Chains Of Volcanoes Discovered On Moon

Press, Volume CVIII, Issue 31678, 14 May 1968, Page 21

Chains Of Volcanoes Discovered On Moon

Knowledge of the Moon and our neighbouring planets is increasing greatly. The data returned by American and Soviet spacecraft over the last two years have given a tremendous fillip to planetary scientists who, for decades if not centuries, have laboured at studies based on meagre and inconclusive information. The situation has now changed dramatically. Already this year scientific journals have carried papers with descriptions of extensive chains of dormant volcanoes on the Moon, calculations of lunar soil depth at a wide variety of locations and even a paper giving evidence for polar icecaps on Venus.

Before the space-age these papers could only have contained guesswork—enlightened guesswork maybe —but now they are based upon sound on-the-spot evidence which in many cases will only be bettered when man himself goes to take a closer look. An exception is the Venus ice-cap hypothesis which, if verified by a further unmanned landing near the poles rather than the equator of Venus, may open the possibility of manned landings on that inscrutable, cloud-shrouded planet Venus Data

Professor Willard Libby, Nobel Prize winning geochemist, has interpreted the data on Venus obtained last year by Venera 4 and Mariner 5. as evidence in favour «'f giant polar ice-caps holding the water that must have come out of Venusian volcanoes when the observed large amount of carbon dioxide gas was liberated into the atmosphere. The surface temperature near the equator of Venus was found by Venera 4 to be close to 270 degrees Centi-grade-far above the boiling point of water. However the extremely thick atmosphere of Venus prevents solar energy penetrating slantwise to the polar regions which could therefore be much cooler and subject to continuous snowfall. Melting must occur at the edge of the icecap and streams of water flow toward lower latitudes until they evaporated to replenish the clouds.

Professor Libby considers that the temperature region between the ice and the equatorial desert would be a likely abode for life. Turning from Venus to the Moon we encounter much harsher conditions due to the absence of a blanketing atmosphere. Even so, the conditions prevailing in the lunar surface layers are much closer to terrestrial conditions than we might have expected. The digging appa-

ratus on the Surveyor spacecraft, for example, found that the lunar topsoil has physical properties rather similar to damp sand, except that the “dampness” cannot be caused by water. Lunar Surface

The upper layer of the lunar surface is composed almost entirely of fragments ejected from meteorite impact craters. The depth of this ejecta varies from a few inches to as much as 60 feet, depending on the locality. This wide variation is revealed by the differences from place to place in the shapes of small craters as photographed by Lunar Orbiter spacecraft. The key to the relationship between crater size and the depth of the fragmented surface layer of the Moon was found by laboratory experiments in which projectiles were fired into simulated lunar soil targets under controlled conditions matching those on the Moon. As a result of this investigation it has been found that the thickest layers of fragmented material coincide with the most densely cratered areas of the Moon. On the relatively smooth, flat maria the layer of ejecta is thin, suggesting lava flows at a stage of the Moon’s history later than the time when the majority of craters were formed. Some lunar craters are ■ observed to have up to four levels of terraces on their inner slopes. This is taken to indicate successive lava flows over thin layers of ejecta. Unusual Craters

Other clear indications of lunar volcanism have emerged from close studies of the Ranger and Lunar Orbiter photographs which reveal details not apparent through terrestrial telescopes. Some high resolution pictures obtained from Ranger 9 drew attention to some unusual craters with surrounding ! haloes of blackened ground i rather than the predomi- ■ nant light-coloured material, i Although many dark-haloed craters can be detected on telescope photographs their nature was quite uncertain until the Ranger 9 and Lunar Orbiter pictures revealed that they are definitely volcanic in origin, with many characteristics setting them apart from meteorite-impact craters.

This conclusion triggered an extensive search for darkhaloed craters on specially selected plates taken recently at the U.S. Naval Observatory. The search brought to light several hundred of these craters and led to the discovery that they are particularly numerous around the borders of the lunar maria.

This distribution parallels the occurrence of volcanoes here on Earth where they mostly are found around the margin of basin areas—particularly the Pacific basin. Some of the dark crater haloes overlie the bright rays of fragmented material flung from large impact craters such as Copernicus and Tycho. These two craters are younger than most, so the dark haloes must be younger still, indicating that volcanic processes producing darkhaloed craters were not confined to a single brief era in the history of the Moon. There are, in fact, many signs that the Moon is still volcanically active. It is certainly not the completely dead world that had been imagined by astronomers not so very long ago.