would, under certain
conditions thought to be reasonable, become unstable and eventually
divide into two bodies revolving around their common center
of mass, at first with their surfaces nearly in contact. Here would
begin to act a tide-raising force which must have played, according to
Darwin's deductions, a most important part in the further history of
the Earth and Moon. The Earth would produce enormous tides in the
Moon, and the Moon much smaller tides in the Earth. Both bodies
would contract in size, through loss of heat, and would try to rotate
more and more rapidly. The two rotating bodies would try to carry
the matter in the tidal waves around with the rest of the materials
in the bodies, but the pull of each body upon the wave materials in the
other would tend to slow down the speed of rotation. The tidal resistance
to rotation would be slight if the bodies at any time were attenuated
gaseous masses, for the friction within the surface strata
would be slight. Nevertheless, there would eventually be a gradual
slowing down of the Moon's rotation, a gradual slowing down of the
Earth's rotation, and a slow increase in the distance between the two
bodies. In other words, the Moon's day, the Earth's day and our
month would gradually increase in length. Carried to its logical
conclusion, the Moon would eventually turn the same face to the Earth,
the Earth would eventually turn the same face to the Moon, and the
Earth's day and the Moon's day would equal the month in length. The
central idea in this logic is as old as Kant: in 1754 he published an
important paper in which he said that tidal interactions between Earth
and Moon had caused the Moon to keep the same face turned toward
us, that the Earth's day was being very slowly lengthened, and that our
planet would eventually turn the same face to the Moon. Laplace, a
half-century later, proposed the action of such a force in connection
with the explanation of lunar phenomena, and Helmholtz, just 100
years after Kant's paper was published, lent his support to this principle;
but Sir George Darwin has been the great contributor to the subject.
His popular volume, "The Tides,'' devotes several chapters to
the effects of tidal friction upon the motions of two bodies in mutual
revolution. We must pass over the difficult and complicated intermediate
steps to Darwin's conclusions concerning the Earth and Moon,
which are substantially as follows: the Earth and Moon were originally
much closer together than they now are: after a very long period of
time, amounting to hundreds of millions of years, the Moon will revolve
around the Earth in 55 days instead of in 27 days as at present;
and the Moon and Earth will then present the same faces constantly
to each other. The estimated period of time required, and the final
length of day and month, 55 days, are of course not insisted upon as
accurate by Darwin.
These tidal forces were unavoidably active, it matters not if the
Earth and Moon were originally one body, as Laplace and Darwin have
postulated, or originally two bodies, growing up from two nuclei, in
accordance with the Kantian school. Whether these forces have been
sufficiently strong to have brought the Earth and Moon to their present
relation, or will eventually equalize the Moon's day, the Earth's day,
and the month, is a vastly more difficult question. Moulton's researches
have cast serious doubt upon this conclusion. All such investigations
are enormously difficult, and many questionable assumptions
must be made if we seek to go back to the Moon's origin, or forward
to its ultimate destiny.
Tidal waves, in order to be effective in reducing the rotational speed
of a planet, must be accompanied by internal friction; and this requires
that the planet be to some extent inelastic. It was the view of Darwin
and others that the viscous state of the Earth and Moon permitted
wave friction to come into play. Michelson has recently proved that
the Earth has a high degree of elasticity. It deforms in response to
tidal forces, but quickly recovers from the action of these forces. It
therefore seems that the rate of tidal evolution of the Earth-Moon system
at present and in the future must be extremely slow, and possibly
almost negligible. What the conditions within the Earth and Moon
were in the distant past is uncertain, but these bodies probably passed
through viscous stages which endured through enormously long periods
of time. No one seriously doubts that Jupiter, Saturn, Uranus and
Neptune are now largely gaseous, and that they will evolve, through
various degrees of viscosity, into the solid and comparatively elastic
state. It is natural to assume that the Earth has already passed
through an analogous experience.
The Moon turns always the same hemisphere toward the Earth.
Observations of Venus and Mercury are prevailingly to the effect that
those planets always turn the same hemispheres toward the Sun.
Many, and perhaps all, of the satellites of Jupiter and Saturn seem
to turn the same hemispheres always toward their respective planets.
This widely prevailing phenomenon is no doubt due to a widely prevailing
cause, which astronomers have all but unanimously attributed
to tidal action.
