The South Pole-Aitken Basin is the most prominent feature of the far side. Centered at 54° south latitiude and 169° west longitude, its rim extends from 15° south latitude (just south of the Korolev Basin) to beyond the South Pole of the Moon, at 80°south latitude, over on the near side. The crater Aitken is not especially notable in itself; it simply happens to be near the n rim of the basin, opposite the South Pole (also near the rim), o and so both Aitken and the pole are convenient references. ^ Because this enormous basin extends to and beyond the ■£ South Pole, the coverage of the entire South Polar Region is o included in this chapter.

0 ^ First Lunar Orbiter and then Apollo photography revealed r parts of the mountainous rim of the South Pole-Aitken de Basin. Because the basin is so large, it can only be seen in w part in individual photos. Several analysts contributed to the o description of this basin and the USGS mapped it as a definite jji feature in 1978. At the Clementine postmission press confer-|- ence in 1994, Gene Shoemaker enthusiastically presented the Lidar data that confirmed its remarkable size and depth.

Figure 8.1 shows an elevation map centered on the South Pole-Aitken Basin. Note that it is an elliptical basin, indicating that the impactor had a low angle of approach. Its major axis is tilted about 10° toward the west from the north. Mul-tispectral data from Clementine revealed an asymmetrical distribution of heavy elements that suggests that the impac-tor arrived from the south (Garrrick-Bethell, 2004).

The area of greatest long-range slope on the Moon extends from the deepest part of the Apollo Basin to the mound of Near Side Megabasin ejecta, surmounted by the rim of the Korolev basin. The impact of the South Pole-Aitken Basin into this region of deep, light, porous ejecta may help explain its extraordinary depth, which was even greater before it underwent isostatic compensation.

The South Pole-Aitken Basin is darkened by mare basalt in the floors of the deeper craters and basins within its depression. The South Pole-Aitken Basin was formed early in the pre-Nectarian Period. The incursion by mare basalt was completed in the Late Imbrian Period, which allowed time for the maria here to be extensively overlain with rays and ejecta from craters and basins outside of the region.

Clementine's near-polar orbit was aligned in such a way that the sun was always nearly in the plane of the orbit, so that brightness variations due to topography were essentially lacking near the equator, although there were strong shadows near the poles.

Figure 8.3 shows the South Polar Region, part of which is within the cavity of the South Pole-Aitken Basin. The floors of the deep craters very near the South Pole are always in shadow, which makes them the coldest places on the Moon. The Prospector spacecraft's instruments detected hydrogen in those craters but could not determine whether the hydrogen is frozen or adsorbed or has combined with oxygen to form water ice. Both hydrogen and water arrive on the Moon, the hydrogen in the solar wind and the water from comets, and so many think that both forms may be held in the cold-traps of the polar craters.

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