UPHEAVAL OR IMPACT?

Taranaki Daily News, 7 February 1931, Page 1 (Supplement)

UPHEAVAL OR IMPACT?

CAUSE OF LUNAR CRATERS

(Rev.

B. Dudley,

F.R.A.S.)

The moon presents' the astronomer with some of his most embarrassing problems. It has for that reason been called “a most troublesome little member of the sun’s family.” There is, for one thing, the question of its origin. No theory on this subject has yet been accepted as conclusive. Whether the moon was once, as some experts think, part of the earth, from which it was subsequently wrenched away, in keeping with the fact that it is slowly, under the stress of tidal strain, increasing its distance from us; or whether it has always been independent of the earth—captured as some believe by the gravitational pull of our planet —are among the questions still undetermined.

» Another problem relates to certain unaccountable movements to which our satellite is subject. It has for long been known that the motion of the moon does not fit in with theory. This is a problem of greater importance in the astronomical sense than any other our satellite presents; though not capable perhaps of being made so interesting to the general public a<s the one to which attention is now being called. This difference between theory and observation has led to the suspicion that there is some unknown law at work which may prove to ba as fundamental as the law of gravitation. It may even be that just as an unexplained discrepancy in the positions of Neptune led to the recent discovery of a ninth planet (Pluto), so the lunar deviations may result in some important 'discovery, not indeed of a new planet, but of something in the nature of gravitation itself.

An easily understood problem in connection with the moon concerns the origin of the craters on its surface. Most people, from photographs occasionally published in the popular .magazines, are familiar with the general appearance of • these craters and crater rings. And it is commonly taken for granted that, like those of the earth, they are of volcanic origin. This, theory may be said to have held the field until recently. James Nasmyth worked it out with characteristic thoroughness, basing his investigations upon the striking resemblances between the lunar formations and the volcanoes of our own planet. “We have upon the earth,” he wrote, -‘some few instances in which the geological conditions which have determined the surface formation have been identical with those that have obtained upon t'he moon; and as, a result we -have’ some terrestrial volcanic districts that, could we view them under the same conditions, would be identical in character with that we see by telescopic aid upon our satellite.” He quoted, as the most remarkable instance of this similarity, the volcanic area about Naples known as Campi Phlegraei, or Burning Fields, a name given to them in the early days because,, as he stated, “they showed traces of ancient earth-fire, or because there wpre attached to the localities traditions concerning hot springs and sulphurous exhalations, if not of actual fiery eruptions.”

Lunar craters generally consist of a basin with a conical elevation rising from the centre. The walls.or ramparts enclosing this area are usually circular, or nearly so. The craters vary widely in shake and size, the “walled plains,” as some of them are described, being the largest enclosures, ranging from 60 to 150 miles in diameter. In the matter of size the lunar formations on the average depart considerably from their likeness to those of the earth, inasmuch as they are. larger. The ramparts in -some cases rise to a height of 12,000 feet above the central plateau. From the top to the interior floor the declivity is as a rule exceedingly steep, while the outer slope is more gradual. The craters and craterlets (of which there are altogether about 100,000) are found on every part of the visible surface of the moon; and they frequently overlap or, as it were, jostle against one another. The volcanic theory of their origin does not by any means commend itself to all who have made a special study of the subject; for, although the resemblances between lunar and terrestrial formations are very striking, there are marked differences. So great, indeed, are these divergencies that experts who are best acquainted with our own volcanoes find that in essential features, there is a total dissimilarity. The theory has for this and other reasons fallen into disfavour.

The answer commonly given to the objection raised on the ground that the lunar craters are excessively large is that the force of gravity in the. moon is only a fraction of that which obtains here, and that in the event of the eruptive force being the same in both cases, it is not unlikely that the moon-craters could be proportionately greater than those of this planet. 1 In the mind of many astronomers the theory of meteoritic impact has found a measure of acceptance. Foreign bodies, it is surmised, have met the moon in its onward path, and ploughed themselves into its surface, giving rise to the pitted appearances we see so clearly with the telescope. A constant rain of huge meteorites, or small planetary bodies of from 10 to I's miles in diameter, would probably be. sufficient to produce such results. It is known, too, that space in the neighbourhood of sun, earth and moon is still strewn with cosmic bodies that move, in swarms. Some even think that it is precisely by the gravitational netting of such 'bodies that the satellite has grown to its present dimensions. The suggestion has inspired numerous experiments, as for example the throwing of balls of mud or clay on to a flat surface, and observing the results, when the required shapes do actually occur. Dr. See speaks of this as the “capture theory,” and is himself a strong supporter of it, as also is Dr. Shaler, the noted American geologist. Richard Proctor at one time accepted this theory of bombardment, but afterwards abandoned it. Astronomers who adopt it recognise in common that the eruptions must have been on an enormous scale, that they took place untold ages ago, and that if there are still any manifestations of activity at work, it takes the form of a quiet emission of gases. But in general it is believed that the disturbances long since practically ceased.

Here, then, for the present, the matter stands, scientific thought being divided upon the subject, with the balance of opinion in favour of the volcano, as against the meteorite.