The Moon sports a thick basaltic crust that differentiated from a denser, underlying mantle at least 4.4 billion years ago. Therefore, the South Pole-Aitken basin is an impact feature so large that it has likely "unearthed" portions of the Moon's upper mantle.
By returning material gleaned from the South Pole-Aitken Basin the hope is to better gauge the early impact history of the inner solar system. By scientists getting their hands on samples they also might get a handle on the nature of the Moon's upper mantle, as well as gain insight into early history of the Earth-Moon system.
"With the little bit of lunar remote sensing accomplished in the last decade, we now know that South Pole-Aitken Basin represents a compositional terrain completely different from that sampled on the lunar nearside decades ago," said Carl Pieters, a space geologist at Brown University in Providence, Rhode Island.
"Nevertheless, Apollo taught us the importance of targeted sampling," Pieters told SPACE.com . "Samples returned to Earth-based laboratories allow extraordinarily valuable studies of mineralogy, geochemistry and age dating that simply cannot be achieved otherwise," she said.
New Frontiers price tag
For its part, NASA has responded to the Decadal Study via a recent draft Announcement of Opportunity (AO) for the space agency's New Frontiers lineup of prospective space probes.
Amongst a set of directives, the NASA AO has asked scientists to put in high gear their ideas on the SPA-SR mission.
"The surface of the Aitken basin located on the Moons far side southern polar region is likely to contain some fraction of the mineralogy of the Moons lower crust. Samples of these ancient materials that are not biased by nearside impact basin formation are highly desirable to further understand the history of Earths celestial companion," explains the NASA document.
The Decadal Survey outlined a "strawman" SPA-SR mission calling for a rover stuffed with science gear to provide context to sampled areas and to rocket back at least a couple of pounds (one-kilogram) of lunar specimens.
Another scenario being discussed is having two different landers put down at two different basin locales, then return both sample caches to Earth.
Engineers and scientists are now hard at work trying to hammer out ways to accomplish the SPA-SR mission - all within the New Frontiers price tag of $650 million.
"NASA has been to the Moon several times. The Russians have returned lunar samples with automated probes decades ago. There is no question that this mission can be done, and with current technology it can be done easily within the New Frontiers budget cap," Pieters said. "A simple scoop sample is at the low end, and sophisticated rovers or multiple landers with an instrumented orbital relay is at the high end," she said.
Rekindling important science
A robotic return to the Moon is a chance to pick up and build upon Apollo and past lunar science missions said Bevan French, a visiting scientist at the Smithsonian Institution's Natural History Museum. He is a former NASA lunar expert specializing in the geology of impact craters.
"I think we now have a chance not only to digest the huge amount of Apollo data that came back, but to add to it systematically," French said.
On the one hand, French added, the wonder and puzzlement about what might be found on the Moon is somewhat diminished by earlier exploration. But going back to the Moon will mean rekindling still important science that needs to be done, he told SPACE.com .
French said that an important revelation that came out of the post-Apollo lunar program was the discovery of the South Pole Aitken basin. "The idea of being able to explore an impact basin so huge is really exciting," he said.
Hauling back small amounts of lunar material via robot sampling would be a big deal.
"The analytical methods have gotten so great that when you talk about a 'big rock'we are talking about something about the size of an aspirin tablet," French said.
"I'd be delighted to see spacecraft and especially human beings going back to the Moon," French said.
Full of mystery
Depending on how NASA budgets and decision-making go, a robotic lunar sample return mission could be underway in the 2009-2010 time period, said David Lawrence, staff scientist in the Space and Atmospheric Sciences Group at Los Alamos National Laboratory in New Mexico.
"The nice thing about the Moon is that there's not a single launch window you have to make. It's not like racing to Pluto before the atmosphere freezes out," Lawrence said.
"There's still a lot of mystery at South Pole Aitken," said Peter Schultz, a space geologist at Brown University. Still debatable is whether or not precious mantle material in that basin can be found. There's more to South Pole Aitken than meets the eye, he noted.
Any Aitken basin landing site needs to be well targeted and reconnoitered, Schultz said. A lesson learned from Apollo is that going-in assumptions regarding a geological setting on the Moon can prove wrong.
"If everyone is working with a single hypothesisthen I think you can be smoked," Schultz said.
"The Moon is not dead. It's an extremely exciting place. The Moon is full of mysteryand we don't know everything," Schultz said.
No-fly zone
"When the Apollo program ended in 1972, the Moon appeared to have been declared a no-fly zone," observes G. Jeffrey Taylor of the Hawai'i Institute of Geophysics and Planetology. But then came the Pentagon's Clementine mission in 1994, followed by NASA's Lunar Prospector in 1997.
"What's striking now is just how many spacecraft are going there. The place is going to be swarming," Taylor said.
Soon to be Moonbound is Europe's Small Missions for Advanced Research and Technology (SMART-1).
That craft will test solar electric propulsion and other deep-space technologies, while performing scientific observations of the Moon. Its bevy of science instruments will address some of the most perplexing lunar unknowns, including the formation of the Moon, search for water ice, and scope out the lunar crust's mineral composition.
Along with Europe, Japan is prepping several lunar missions, as is China and India, Taylor said. "If all goes as planned, we will know vastly more about the Moon a decade or less from now," he said.
As for a NASA Lunar South Pole-Aitken Basin Sample Return mission, Taylor said the key is not to sell a bill of goods and hype the science return. "We want to deliver things that we know we can deliver. Everyone is now busy thinking about what those things really will be," he said.
Hard-nosed vision
Lunar science not only benefits by assaying new Moon samples, Taylor said. So too will our understanding of planet formation and the nature of big impacts, among other cosmic processes.
In the larger picture, Taylor said that "hard-nosed vision" on an international scale is needed about the future of the Moon.
"It's time to open up forums on how the Moon can be developedand have humans end up living and working there. There's need to start and continue a process of creating a roadmap for human development of the Moon," Taylor concluded.
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