Part 29

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.

In the direction of motion we see that the satellite

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creates as it passes over the crust a wave of rarefaction or tension as at D, followed by compression just beneath the satellite and by a reversed direction of gravitational pull as the satellite passes onwards. These stresses rapidly replace one another as the satellite travels along. They are resisted by the inertia of the crust, and are taken up by its elasticity. The nature of this succession of alternate compressions and rarefactions in the crust possess some resemblance to those arising in an earthquake shock.

If we consider the effects taking place laterally to the line of motion we see that there are no such changes in the nature of the forces in the crust. At each passage of the satellite the horizontal tearing stress increases to a maximum, when it is exerted laterally, along the line passing through the horizontal projection of the satellite and at right angles to the line of motion, and again dies away. It is always a tearing stress, renewed again and again.