To return to the beauty of the Moon: in what way is it a ruin? Even a glance reminds us that it is far from homogeneous in appearance.2 To a first approximation, in the reflected sunlight much of the Moon's surface is bright, but there are also conspicuous and more-or-less circular dark areas, once thought to be seas across which sailed the lunar inhabitants or selenites. These areas are still referred to as the maria (singular mare). The Moon has no atmosphere and there is thus no weathering of its surface, nor is there any crustal recycling akin to terrestrial plate tectonics. Even the Moon's volcanism is long extinct. Today the ancient surface of the Moon is modified only by the impact of meteorites and comets. The large craters, such as Copernicus, Tycho, and, on the far side of the Moon, Giordano Bruno, exhibit dramatic rays that in radiating from the point of impact provide graphic evidence for collisions. The two craters visible from Earth are, in comparison with the age of the Moon, relatively young. Copernicus was formed about one billion years ago, but Tycho is much younger. It was formed perhaps 50 or so million years before a similar object hit the Earth, spelling out the doom of the dinosaurs. Just like the Earth, the Moon continues to receive high-velocity visitors. One of these, it has been claimed, was recorded as a dramatic eye-witness account in a medieval manuscript, written by the chronicler Gervase of Canterbury in 1178. Possibly it was this impact that formed the Giordano Bruno crater.3
Such impacts are relatively infrequent, and it is agreed that the surface of the Moon was largely shaped by intense episodes of bombardment early in the history of the Solar System. The principal bombardment (Fig. 5.1) literally pulverized the surface with repeated impacts, churning the surface to rubble and bestrewing the Moon with craters. As the meteoritic material was swept up by the Moon (and, as we shall see, the other planets) so the intensity of the impacts diminished. There was, however, a last, violent interval when the Moon experienced a series of very energetic impacts.4 The standard view is that these bodies were presumably also products of continued accretion, and so would have had diameters of the order of a hundred kilometres or so.5 It was the impact of these massive bodies6 which led to the formation of the darker maria, such as the Mare Tranquilli-tatis, where the astronauts of Apollo 11 first stepped on to the Moon. The relative smoothness of the maria, which have rather few craters, shows that they must have been formed after the main bombardment episode. These maria are effectively huge pools of basalt, now solid rock, but molten lava that flowed into the circular depressions formed by the impacts. It appears that the basaltic magma was not formed as a direct result of the collision, but was formed by melting deeper within the Moon and then ascended through marginal cracks to fill the giant craters. In some instances, as in Mare Imbrium, the tops of mountains formed earlier still protrude above the lava plains.
figure 5.1 The inferred bombardment history of the Moon in terms of cumulative density of cratering since the formation of the Moon about 4500 million years ago. Note the intense rate of early cratering, before about 4000 million years, which then declines with the formation of the huge maria, of which four are listed here. Thereafter, the rate of impacts gradually falls off, with a steep decline from about 1000 million years ago. In this interval infrequent giant collisions include those that led to the formation of Copernicus and Tycho. (Reprinted from Icarus, vol. 92, C. F. Chyba, Terrestrial mantle siderophiles and the lunar impact record, pp. 217-233, fig. 1, copyright (1991), with permission of Elsevier Science, and also the author.)
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