BINARY STAR SYSTEMS
That an original mass actually divided to form the Earth and
Moon, according to the Laplacian or the Darwin-Poincaré principle,
seems to be extremely doubtful, especially on account of their diminutive
sizes, and I greatly prefer to think that the Earth and Moon were
built up from two nuclei; but that very much greater masses, masses
larger on the average than our Sun, composing highly attenuated stars,
have divided each into two masses to form many or most of our double
stars, I firmly believe. The two component stars would in such a case
at first revolve around each other with their surfaces almost or quite
in contact. Tidal forces would very gradually cause the bodies to
move in orbits of larger and larger size, with correspondingly longer
periods of revolutions, and the orbits would become constantly more
eccentric. While these processes were under way the component bodies
would be radiating heat and growing smaller, and their spectra would
be changing into the more advanced types. We can not hope to watch
such changes as they occur, but we can, I think, find abundant illustrations
of these processes in the double stars. I have given reasons
for believing that one star in every two and one half, as a minimum
proportion, is not the single star which it appears to be to the eye or in
the telescope, but is a system of two or more suns in mutual revolution.
The formation of double stars, therefore, is not a sporadic process: it is
one of the straightforward results of the evolutionary process.
Some of the variable stars offer strong evidence as to the early life
of the double stars. The so-called β Lyræ variables vary continuously
in brightness, as if they consist in each case of two stars so close together
that their surfaces are actually in contact in some pairs and
nearly in contact in others, so that from our point of view the two
stars mutually eclipse each other. When the two stars are in line with
us we have minimum brightness. When they have moved a quarter-revolution
farther, and the line joining them is at right angles to our
line of sight, so to speak, we have maximum brightness. In every
known case the β Lyræ pairs of stars have spectra of the very early
types. Some of them even contain bright lines in their spectra. The
densities of these great stars are known to be exceedingly low, in some
cases much lower on the average than that of the atmosphere which we
breathe.
About 80 Algol variable stars are known. These are double stars
whose light is constant except during the short time when one of the
components in each system passes between us and the other component.
All double stars would be Algol variables if we were exactly in the
planes of their orbits. That so few Algols have been observed amongst
the tens of thousands of double stars, is easily explained. The two
component stars in the few known Algol systems are so great in diameter,
in proportion to the size of their orbits, that eclipses are observable
throughout a wide volume of space, and the eclipses are of long
duration relatively to the revolution period. Their densities are, so
far as we have been able to determine them, on an average less than
1/10th of the Sun's density. Let us note well that their spectra, so
far as we have been able to determine them, are of the early types;
mostly helium and hydrogen stars, and a very few of the Class F,
intermediate between the hydrogen and solar stars. There are no known
Algols of the Classes G, K, and M: these stars are very condensed and
therefore small in size, as compared with stars of Classes B and A; and
the components of double stars of these classes are on the average much
denser and therefore smaller in size than the components in Classes B
and A double stars; the components are much farther apart in Classes
G to M doubles than in Classes B and A doubles; and for these reasons
eclipses in Classes G to M doubles occur but rarely for observers scattered
throughout space. It is difficult to avoid the conclusion that
the components of double stars separate more and more widely with
the progress of time. The conclusions which we have earlier drawn
from visual double stars are in full harmony with the argument.
It is agreed by all, I think, that tidal action has been responsible
for at least a part of the separation of the Earth and Moon, for at least
a part of the gradual separation of the components of double stars, and
for at least a part of the eccentricity of their orbits. See's investigations
of 25 years ago led him to the conclusion that this force is sufficient to
account for all the observed separation of the components of double
stars, and for the well-known high eccentricities of their orbits. In
recent years Moulton and Russell have seriously questioned the sufficiency
of this force to account for the major part of the separation
and eccentricity in the double star systems. I think, however, that if
the tidal force is not competent to account for the observed facts as
described, some other separating force or forces must be found to supply
the deficiency.
THE FORMATION OF THE EARTH
Does the condition of the Earth's interior give evidence on the question
of its origin? There are certain important facts which bear upon
the problem.
1. The evidence supplied by the volcanoes, by the hot springs, and
by the rise in temperature as we go down in all deep mines, is unmistakably
to the effect that there is an immense quantity of heat in the
Earth's interior. Near the surface the temperature increases at the
average of 1° Centigrade for every 30 meters of depth. If this rate
were maintained we should at 60 km. in depth arrive at a temperature
high enough to melt platinum, the most refractory of the known metals.
