IN STARRY SKIES

Evening Post, Volume CX, Issue 104, 29 October 1925, Page 17

IN STARRY SKIES

THE LUNAR CRATERS METEORIC BOMBARDMENT OF THE MOON (By "Omega Centauri.") As soon as the surface of the moon was Gxamined through large telescopes it was realised that the volcanic theory was beset with difficulties. R. A. Proctor was the first to point out clearly that no plausible theory of the origin of tho craters had been proposed. He saw that the problem was but a part of a still larger one, and depended, on tho view taken of the origin of the moon:itself and indeed of the whole Solar System. Ho realised, more clearly than mostj tlie impossibility of accepting in its entirety the nebular hypothesis of Laplace. He showed that many features that otherwise appeared inexplicable might be explained it' wo regarded the Solar System as formed by the gathering in from outer space of materials onco widely scattered. Unfortunately he did not explain where these materials came from. According to his view, the moon must have had its heat generated to a considerable extent by meteoric impact, and if the fall of me- I teorites continued after a crust began to form, scars produced by the bombardment might be expected. The assumption made by Proctor that the craters were formed whilst the surface was still in a plastic condition retarded the development of the meteoric theory. All its exponents for many years regarded the craters and central peaks as a record of splash and recoil. The vast holes in the surface were supposed by them to have been formed by the meteorite plunging in. The semi-plastic material, they thought, was drawn outwards, solidifying before it could, all return, and thus forming a mountain ring, whilst tho portion which surged back formed tho group of mountains iv tho centre. There were insuperable objections to this view. Proctor saw the difficulties and applied his hypothesis to the smaller craters only, and even withdrew all reference to the theory in later editions of his book on "The Moon." The period during which the surface waa plastic must have been too short geologically, unless the meteoric bombardment was an improbably brief and intense one.

The consideration which completely killed tho hypothesis was that nearly all tho meteorites must have struck the moon obliquely and could not by their entrance* have produced such uniformly circular scars. Another argument, which carries much less

weight, was frequently urged, that, the earth ought to show evidence of a similar bombardment. Tho au-

swer to tho latter objection is twofold. First, tho earth is protected by its atmosphere, which vapourises ail small meteorites, and reduces to a small fraction of its former value the speed of any larger one. Secondly, the earth was probably still liquid tit the time tho surface of the moon was sculptured, and was thus incapable of retaining any record of. tho blows it received. But it is little use answering individual objections whilst a single one remains which is sufficient of itself to disprove the hypothesis, i It was reported lately, for instance, that in a certain accident two of tho wounds which •£ man suffered were fatal, but the third was fortunately of a much less dangerous character. Now it is quite incredible that all meteorites which struck tho moon should have fallen perpendicularly to its surface.

The vulcanists wore, therefore, perfectly justified in refusing to believe such an absurdity. Meydenbaur (1882) and- Dr. G. K. Gilbert (1892) suggested that the meteorites which scarred the moon's surface were originally a ring of moonlets which revolved round the earth, and which, entering the neutral " region, dropped vertically on to the moon. But' this idea does little to remove the improbability. Gilbert's most valuable work in this cob nection was his proof of the essential difference between a lunar crater and a terrestrial volcano. But his explanation, as in fact those of nearly all other advocates of external action, demanded meteorites of very great size. T. J. J. See pictured a satellite fifty miles in diameter, sinking down into the uncompacted material, flattening and spreading out at the base, and thus making a broad saucer-shaped depression three times its original diameter, the peak in the centre being a remnant of the satellite itself. Gilbert made experiments by dropping pebbles into pasty mud, and by firing projectiles into plastic materials. He took the velocity of impact as simply that due to the attraction of the moon itself, and calculated that a temperature of 3500 degrees F. or higher would be produced, which, would melt much of the rock and so produce the level floor of the crater.

In all these theories, one fact, and that the most important of all, was overlooked, and that is tho explosive character of a meteorite when' its cosmic velocity is suddenly destroy-

cd. The kinetic energy of a moving body is proportional to tho square of its velocity. A body travelling at the rate of two miles per second has more energy than an equal mass of dynamite. If it had a speed of 40 miles per second, its energy would be 400 times as great as when its speed was only two miles per second. Now meteorites entering the earth's atmosphere arc found to have speeds up to forty miles per second. It is probable that those meeting the moon have similar rates. But the moon has no blanket of air to protect it. A meteor reaches the surface with its original speed slightly increased by the moon's attraction. On account of the small mass of the moon its materials are probably far less consolidated than those of the crust of the earth. If an appreciable portion of the' moon's mass has been built up of meteoric material, it is likely to be quite loosely aggregated. A meteorite striking this material with a speed of forty miles per second, and stopped by it in one-tenth of a second, will penetrate two miles. In that short time the energy of motion is changed into heat energy, and the material of the meteorite explodes with an intensity tour hundred times that of one of our high explosives. The fact may be verified by calculation that such an explosion will produce a crater of tynical lunar form, with a mountain ring'and central peak. Owing to the small value of the gravitation of the moon (onesixth that of the earth), a speed of ejection of a quarter of a mile per second would produce a crater 123 miles in diameter, whilst to produce a crater liko Copernicus 56 miles in diameter the material would have to be thrown out at the speed of only one-sixth of a mile per second. If the meteorite entered at 40 miles per second, it had 57,600 times tho energy TJer unit mass of the ejected material. The vast meteorites pictured by Gilbert and by See are not, therefore required to account for lunar craters, though the total mass needed to cover the lunar surface- with gigantic rings was doubtless very great.

Whero did all this material come from? Proctor pictured tho build-

ing up of planets and satellites by material from without. Gilbert and See postulated inoonlets and gigantic meteorites, Chamberlain and Moulton start with phinetesimals, but none of these writers clearly explain the past history of the material. Now there is one possible explanation which gets over this difficulty and appears to account for innumerable facts. Let us suppose two small .suns meeting in space. If their masses are known it is easy to calculate the speed with which they c|ash. As they approach one another enormous tides will be generated. If they maet sufficiently directly the bulk of the material will be whirled into one rotating mass. The inner tides wit! be involved in the central whirl, but the outer ones will bo flung off. The whole of the region of the plane of relative motion, the ecliptic plane of the system produced, will thus be strewn with meteoric "material. If the original suns differed in density, but were- approximately equal in size, 'the lighter one would have the higher velocity, and the greater tides. The material flung from it would therefore be greater in quantity than that from the other, and would also bo Hung farther. Thus tho nuclei of the great outer planets would come from one sun and those.of tho smaller denser inner planets from the other. On this hypothesis tho moon was separated from the earth by tidal action during birth. Ever since the event which formed the solar system, the sun and the planets and their satellites have been gathering up the scattered meteoric material, making their orbits more nearly circular by doing so. The twenty million meteors per day which enter the earth's atmosphere represent tho tiny gleanings'of the immense harvest reaped in the distant past. The surface of the moon bears silent but impressive testimony to the intensity of the bombardment which it suffered in those days when the solar system was in its early youth.

A snake's fang is a sort of hollow tooth, nnd -when the snake strikes at anything tho pressure of this hollow tooth against Ihe gland above it forces the poison through the tooth into tho wound tho fang has made.