We have reviewed the theory of the artificial construction of the
Martian lines. The amount of consideration we are disposed to
give to the supposition that there are upon Mars other minds than
ours will--as I have stated--necessarily depend upon whether or not
we can assign a probable explanation of the lines upon purely
physical grounds. If it is apparent that such
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lines would be formed with great probability under certain
conditions, which conditions are themselves probable, then the
argument by exclusion for the existence of civilisation on Mars,
at once breaks down.
{Fig. 10}
As a romance writer is sometimes under the necessity of
transporting his readers to other scenes, so I must now ask you
to consent to be transported some millions
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of miles into the region of the heavens which lies outside Mars'
orbit.
Between Mars and Jupiter is a chasm of 341 millions of miles.
This gap in the sequence of planets was long known to be quite
out of keeping with the orderly succession of worlds outward from
the Sun. A society was formed at the close of the last century
for the detection of the missing world. On the first day of the
last century, Piazzi--who, by the way, was not a member of the
society--discovered a tiny world in the vacant gap. Although
eagerly welcomed, as better than nothing, it was a disappointing
find. The new world was a mere rock. A speck of about 160 miles
in diameter. It was obviously never intended that such a body
should have all this space to itself. And, sure enough, shortly
after, another small world was discovered. Then another was
found, and another, and so on; and now more than 400 of these
strange little worlds are known.
But whence came such bodies? The generally accepted belief is
that these really represent a misbegotten world. When the Sun was
younger he shed off the several worlds of our system as so many
rings. Each ring then coalesced into a world. Neptune being the
first born; Mercury the youngest born.
After Jupiter was thrown off, and the Sun had shrunk away inwards
some 20o million miles, he shed off another ring. Meaning that
this offspring of his should grow up like the rest, develop into
a stable world with the
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potentiality even, it may be, of becoming the abode of rational
beings. But something went wrong. It broke up into a ring of
little bodies, circulating around him.
It is probable on this hypothesis that the number we are
acquainted with does not nearly represent the actual number of
past and present asteroids. It would take 125,000 of the biggest
of them to make up a globe as big as our world. They, so far as
they are known, vary in size from 10 miles to 160 miles in
diameter. It is probable then--on the assumption that this failure
of a world was intended to be about the mass of our Earth--that
they numbered, and possibly number, many hundreds of thousands.
Some of these little bodies are very peculiar in respect to the
orbits they move in. This peculiarity is sometimes in the
eccentricity of their orbits, sometimes in the manner in which
their orbits are tilted to the general plane of the ecliptic, in
which all the other planets move.
The eccentricity, according to Proctor, in some cases may attain
such extremes as to bring the little world inside Mars' mean
distance from the sun. This, as you will remember, is very much
less than his greatest distance from the sun. The entire belt of
asteroids--as known--lie much nearer to Mars than to Jupiter.
As regards the tilt of their orbits, some are actually as much as
34 degrees inclined to the ecliptic, so that in fact they are
seen from the Earth among our polar constellations.
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From all this you see that Mars occupies a rather hot comer in
the solar system. Is it not possible that more than once in the
remote past Mars may have encountered one of these wanderers? If
he came within a certain distance of the small body his great
mass would sway it from its orbit, and under certain conditions
he would pick up a satellite in this manner. That his present
satellites were actually so acquired is the suggestion of Newton,
of Yale College.
Mars' satellites are indeed suspiciously and most abnormally
small. I have not time to prove this to you by comparison with
the other worlds of the solar system. In fact, they were not
discovered till 1877--although they were predicted in a most
curious manner, with the most uncannily accurate details, by
Swift.
One of these bodies is about 36 miles in diameter. This is
Phobos. Phobos is only 3.700 miles from the surface of Mars. The
other is smaller and further off. He is named Deimos, and his
diameter is only 10 miles. He is 12,500 miles from Mars' surface.
With the exception of Phobos the next smallest satellite known in
the solar system is one of Saturn's--Hyperion; almost 800 miles in
diameter. The inner one goes all round Mars in 7½ hours. This is
Phobos' month. Mars turns on his axis in 24 hours and 40 minutes,
so that people in Mars would see the rise of Phobos twice in the
course of a day and night; lie would apparently cross the sky
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going against the other satellite; that is, he would move
apparently from west to east.
We may at least assume as probable that other satellites have
been gathered by Mars in the past from the army of asteroids.
Some of the satellites so picked up would be direct: that is,
would move round the planet in the direction of his axial
rotation. Others, on the chances, would be retrograde: that is,
would move against his axial rotation. They would describe orbits
making the same various angles with the ecliptic as do the
asteroids; and we may be sure they would be of the same varying
dimensions.
We go on to inquire what would be the consequence to Mars of such
captures.
A satellite captured in this manner is very likely to be pulled
into the Planet. This is a probable end of a satellite in any
case. It will probably be the end of our satellite too. The
satellite Phobos is indeed believed to be about to take this very
plunge into his planet. But in the case when the satellite picked
up happens to be rotating round the planet in the opposite
direction to the axial rotation of the planet, it is pretty
certain that its career as a satellite will be a brief one. The
reasons for this I cannot now give. If, then, Mars picked up
satellites he is very sure to have absorbed them sooner or later.
Sooner if they happened to be retrograde satellites, later if
direct satellites. His present satellites are recent additions.
They are direct.
