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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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