s, 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

179

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.

180

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