What the law of temperature-increase at great depths is we do not know,
but the temperature of the Earth's deep interior must be very high.
2. The pressures in the Earth increase from zero at the surface to
the order of 3,000,000 atmospheric pressures at the center. We know
that rock structure, or iron or other metals, can be slightly compressed
by pressure, but the experiments at very high pressures, notably those
conducted by Bridgman, give no indications that matter under such
pressures breaks down and obeys different or unknown laws. It should
be said, however, that laboratory pressure-effects alone are not a safe
guide as to conditions within the Earth, where high pressures are
accompanied by high temperature. Unfortunately it has not been found
possible to combine the high-temperature factor with the high-pressure
factor in the laboratory experiments. It is well known that the melting
points of metals, including rocks, increase with increase of pressure;
and although the temperatures in the Earth's interior are very high, it
is easy to conceive that the materials of the Earth's interior are
nevertheless in the solid state, or that they act like solids, because of the
high pressures to which they are subjected.
3. The specific gravity of the entire Earth is 5.5 on the scale of
water as one, whereas the density of the stratified rocks averages only
2.75; that is, the stratified rocks have but one half the density of the
Earth as a whole. The basaltic rocks underlying the stratified attain
occasionally the density 3.1, and perhaps a little higher. It follows
absolutely that the density of the materials of the Earth's interior must
be considerably in excess of 5.5. If the interior is composed chiefly
of substances which are plentiful in the Earth's surface strata, our
choice of materials which principally compose the interior is reduced
to a few elements, notably the denser ones.
4. The observed phenomena of terrestrial precession can not be explained
on the basis of an Earth with a thin solid surface shell and a
liquid interior, for the attractions of the Moon and Sun upon the
Earth's equatorial protuberance would cause the surface shell to shift
over the fluid interior, instead of swinging the entire Earth.
5. If the Earth consisted of a thin solid shell upon a liquid interior
there would be tides in the liquid interior, the crust would yield to these
tides almost as if it were composed of rubber, and the ocean tides would
be only an insignificant amount larger than the land tides. As a result
we should not see the ocean tides; their visibility depends upon the
contrast between the ocean tides and the land tides. If the Earth were
absolutely unyielding from surface to center the ocean tides would be
relatively 50 per cent. higher than we now see them. The conclusion
from these facts is that the Earth yields to the tidal forces a little less
than if it were a solid ball of steel, supposing that the well-known
rigidity and density existed from surface to center of the ball. This
result is established by Darwin's and Schweydar's studies of ocean
tides, by studies of the tides in the Earth's surface strata made by
Hecker, Paschwitz and others, and by Michelson's recent extremely
accurate comparison of land and water tides. Michelson's results establish
further that the Earth is highly elastic: though distortion is resisted,
there is yielding, but the original form is recovered quickly,
almost as quickly as a perfectly elastic body would recover.
6. Some 25 years ago it was discovered by Küstner that the
latitudes of points on the Earth's surface are changing slowly. Chandler
proved that these variations pass through their principal cycle in a
period of 427 days. The entire Earth oscillates slightly in this period.
The earlier researches of Euler had shown that the Earth would have
a natural oscillation period of 305 days provided it were an absolutely
rigid body. Newcomb showed that the period of oscillation would be
441 days if the Earth had the rigidity of steel. As the observed oscillation
requires 427 days, Newcomb concluded that the Earth is slightly
more rigid than steel.
7. The first waves from a very distant earthquake come to us directly
through the Earth. The observed speeds of transmission are the
greater, in general, the more nearly the earthquake origin is exactly on
the opposite side of the Earth from the observer; that is, the speeds of
transmission are greater the nearer the center of the Earth the waves
pass. Now, we know that the speeds are functions of the rigidity and
density of the materials traversed. The observed speeds require for
their explanation, so far as we can now see, that the rigidity of the
Earth's central volume be much greater than that of steel, and the
rigidity of the Earth's outer strata considerably less than that of steel.
Wiechert has shown that a core of radius 4,900 km. whose rigidity is
somewhat greater than that of steel and whose average density is 8.3,
overlaid by an outer stony shell of thickness 1,500 km. and average
density 3.2, would satisfy the observed facts as to the average density of
the Earth, as to the speeds of earthquake waves, as to the flattening of
the Earth,—assuming the concentric strata to be homogeneous in themselves,—
and as to the relative strengths of gravity at the Poles and at
the Equator. The dividing line, 1,500 km. below the surface—1,600
km. would be just one fourth of the way from the surface to the center—
places a little over half the volmue in the outer shell and a little less
than half in the core. Wiechert did not mean that there must be a
sudden change of density at the depth of 1,500 km., with uniform
density 8.3 below that surface and uniform density 3.2 above that surface.
