Apollo Expeditions to the Moon

CHAPTER 15.3


HOT DEBATES ABOUT VOLCANOES

What have been the relative roles of impact cratering and volcanism? For the crater-ridden Moon the obvious first question is how were the craters formed: by meteorites hitting the surface at high velocity, or as the remains of once active volcanoes? Debates on this subject generated about as much heat as the volcanoes themselves. Many argued for, or gave the appearance of arguing for, one extreme or the other. More sober debaters recognized that both cratering and volcanism played a role and sought to discern their relative importance. It is clear that the Moon has changed considerably since it was first formed, but most of that change occurred more than three billion years ago. Apparently the first 1.5 billion years were a period of violent evolution, involving the Mare Imbrium event and other cataclysmic impacts, followed by the vast flooding of the mare plains with a series of lava flows. In contrast, the last three billion years have been relatively quiet, with occasional impacts like those that generated the craters Copernicus and Tycho, but no great lava events.

The implication of this for Earth is that during its first billion years Earth also must have been subjected to severe bombardment, generating huge craters like those still seen on the Moon, and this may also have been true for other planets as well. Certainly the Mariner 9 pictures of Mars and Mariner 10 pictures of Mercury show that both planets experienced substantial cratering, while terrestrial radars indicate the same for Venus. The evidence mounts that violent meteorite bombardment was widespread in the solar system in times past. On Earth, however, erosion, crumpling of the crust, and subsequent volcanism have erased most of the evidence of this early catastrophic period.

Is the Moon still active today? Infrequent observations of sudden localized glows or hazes on the Moon have caused a stir when they occurred, and gave rise to speculation as to whether these were due to current volcanic activity, or were merely trapped gases shaken loose by moonquakes or meteorite impacts. There was speculation about whether our satellite was a dead planet or still a live one. Five Apollo seismometers were set up at different landing sites, and four of them are still working. At times of lunar perigee, these detect moonquakes of very deep foci centered at 500 to 620 miles below the lunar crust. The energy released over a year by these moonquakes, however, is a billion times less than that released by earthquakes over a similar period. No evidence of current volcanic activity on the Moon has been found from either the unmanned or manned space missions In most places the soil just below the surface appears to have been relatively undisturbed for millions of years, which is consistent with the rarity of large-scale meteorite impacts that Earth experiences today. At the very surface a slow erosion takes place by micrometeorite impacts and solar-wind particles, wearing away on the average a few molecular layers a year on exposed surfaces. Material does move around. Some material falls and slides down slopes, some may be moved around by electrostatic forces, and much of what movement occurs is due to splashes from meteorite impacts. But all in all, the Moon appears to be extremely quiet now, in comparison with its earliest history or with Earth today.

Is the interior of the Moon hot or cold? Discussion of this subject once made the sparks fly. Assuming the Moon contains radioactive elements, as does Earth, then the heat generated by their decay should warm up the interior. But would this source provide enough heat to melt the Moon? Whatever the cause, the lava flows apparent on the surface of the Moon show that at least part of the Moon's interior actually was molten at one time. But on the other side of the picture, the distinctly out-of-round shape implies a rigidity that a very hot and plastic Moon would not have. The Apollo measurements have added fuel to this flaming controversy. Anomalous concentrations of mass called mascons have been discovered in the great circular mare basins, detected by the way in which they distort the Moon's gravitational field. In a hot, plastic Moon, these mascons would have sunk until the gravity field was restored to equilibrium. The fact that they persist today indicates a rigid, cool Moon. Yet melting and lava flooding in the upper layers of the Moon are widespread. Electrical conductivities inside the Moon, deduced from the Moon's reaction to the electrical charges and magnetic fields in the solar wind, indicate that the outer layers, down to 500 to 620 miles in depth, are now well below the melting point. Below those levels, however, is a region where seismic shear waves are markedly attenuated, indicating partial melting of the lunar material there. Moreover while magnetic measurements do not reveal any dipole magnetic field at present like that of the Earth, remnant magnetization in rock samples, and substantial local magnetic fields. suggest that the Moon may well have had a dipole field in the past. Since it is believed that such a planetary magnetic dipole field is generated by circulation in a liquid core, this implies that at one time the core of the Moon was molten. This would further imply that the Moon was at one time quite hot throughout, which gets us right back to the difficulty of explaining how the mascons and the aspherical shape can continue to exist. There is a real puzzle here that needs sorting out.

Does the Moon have any atmosphere at all? Before Apollo it was already clear that the Moon could have very little atmosphere. It was expected that heavier gases like argon and krypton would be found clinging close to the surface, but in the lower gravitational field lighter gases would long since have escaped. In large measure this has been confirmed. Argon generated by radioactivity in the lunar crust, and hydrogen, helium, and neon from the solar wind, account for most of what little atmosphere there is, and that is over a billion times less than the Earth's atmosphere.