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The path of an expiring satellite will be a slow spiral described
round the planet. The spiral will at last, after many years,
bring the satellite down upon the surface of the primary. Its
final approach will be accelerated if the planet possesses an
atmosphere, as Mars probably does. A satellite of the dimensions
of Phobos--that is 36 miles in diameter--would hardly survive more
than 30 to 60 years within seventy miles of Mars' surface. It
will then be rotating round Mars in an hour and forty minutes,
moving, in fact, at the rate of 2.2 miles per second. In the
course of this 30 or 60 years it will, therefore, get round
perhaps 200,000 times, before it finally crashes down upon the
Martians. During this closing history of the satellite there is
reason to believe, however, that it would by no means pursue
continually the same path over the surface of the planet. There
are many disturbing factors to be considered. Being so small any
large surface features of Mars would probably act to perturb the
orbit of the satellite.
The explanation of Mars' lines which I suggest, is that they were
formed by the approach of such satellites in former times. I do
not mean that they are lines cut into his surface by the actual
infall of a satellite. The final end of the satellite would be
too rapid for this, I think. But I hope to be able to show you
that there is reason to believe that the mere passage of the
satellite, say at 70 miles above the surface of the planet, will,
in itself, give rise to effects on the crust of the planet
capable
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of accounting for just such single or parallel lines as we see.
In the first place we have to consider the stability of the
satellite. Even in the case of a small satellite we cannot
overlook the fact that the half of the satellite near the planet
is pulled towards the planet by a gravitational force greater
than that attracting the outer half, and that the centrifugal
force is less on the inner than on the outer hemisphere. Hence
there exists a force tending to tear the satellite asunder on the
equatorial section tangential
{Fig. 11}
to the planet's surface. If in a fluid or plastic state, Phobos,
for instance, could not possibly exist near the planet's surface.
The forces referred to would decide its fate. It may be shown by
calculation, however, that if Phobos has the strength of basalt
or glass there would remain a considerable coefficient of safety
in favour of the satellite's stability; even when the surfaces of
planet and satellite were separated by only five miles.
We have now to consider some things which we expect will happen
before the satellite takes its final plunge into the planet.
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This diagram (Fig. 11) shows you the satellite travelling above
the surface of the planet. The satellite is advancing towards, or
away from, the spectator. The planet is supposed to show its
solid crust in cross section, which may be a few miles in
thickness. Below this is such a hot plastic magma as we have
reason to believe underlies much of the solid crust of our own
Earth. Now there is an attraction between the satellite and the
crust of the planet; the same gravitational attraction which
exists between every particle of matter in the universe. Let us
consider how this attraction will affect the planet's crust. I
have drawn little arrows to show how we may consider the
attraction of the satellite pulling the crust of the planet not
only upwards, but also pulling it inwards beneath the satellite.
I have made these arrows longer where calculation shows the
stress is greater. You see that the greatest lifting stress is
just beneath the satellite, whereas the greatest stress pulling
the crust in under the satellite is at a point which lies out
from under the satellite, at a considerable distance. At each
side of the satellite there is a point where the stress pulling
on the crust is the greatest. Of the two stresses the lifting
stress will tend to raise the crust a little; the pulling stress
may in certain cases actually tear the crust across; as at A and
B.
It is possible to calculate the amount of the stress at the point
at each side of the satellite where the stress is at its
greatest. We must assume the satellite to be a certain size and
density; we must also assume the crust of
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Mars to be of some certain density. To fix our ideas on these
points I take the case of the present satellite Phobos. What
amount of stress will he exert upon the crust of Mars when he
approaches within, say, 40 miles of the planet's surface? We know
his size approximately--he is about 36 miles in diameter. We can
guess his density to be between four times that of water and
eight times that of water. We may assume the density of Mars'
surface to be about the same as that of our Earth's surface, that
is three times as dense as water. We now find that the greatest
stress tending to rend open the surface crust of Mars will be
between 4,000 and 8,000 pounds to the square foot according to
the density we assign to Phobos.
Will such a stress actually tear open the crust? We are not able
to answer this question with any certainty. Much will depend upon
the nature and condition of the crust. Thus, suppose that we are
here (Fig. 12) looking down upon the satellite which is moving
along slowly relatively to Mars' surface, in the direction of the
arrow. The satellite has just passed over a weak and cracked part
of the planet's crust. Here the stress has been sufficient to
start two cracks. Now you know how easy it is to tear a piece of
cloth when you go to the edge of it in order to make a beginning.
Here the stress from the satellite has got to the edge of the
crust. It is greatly concentrated just at the extremities of the
cracks. It will, unler such circumstances probably carry on the
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tear. If it does not do so this time, remember the satellite will
some hours later be coming over the same place again, and then
again for, at least, many hundreds of times. Then also we are not
limited to the assumption that the
{Fig. 12}
satellite is as small as Phobos. Suppose we consider the case of
a satellite approaching Mars which has a diameter double that of
Phobos; a diameter still much less than that of the larger class
of asteroids. Even at the distance
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of 65 miles the stress will now amount to as much as from 15 to
30 tons per square foot. It is almost certain that such a stress
repeated a comparatively few times over the same parts of the
planet's surface would so rend the crust as to set up lines along
which plutonic action would find a vent. That is, we might expect
along these lines all the phenomena of upheaval and volcanic
eruption which give rise to surface elevations.
The probable effect of a satellite of this dimension travelling
slowly relatively to the surface of Mars is, then, to leave a
very conspicuous memorial of his presence behind him. You see
from the diagram that this memorial will consist o: two parallel
lines of disturbance.
The linear character of the gravitational effects of the
satellite is due entirely to the motion of the satellite
relatively to the surface of the planet. If the satellite stood
still above the surface the gravitational stress in the crust
would, of course, be exerted radially outwards from the centre of
the satellite. It would attain at the central point beneath the
satellite its maximum vertical effect, and at some radial
distance measured outwards from this point, which distance we can
calculate, its maximum horizontal tearing effect. When the
satellite moves relatively to the planet's crust, the horizontal
tearing force acts differently according to whether it is
directed in the line of motion or at right angles to this line.