The change of density is probably fairly continuous. It was
necessary in such a preliminary investigation to simplify the assumptions.
The observational data are not yet sufficiently accurate to let us
say what the law of increase in density and rigidity is as we pass from
the surface to the center.
8. The phenomena of terrestrial magnetism indicate that the distribution
of magnetic materials in the Earth is far from uniform or
symmetrical; the magnetic poles are distant from the Earth's poles of
rotation; the magnetic poles are not opposite each other; the lines of
equal intensity as to all the magnetic components involved run very
irregularly over the Earth's surface. There is reason to believe that
iron in the deep interior of the Earth, in view of its high temperature,
is devoid of magnetic properties, but we must not state this as a fact.
We know that iron is very widely, but very irregularly spread throughout
the Earth's outer strata. Whatever may be the main factors in
making the Earth a great magnet, to whatever extent the rotation factor
may be important, the Earth's magnetic properties point strongly to a
very irregular distribution of magnetic materials in the outer strata
where the temperatures are below that at which magnetic materials
commonly lose their polarity.
9. Irregularities in the direction of the plumb-line and in the force
of gravity as observed widely and accurately over the Earth's surface
indicate that the surface strata are very irregular as to density. To
harmonize the observed facts Hayford has shown the need of assuming
that the heterogeneous conditions extend down to a depth of 122 km.
from the surface. Below that level the Earth's concentric strata seem
to be of approximately uniform densities.
10. The radio active elements have been found by Strutt and others
in practically all kinds of rock accessible to the geologists, but they are
not found in significant quantities in the so-called metals which exist
in a pure state. These radioactive elements are liberating heat.
Strutt has shown that if they existed down to the Earth's center in the
same proportion that he finds in the surface strata they would liberate
a great deal more heat than the body of the Earth is now radiating to
outer space. The conclusion is that they are restricted to the strata
relatively near the Earth's surface, and are not in combination with the
materials composing the Earth's core. They have apparently found
some way of coming to the higher levels. Chamberlin suggests that as
they liberate heat they would raise surrounding materials to temperatures
above the normals for their strata, and that these expanded materials
would embrace every opportunity to approach the surface of the
Earth, carrying the radioactive substances with them.
The evidence is exceedingly strong, and perhaps irresistible, to the
effect that the Earth is now solid, or acts like a solid, from surface to
center, with possibly local, but on the whole negligible, pockets of molten
matter here and there; and further, that the Earth existed in a molten,
or at the least a thickly plastic, state throughout a long part of its
life. The nucleus, whether gaseous or meteoric, from which I believe
it has grown, may have been fairly hot or quite cold, and the materials
which were successively drawn into the nucleus may have been hot or
cold: heat would be generated by the impacts of the incoming materials;
and as the attraction toward the center of the mass became strong,
additional heat would be generated in the contraction process. The denser
materials have been able, on the whole, to gravitate to the center of the
structure, and the lighter elements have been able, on the whole, to rise
to and float upon the surface very much as the lighter impurities in an
iron furnace find their way to the surface and form the slag upon the
molten metal. The lighter materials which in general form the surface
strata are solid under the conditions of solids known to us in every-day
life. The interior is solid or at least acts as a solid, because the
materials, though at high temperatures, are under stupendous pressures.
If the pressures were removed the deep-lying materials would quickly
liquefy, and probably even vaporize.
If the Earth grew from a small nucleus to its present size by the
extremely gradual drawing-in of innumerable small masses in its neighborhood,
the process would always be slow; much slower at first when
the small nucleus had low gravitating powers, more rapid when the
body was of good size and the store of materials to draw upon plentiful,
and gradually slower and slower as the supply of building materials
was depleted. Meteoric matter still falls upon and builds up the Earth,
but at so slow a rate as to increase the Earth's diameter an inch only
after the passage of hundreds of millions of years. If the Earth grew
in this manner, the growth may now be said to be essentially complete,
through the substantial exhaustion of the supply of materials.
Whether the Earth of its present size was ever completely liquefied,
that is, from center to surface, at one and the same time, is doubtful.
