December 5 1960 – Tektites

Tektites are certainly among the most mysterious objects known to science, and it has been suggested that they come from the Moon. Proof of this fascinating speculation is lacking, but it is now widely considered that what­ever they may be, tektites are extra-terrestrial. In discussing them during the programme of December 5, I was accompanied by Dr M. H. Hey, a Senior Principal Scientific Officer of the Department of Mineralogy at the British Museum (Natural History). At the present time - 1964 - the tektite mystery remains as deep as ever.

The Moon, so much the nearest of our companion worlds in space, is rapidly becoming of practical as well as theoretical importance to us. Studies of it are making quick progress, and during the past few months two investigations have been made which are worthy of special mention.

The first of these relates to the Moon's averted hemisphere. As everyone will remember, the first photographs of these new regions were secured in October 1959, when the Russian vehicles Lunik III photographed them and sent back the pictures by means of com­plex television techniques. The two best photographs have been widely published in Britain, but over thirty others exist; they are not so spectacular, but are quite good enough for analysis. I was able to examine them during my recent visit to the USSR, and it was at once clear that many features were shown.

A comprehensive atlas of the new regions, based on these photographs, has now been completed by Y. N. Lipski, in Moscow. Several hundreds of craters are charted, as well as various other features. As expected, the averted hemisphere is basically similar to the hemisphere we know, though there are fewer of the grey maria, or 'seas'. In 1953 I suggested that seas might be rare on the far side of the Moon. This suggestion was based on a volcanic theory about the origin of the craters. At that time I had no expectation that the problem would be cleared up for many decades; the progress of space research has certainly been unex­pectedly quick. However, the arrangement of the features on the hidden side of the Moon may give some extra clues to the genesis of the surface formations. Positive knowledge about this oft- discussed puzzle would obviously be important. It is linked, too, with another question which has been discussed recently: the problem of the mysterious objects known as tektites.

Tektites are relatively small. Most of them measure less than an inch in length, and even the largest example known to us is inferior in size to a hen's egg. In composition they are 'glassy', and contain about 80 per cent silica; they are easy to recognize, since they are unlike any other natural objects so far discovered.

The first really remarkable thing about them is their distribu­tion. They are not found everywhere - for instance, none has ever been discovered in Britain - but occur in various well-defined areas. The largest of these fields is in Australia, and covers much of the southern part of the continent. Two more, at least, occur in the area of the East Indies, and there are a few others in Asia and the American continent. The main European tektite field is in Moravia.

Yet the areas in which tektites are found are extremely limited, and this must influence our ideas about the origin of the tektites themselves. Geologically, the Australian field is recent, while most of the rest belong to the Tertiary period. The fact that tektites are particularly numerous in Australia is partly because the country has been less developed, and partly because the tektites themselves have not been there for so long.

Originally, it was assumed that tektites were of terrestrial origin. Even a casual glance showed that at one stage they had been very strongly heated, and it was natural to suppose that they had been hurled out of active volcanoes. It is of course true that volcanic outbreaks may be extremely violent. Active volcanoes are, more­over, numerous - and some of them are just as destructive as the famous Vesuvius and Etna. In 1947, for instance, there was a tremendous outbreak from Hekla, in Iceland, which had previously been quiescent for many years.

One immediate difficulty is that some of the tektite fields are well away from either active or extinct volcanoes. Even more significant is the discovery that tektites seem to have been heated not once, but twice. The original heating must have been very thorough, but the second was less extreme, and affected only the layers near the surface. This could conceivably be explained on the volcanic hypothesis. Suppose that a tektite were heated inside the volcano, and then ejected at a speed sufficient to carry it clear of the dense part of the Earth's atmosphere. As it fell back toward the ground, it would encounter friction against the air-particles, in the same manner as a meteorite, which would account for the second (surface) heating.

D. Chapman, in California, has now examined this idea thoroughly. New data are available, based on experiments with the nose-cones of rockets which have been launched from the surface of the Earth. These nose-cones can be studied after landing, and the effects of frictional heating analysed. This is surely another instance of the close association between rocket research and other branches of science; the problems of nose-cones are intimately associated with those of meteoric bodies.

Chapman's work has shown that the second heating of tektites was too strong to be explained by the simple volcanic hypothesis. When a body is hurled upward - either by a volcano, a rocket booster, or in any other manner - it can behave in one of three ways. If it is moving at escape velocity (approximately 7 miles per second), it will never return, and will continue moving away into space. If it is moving at a somewhat lesser speed, it may enter an orbit round the Earth, as with the numerous artificial satellites - though of course the launching of a satellite must always be a highly complicated process. If the body is moving slower still, it will rise to a certain height, stop, and then fall back to the ground. The maximum height attained will depend upon the initial velocity.

