Chapter 1 presented the history of the discovery of major differences between the near side and the far side of the Moon, based on images returned from spacecraft. In this chapter, additional evidence of difference is provided and a new view of the cause of the difference is discussed. There is good evidence that very early in the history of the Moon, a giant impact created the Near Side Megabasin. The near side is nearly all within the depression of this giant basin, and its rim and ejecta cover the far side.
A photograph of the eastern limb from the Apollo camera in the Command Module of Apollo mission 16, taken as it awaited return of the Lunar Module ascent stage, is shown in Figure 13.1. The terminator is about 35 beyond the limb in this picture. Although some of the impression of ruggedness on the far side is due to its nearness to the terminator (where the low sun angle increases the length of shadows), most of the impression is due to differences in the topography.
The Clementine and Lunar Prospector spacecraft missions gave us a wealth of new information about the Moon, but the mystery deepened. Clementine's laser altimeter revealed the remarkable depth of the giant basin in the southern far side, the South Pole-Aitken Basin, which extends from the South Pole up to 17° south latitude. It is about 6.8 km deep, deeper than any other lunar crater but mare floods only the deepest craters and basins within it. The altimeter also confirmed, with additional precision, that the Moon is irregularly shaped, with a bulge of several kilometers on its far side (see Figures 13.2 and 13.3). Further, its center of gravity (the center of spacecraft orbits) is shifted about 2 km toward the near side from the center of its surface figure.
The Lunar Prospector spacecraft provided additional news of the unusual surface distributions of relatively heavy elements like titanium, iron, and thorium. These materials tended to be more abundant at the surface of the near side. The only unusual concentrations on the far side were found to be within the South Pole-Aitken Basin.
We have known since 1984 that the Moon owes its origin to the Mars-sized planet Theia. The name refers to the Titan goddess in Greek mythology who created light and was the mother of Helios (the sun), Eo (dawn), and Selene, who personified the Moon. The orbit of Theia was disrupted until it struck Earth a glancing blow and was essentially vaporized, along with part of the Earth's crust. The debris formed a temporary ring around Earth. The Moon coalesced from this ring, having lost its volatile material to space and part of its iron core to Earth. So why did not the Moon form a nearly spherical, uniform ball, perhaps slightly ellipsoidal because of its spin about its poles?
We know why the Moon has a near and a far side; that is, why we consistently see the same side throughout the lunar month. Just as the Moon's gravity causes tides in the oceans of Earth, the Earth's much stronger gravity causes a tidal effect on the solid (once partially molten) Moon. While the Moon was still spinning in relation to Earth's gravity field, the resultant dissipation of energy damped the spin until the Moon fell into synchronism. That is, its period of rotation is equal to its period of revolution. This has happened with all the large moons of the planets. But until recently, there has been no explanation of the strongly asymmetric shape of the Moon.
Cadogan and Whitaker each proposed that a large impact had hit the near side, creating a basin much larger than the known basins and spreading its ejecta on the far side. Cadogan proposed the Gargantuan Basin and Whitaker proposed the Procellarum Basin. Both basins share the arc of the west edge of Oceanus Procellarum, but the proposed Procellarum Basin extended further to the east. Their thinking was essentially correct, but the geologic evidence of their proposed basin boundaries was not confirmed.
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