THE WORLD OF SCIENCE

Te Awamutu Courier, Volume 55, Issue 3938, 11 August 1937, Page 3

THE WORLD OF SCIENCE

WHAT DO WE KNOW OF LUNAR CRATERS.

The ancient and almost universal myth of the “Man in the Moon” testifies, at least, to general recognition of a diversity of features in that face of our satellite which is turned over towards us. In fact, the most casual glance at the full moon on a clearnight is sufficient to show a division of the moon’s surface into well-de-fined areas of lighter and darker shade, indicative, of course, of differences in their power of reflecting the sunlight that falls upon them. Wii‘l the aid of a good pair of field glasses more detail appears, in the form especially of bright spots and streak.*, and also of darker lines suggestive of mountain ranges, as, indeed, they are.

Viewed through an astronomical telescope of moderate power, mountain peaks and mountain ranges can be definitely recognised. The large dark patches, formerly thought to be actual seas, and still called by that name, are now identifiable merely as portions of the solid surface of a darker Hue than the rest.

But the most interesting item added to our knowledge 'of lunar topography by the telescope is that the bright circular patches, of which only a few were clearly discernible in the field glasses, are now seen to exist in great profusion. One astronomer has counted over 30,000. These round spots are the Lunar Craters, the nature and mode of origin of which has long been an unsolved problem of selenography. Although these lunar craters owe their name to their superficial resemblance to the craters of terrestrial volcanoes, being quite definitely depressions surrounded ay high and precipitous walls, yet they differ so markedly from volcanic craters in other respects that the theory of their identity with these is extremely difficult to uphold; for whereas our own volcanic craters are of the nature of deep pits sunk in a volcanic cone, which often rises to great heights above the earth’s surface, these lunar craters resemble flatbottomed dishes, only the rim of which is elevated above the level of the surrounding plain.

If. size, too, the lunar craters may far exceed their terrestrial fellows, ranging as they do from barely visible holes to huge basins a hundred miles or more across, with walls which may approach twenty thousand feet in height. Another very interesting and peculiar feature of some of the larger craters is the existence of a mountam peak in the very centre of the basin, that in the giant crater called “Copernicus” towering 11,000 feet above the floor; and yet another is the oc-

casional association with a system of bright streaks or “rays,”’ all radiating outwards from the crater along the moon’s surface. Now, how did these craters come into existence, and how are the striking peculiarities of their structure to be explained? Many and various are the theories which have been advanced since the day when Robert Hooke compared them to “bubbles rising from a pot of boiling alabaster” (for which we may substitute the homely porridge pot). Only two, I think, deserve serious consideration to-day; firstly, the volcanic theory already alluded to, and secondly, that which attributes the craters as the impacts of meteorites on the moon’s surface. Some of the objections to the volconic theory of origin have beer already mentioned, and there are others just as formidable. For example, steam plays a most important, if not essential, part in terrestrial volcanism, but the moon is to-day, and probably always has been, entirely waterless. The theory of meteoritic impact has its difficulties, too, but in recent years it has gained in favour, mainly, perhaps, by reason of the discovery of a small number of craters of undoubtei meteoric origin upon the earth. The most notable and one of the earliest known of these is the crater known as “Coon Butte” in Arizona, which is nearly a mile across and several hundred feet in depth from floor to top of enclosing wall. Then there is the Central Australian group of craters, first scientifically surveyed and described a few years ago by Dr Arthur Aiderman, thirteen in numbe i-, the largest about 200 yards across. In these and in several other craters the correctness of the meteoric theory of origin is proved by the existence of masses of meteoric iron, both within the crater and scattered over the surrounding country. The circular shape of these craters, the precipitous inner wall and the gently sloping elevation of the outer ground to the top of the wall are features which they share with the lunar ones, and these common characteristics, supported by a very plausible theory of the mode of formation of meteoric craters have led Dr L. J. Spencer, late head of the mineralogical department of the British Museum, lately to declare his unreserved acceptance of their meteoric origin. Yet the evidence from terrestrial analogy is perhaps not unequivocally favourable. Why, it has been asked, is not the surface of the earth pock-marked, like that of the moon, with thousands of such craters, why are the dimensions of the larger lunar craters to be measured in miles and those known on earth in yards only; why, also, should the indentation due to the impact of a meteorite on the ground be invariably circular? Dr A. C. Gifford, a well-known astronomer, of New Zealand, has dealt

very faithfully with these and other difficulties of the meteorite theory in an article published several years ag?. He points out that, in all probability, meteoric masses in that part of the solar system which earth and moon occupy would have been much more abundant and larger in the days of their youth than now is the case, and, consequently, the number of meteoric impacts on both moon and earth would in those remote days be vastly greater than it is to-day. But, whereas on the earth the destructive and levelling effects of wind and water have long since obliterated all but the most recent of the craters so formed, the absence of these erosive agencies on the moon has permitted the survival of the majority, if not of all? (There are signs of distinct difference of age in the lunar craters.)

Gifford advocates forcibly an “explosion theory” of crater-formation which Spencer, too, supports on the basis of evidence furnished by terrestrial craters. According to the explosion theory the crater is not produced directly by impact of the meteorite, but rather by the explosive force of the vapours generated by the conversion of its colossal energy of motion into heat. A rifle bullet, it is known, can be partly melted by its impact on a target, and if this is so with a body travelling only one-third of a mile per second, it is obvious that not by the impact, but by the outburst may be expected to ensue when that speed is increased, as it is in an average meteorite, a hundredfold or morj.

There is, in fact, not merely conclusive argument, but actual evidence for the occurrence of such volatilisation both of the substance and the meteorite, and of the rocks into which it penetrates. Hence the hole is made not by the impoct, but by the outburst of a fiery blast of incandescent vapour. The explosive wave from the great meteorite which fell within the Arctic circle in Siberia in June, 1908, actually levelled every tree in the forest around for nearly forty miles. It is not difficult to believe—and actual experiments which have been made on a small scale support the •belief—that all the major features of the lunar craters the circular form, the precipitous inner wall, the the upheaval of the surrounding earth, the radial ridges or cracks, the central peak on an otherwise flat floor (the counterpart possibly of the jet thrown up in the hollow made by a raindrop falling into water), would follow as natural effects of such a cataclysm.

Although certain features, in particular the extreme irregularity of the distribution of the craters over the moon’s surface, are still left unexplained, these arguments and others put forward by Gifford and his supporters are perhaps sufficient to justify acceptance of the theory of meteorite formation at least until a better is advanced. —K.G.