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conditions would be if they arose at the surface of our own
planet. We assume a horizontal force in the crust adequate to set
up tensile stresses of the order, say, of fifteen tons to the
square foot and these stresses to be repeated every few hours;
our world being also subject to the dynamic effects we recognise
in and beneath its crust.
It is easy to see that the areas over which the satellite exerted
its gravitational stresses must become the foci --foci of linear
form--of tectonic developments or crust movements. The relief of
stresses, from whatever cause arising, in and beneath the crust
must surely take place in these regions of disturbance and along
these linear areas. Here must become concentrated the folding
movements, which are under existing conditions brought into the
geosynclines, along with their attendant volcanic phenomena. In
the case of Mars such a concentration of tectonic events would
not, owing to the absence of extensive subaerial denudation and
great oceans, be complicated by the existence of such synclinal
accumulations as have controlled terrestrial surface development.
With the passage of time the linear features would probably
develop; the energetic substratum continually asserting its
influence along such lines of weakness. It is in the highest
degree probable that radioactivity plays no less a part in
Martian history than in terrestrial. The fact of radioactive
heating allows us to assume the thin surface crust and continued
sub-crustal energy throughout the entire period of the planet's
history.
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How far willl these effects resemble the double canals of Mars?
In this figure and in the calculations I have given you I have
supposed the satellite engaged in marking the planet's surface
with two lines separated by about the interval separating the
wider double canals of Mars--that is about 220 miles apart. What
the distance between the lines will be, as already stated, will
depend upon the height of the satellite above the surface when it
comes upon a part of the crust in a condition to be affected by
the stresses it sets up in it. If the satellite does its work at
a point lower down above the surface the canal produced will be
narrower. The stresses, too, will then be much greater. I must
also observe that once the crust has yielded to the pulling
stress, there is great probability that in future revolutions of
the satellite a central fracture will result. For then all the
pulling force adds itself to the lifting force and tends to crush
the crust inwards on the central line beneath the satellite. It
is thus quite possible that the passage of a satellite may give
rise to triple lines. There is reason to believe that the canals
on Mars are in some cases triple.
I have spoken all along of the satellite moving slowly over the
surface of Mars. I have done so as I cannot at all pronounce so
readily on what will happen when the satellite's velocity over
the surface of Mars is very great. To account for all the lines
mapped by Lowell some of them must have been produced by
satellities moving relatively to the surface of Mars at
velocities so great
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as three miles a second or even rather more. The stresses set up
are, in such cases, very difficult to estimate. It has not yet
been done. Parallel lines of greatest stress or impulse ought to
be formed as in the other case.
I now ask your attention to another kind of evidence that the
lines are due in some way to the motion of satellites passing
over the surface of Mars.
I may put the fresh evidence to which I refer, in this way: In
Lowell's map (P1. XXII, p. 192), and in a less degree in
Schiaparelli's map (ante p. 166), we are given the course of the
lines as fragments of incomplete curves. Now these curves might
have been anything at all. We must take them as they are,
however, when we apply them as a test of the theory that the
motion of a satellite round Mars can strike such lines. If it can
be shown that satellites revolving round Mars might strike just
such curves then we assume this as an added confirmation of the
hypothesis.
We must begin by realising what sort of curves a satellite which
disturbs the surface of a planet would leave behind it after its
demise. You might think that the satellite revolving round and
round the planet must simply describe a circle upon the spherical
surface of the planet: a "great circle" as it is called; that is
the greatest circle which can be described upon a sphere. This
great circle can, however, only be struck, as you will see, when
the planet is not turning upon its axis: a condition not likely
to be realised.