The lack of homogeneity, as indicated by the plumb-line, gravity,
terrestrial magnetism and radiaoctive matter, extending in a perceptible
degree down to 122 km., and quite probably in lesser and imperceptible
degree to a much greater depth, is opposed to the idea.
Solidification would respond to the fall of temperature down to the
point required under the existing high pressures, and it is probable
that the solidification began at the center and proceeded outwards. It
is natural that the plastic state should have developed and existed
especially during the age of most rapid growth, for this would be the age
of most rapid generation of heat. Later, while the rate of growth was
declining, the body could probably have solidified slowly and successively
from center out to surface. In later slow depositions of materials,
the denser substance would not be able to sink down to the
deepest strata: they must lie within a limited depth and horizontal distance
from where they fell, and the outer stratum of the Earth would be
heterogeneous in density.
The simplest hypothesis we can make concerning the Earth's deep
interior is that the chief ingredient is iron; perhaps a full half of the
volume is iron. The normal density of iron is 7.8, and of rock formations
about 2.8. If these are mixed, half and half, the average density
is 5.3. Pressures in the Earth should increase the density and the heat
in the Earth should decrease the density. The known density of the
Earth is 5.5. We know that iron is plentiful in the Earth's crust, and
that iron is still falling upon the Earth in the form of meteorites.
The composition of the Earth as a whole, on this assumption, is very
similar to the composition of the meteorites in general. They include
many of the metals, but especially iron, and they include a large proportion
of stony matter. Iron is plentiful in the Sun and throughout
the stellar universe. Why should it not be equally plentiful in the
materials which have coalesced to form the Earth? It is difficult to
explain the Earth's constitution on any other hypothesis.
The Earth's form is that which its rotation period demands. Undoubtedly
if the period has changed, the form has changed. Given a
little time, solids under great pressure flow quite readily into new forms.
Now any great slowing-down of the Earth's rotation period within
geological times would be expected to show in the surface features. The
strata should have wrinkled, so to speak, in the equatorial regions and
stretched in the polar regions, if the Earth changed from a spheroid that
was considerably flatter than it now is, to its present form. Mountains,
as evidence of the folding of the rock strata, should exist in profusion
in the torrid zone, and be scarce in or absent from the higher
latitudes of the Earth. Such differential effects do not exist, and it
seems to follow that changes in the Earth's rotation period and in its
form could have been only slight while the stratification of our rocks
was in progress.
Geologists estimate from the deposition of salt in the oceans, and
from the rates of denudation and sedimentation, that the formation of
the rock strata has consumed from 60,000,000 to 100,000,000 years. If
the Earth had substantially its present form 80,000,000 years ago we
are safe in saying that the period of time represented in the building
up of the Earth from a small nucleus to its present dimensions has
been vastly longer, probably reckoned in the thousands of millions of
years.
For more than a century past the problem of the evolution of the
stars, including the solar system and the Earth, has occupied the central
place in astronomical thought. No one is bold enough to say that the
problem has been solved. The chief difficulty proceeds from the fact
that we have only one Earth, one solar system and one stellar system
available for tests of the hypotheses proposed; we should like to test
them on many systems, but this privilege is denied us. However, the
search for the truth will undoubtedly proceed at an ever increasing
pace, partly because of man's desire to know the truth, but chiefly, as
Lessing suggested, because the investigator finds an irresistible satisfaction
in the process. There is always with him the certainty that the
truth is going to be incomparably stranger and more interesting than
fiction.
[1.]
The average eccentricity of the orbits of the four
inner planets (per unit mass) is 0.0221, and of the four outer planets
is 0.0489.
[2.]
It would seem that all rotating nebulæ should in
reality possess some of the attributes of spiral motion. Whether the
spiral structure should be visible or invisible to a terrestrial
observer would depend upon the sizes and distances of the nebulæ,
upon the distribution of materials composing them, and perhaps upon
other factors. See developed the hypothesis that spiral nebulæ owe
their origin to the collision of two nebulæ. Collisions of this
kind could readily occur because of the enormous dimensions of the
nebulæ, and motions of rotation and consequently spiral structure
might readily result therefrom. The abnormally high speeds of the spiral
nebulæ are apparently a very strong objection to the
hypothesis.
[3.]
Bohlin found a parallax of 0"17 for the Andromeda Nebula,
and Lampland thinks that Nebula N.G.G. 4594 has a proper motion of approximately 0"05
per annum.