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 whe