As the Moon moves around Earth in its monthly orbit, changes in its orbital speed and the inclination of its orbit and tilt of its axis cause it to appear to nod back and forth and up and down giving us tantalizing glimpses of what lies around the rim on its far side. As the Moon's south pole tips into view, we gain a glimpse at what has become one of the most fascinating places in the universe over these past ten years.
Scientists had long speculated about the possibility that some craters on the Moon may have been formed by comet impacts over the millennia. Since comets are made mostly of water, large amounts of water ice may have been deposited on the lunar surface during those impacts. Exposed to the Sun and to the effects of daytime temperatures well above the boiling point of water, the ices would have boiled off and escaped to space eons ago. But at the south pole, many crater bottoms reside in permanent shadow and the temperatures there are always around -130 degrees Celsius. Any ice deposited in these areas could then remain frozen leaving perhaps millions of gallons of frozen water available for use by future human explorers either for consumption or for distillation into the components of rocket fuel. In the mid-1990s, the NASA/DOD Clementine spacecraft turned speculation into the very exciting possibility that lunar ice might be for real when its onboard spectrograph signaled the presence of hydrogen in those permanently shadowed craters. NASA's late 1990s follow-on mission, Lunar Prospector, also confirmed the presence of hydrogen and attempted to detect the presence of water by crashing itself at the end of its mission into one of those craters. Researchers hoped that the probe's impact would throw up a cloud of vaporized water that could in turn be detected by instruments on Earth. No such cloud appeared, but the evidence for water on the Moon remains compelling enough to warrant future investigation. NASA's upcoming Lunar Observer probe will further these investigation in advance of the return of human exploration of the lunar surface during the next ten to fifteen years.
The South Pole-Aitken Basin is L98 on the Lunar 100 list. It is placed so high on the list because its position on the lunar rim makes it difficult to see under the best of conditions and requires a favorable libration to bring it into view at all. The view is foreshortened by the viewing angle, so viewing into the crater bottoms is impossible. Since sunlight cannot reach the crater bottoms either, there is nothing to see anyway in the bottoms of these craters. Since the viewing angle is so shallow, we get a much truer representation here of just how rough the Moon really is. Here you can see that the Moon has been battered and tortured by meteor impacts to a point where it is considerably "out of round." And as the Moon revolves around Earth, each month the south pole will show us a slightly different perspective which makes it worth returning here every month because during no two months will it ever look exactly the same. NASA will undoubtedly be looking at this area as a future landing site as it begins to formulate its return to the Moon over the next decade. Here is your chance to get an early preview of what might be one of the most exciting space discoveries ever.
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