One primary goal of the LRO mission is to acquire the amazing bounty of scientific data necessary to enable future human lunar exploration and utilization. But why should we even bother going back? Isn't returning humans to the Moon a waste of time and money? Didn't we learn everything we need to know about the Moon from Apollo?
Well, nothing could be further from the truth. The Moon is a geologic wonderland, with gigantic lava flows, enormous mountains, abundant resources, and most importantly, forty years of questions that need answering. Apollo barely scratched the surface. We only landed human explorers in six places on the lunar surface. To put this in perspective, this would be roughly equivalent to visiting a few spots in my home state of Michigan and then assuming that the geology of the Michigan basin is completely representative of Earth! Based on the results of the Apollo expeditions as well as the remote sensing data acquired by Lunar Prospector and Clementine, the lunar science community has a robust, well-defined set of questions that need answering to advance our understanding of the Moon and the rest of the terrestrial planets.
We've also had some experience selecting prospective landing sites for future human lunar exploration. During the late 1980s, there were extensive efforts to identify high-priority sites for future human lunar exploration. With America heading back to the Moon, much of that work was used by NASA's Project Constellation to identify fifty notional design reference sites for possible future human exploration. These sites will be a subject of intense scientific investigation by LROC and the other instruments aboard LRO. One of these fifty sites is located a few kilometers east of Aitken crater on the lunar farside.
Aitken crater (~135 km in diameter) is mapped as an Imbrian-aged feature, and its floor is covered in a small puddle of mare basalt. It's notable claim to fame, besides being a spectacular crater on the lunar farside with intresting geology that would be an awesome place to send human explorers, is the fact that it sits on the northern rim of the South Pole-Aitken (or SPA) basin. SPA is the largest and presumably oldest impact basin on the Moon. SPA's circular rim stretches from the lunar south pole to Aitken crater (Figure 1) on the lunar farside, a distance of almost 2200 kilometers (Figure 2). Although lunar scientists had a fairly good idea that SPA existed from Apollo data and 1960s-era telescopic observations, SPA itself was not fully mapped until the Clementine mission of 1994 returned a global multispectral and topographic dataset. Celementine data revealed that the SPA contains both the lowest elevations on the Moon (almost 12 km deep) and puddles of mare basalt, unlike the rest of the lunar highlands.
Why is SPA so important? Aren't all impact basins just big craters?
SPA is a bit of an enigma, and there are some big scientific questions that lunar scientists would like to answer using data from both the new armada of lunar spacecraft that are returning data from the Moon and future human exploration. SPA's tremendous size implies that the impact should have been large enough to expose lunar mantle materials. However, while the Lunar Prospector spacecraft detected geochemical anomalies within SPA (specifically, elevated amounts of thorium, an element associated with enrichments of the lunar KREEP component) they are nowhere near what models predicted. Lunar scientists aren't sure if the mare basalts found inside SPA are representative of mantle materials beneath the basin or even formed as a delayed response to SPA formation. Nevertheless, SPA is one of the more likely locations to sample materials from the lunar mantle, and exploration and sampling of the SPA basin itself could redefine our admittedly sketchy understanding of the evolution of the lunar crust and interior, which in turn would enable us to better understand the history and formation of all of the terrestrial planets.
Another fundamental question concerns the timing of the SPA impact. Because the Apollo samples were collected by human explorers in the field, their context is relatively well understood. Laboratory analyses of Apollo samples and other techniques such as crater counting using remote sensing data have allowed planetary scientists to derive a rough chronology for the formation of many of the large basins on the lunar nearside, including Imbrium, Serenitatis, and Nectaris. This chronology, in turn, was extended (with some adjustments) to the other terrestrial planets and even the outer Solar System. SPA has been degraded by numerous small impacts, which suggests that it is quite old. But how old? We're not exactly sure, but we definitely need to find out.
Many of the radiometric age dates from Apollo samples determined for major basins on the lunar nearside cluster at about 3.85 billion years old, which has led some lunar scientists to propose the "Lunar Cataclysm" hypothesis. According to this theory, most of the great lunar basins formed between about 3.9 and 3.8 billion years ago as part of a mass influx of material from somewhere else in the solar system, possibly related to the migration of Neptune and Uranus, which could have scattered numerous planetesimals toward the inner Solar System. The other school of thought is that the major basins simply represent the tail-end of a nearly continuous bombardment, starting with the formation of the Earth-Moon system, and continuing until about 3.75 billion years ago.
Answering this important question requires samples and fieldwork within the South Pole-Aitken basin itself, with profound ramifications for science. If SPA really formed as part of a cataclysmic bombardment contemporaneously with the other lunar basins, it would force planetary scientists to reevaluate what is currently thought about the geologic histories of the other terrestrial planets, such as Mars and Mercury. However, just saying "sampling and fieldwork is required" really oversimplifies the challenge. We will make important progress in interpreting the geology of SPA using samples returned by robotic spacecraft--any lunar sample is A Good Thing, and the results of an automated sample return mission will be very useful. But, the geology of SPA is anything but simple. Numerous impacts have scarred the surface of SPA - even other impact basins. Distinguishing samples from the smaller impacts within SPA and the formation of SPA itself will be a major challenge. It will require sample collection from numerous localities within SPA, identified using remotely-sensed data from LRO and other lunar missions, coupled with years of fieldwork. In other words, it is the perfect sort of lengthy scientific investigation enabled by a permanent lunar base. In addition to the Aitken crater site, several more of the notional design reference potential human exploration sites selected by NASA's Project Constellation as areas of intense study by NASA's Lunar Reconnaissance Orbiter are near or within SPA. Future human exploration will settle this important scientific question!