An object moving in this way must therefore start its drop through the atmosphere from rest, and can only reach a certain velocity before it hits the ground. Tektites have been so violently heated that they must have moved more quickly than this. If they did in fact come from space, they must then have entered the upper atmosphere when they were already moving at a considerable speed - and so they cannot be explained so easily as might be thought. Efforts to explain them by normal geological processes have been uniformly unsuccessful. (In less serious vein, mention should be made of some extraordinary suggestions which have been made from time to time. It has been maintained, for instance, that our very remote ancestors were remarkably good at glass-making (!), or that tektites were produced by nuclear explosions initiated by the inhabitants of Atlantis or some equally mythical country. A Russian writer has gone even further, and attributed this alleged nuclear explosion to visiting space-men who landed on Earth in prehistoric times.)

Modern evidence leads us to the view that tektites are not of terrestrial origin at all. One point made by Chapman is of parti­cular significance. Tektites are aerodynamically stable, and this clearly points to a descent through the Earth's atmosphere. On the other hand, they are certainly not ordinary meteoric bodies. They show every sign of intense original heating, as would have taken place in a volcano; and if terrestrial volcanoes are ruled out, one possible alternative is that the tektites began their careers inside volcanoes on the Moon.

There are two main theories about the large lunar craters. Some authorities believe them to be basically igneous, while others regard them as due to meteoric impacts. In any case, there must at one time have been considerable volcanic activity there. During the past few years I have catalogued over fifty objects on the lunar surface which show a remarkable resemblance to conventional volcanoes; G. P. Kuiper has also called attention to more than a dozen of the objects contained in my lists, and it seems that the features are probably quite common. Then, too, there was the observation made on 3 November 1958 by N. Kozirev, at the Crimean Astrophysical Observatory. Kozirev saw, and photo­graphed, a disturbance inside the prominent crater Alphonsus. This outbreak was mild by our standards, and certainly must not be regarded as a full-scale eruption, but at least Kozirev has proved that the Moon is not completely inert even now.

The Moon is less massive than the Earth, and has a lower escape velocity - about if miles per second. There is thus nothing im­probable in the suggestion that active volcanoes could hurl material clear of the Moon altogether. Lava can bring up material which is lighter, and also richer in radioactive elements, than the average lunar composition; this gives a straightforward explana­tion of why the density of tektites (2-3 to 2-5) is lower than the Moon's average (3-3), and also why tektites, like terrestrial eruptive rocks, are richer than meteorites in uranium and thorium.

We can work out a sequence of events. On the lunar-volcano theory, tektites had their origin inside the Moon, and were fiercely heated; they were ejected at a velocity greater than i j miles per second, and so entered the Earth's atmosphere while moving at appreciable velocity. They had cooled during their journey through space, and were re-heated by their drop to ground level. Each tektite-field would be related to some specific lunar eruption. Few eruptions would produce material moving in a suitable manner to hit the Earth, which accounts for the rarity of the tektite-fields.

The whole idea is not new; it seems to have been first proposed by a Dutch scientist, R. D. M. Verbeek, in .1897. However, Chapman's researches have added force to it, and many astrono­mers now regard it as the most probable explanation. This is not to suggest that the theory rests on firm evidence; it is speculative, and for all we know the real answer may be entirely different.

A different mechanism was proposed by H. Nininger in 1940. According to Nininger, tektites were hurled away from the Moon when large meteorites struck the lunar surface, producing the vast craters which we now know. Recent work by Charters and summers has shown that the ratio of 'backsplash' velocity to impact velocity is about 0-3, so that Nininger's process is well within the bounds of possibility. On the other hand, it seems that the total mass of known tektites is rather too great for the 'backsplash' hypothesis to be valid; and if the Moon is involved at all, it seems that active volcanoes are a more probable cause of the ejection of material.

We still do not know definitely how the Earth and other planets came into being. The general view today is that they grew by accretion out of a disk-shaped cloud of matter surrounding the primeval Sun. We are also uncertain of the origin of the asteroids and meteoric bodies. They may represent the broken fragments of an old planet (or planets) which met with disaster, probably by collision, and accordingly it has been maintained that tektites originated in the central, intensely heated, part of this ancient planet.

One fact, at least, emerges from all this research: whatever they may be, the tektites are not of terrestrial origin. It seems over­whelmingly likely that they came from space, and that their second heating was produced by friction against the atmosphere. Of the current theories, that of ejection from lunar volcanoes seems the most probable, even though the evidence is largely indirect and is certainly inconclusive. In this case, we are actually privileged to handle material which once formed part of the Moon itself.

Opinions may change during the coming years, and many problems remain to be solved, but nobody is likely to deny that the tektites - small and unspectacular though they are - are among the most enigmatical objects known to science.