Dating of Shackleton crater suggests it may offer supply of ice.
A piece of prime real estate on the Moon is much older than previously thought, which means there’s been more time for water ice to have collected there. The conclusion, based on analysis of data from the SMART-1 mission, makes the crater a very attractive site for a lunar colony, according to scientists behind the study.
Shackleton crater is 20 kilometres across and sits near the Moon’s south pole. It is being eyed as a site for a lunar base because its bottom is permanently shadowed — a prerequisite for storing ice, if it exists there. Conversely, the crater's rim seems to benefit from almost year-round sunshine, essential for any solar-powered base.
Scientists led by Paul Spudis of the Lunar and Planetary Institute in Houston, Texas, have now used images from the European Space Agency’s SMART-1 probe to work out the crater’s age from a careful count of the smaller impact craters around it. “We found it to be much older than previously thought,” says team member Ben Bussey of the Applied Physics Laboratory in Laurel, Maryland.
The Solar System is full of debris that bombards all the bodies within it at roughly the same rate, so counting the craters and noting how they overlap can give an indication of age. Previous estimates of the crater’s age had ranged from less than 1 billion to 3.3 billion years old.
But the more detailed images from SMART-1's Advanced Moon micro-Imager Experiment (AMIE) allowed the team to age Shackelton as roughly 3.6 billion years old. The work is published in Geophysical Research Letters1.
Home from home
This is good news for humans thinking about staying on the Moon for a while. “There’s been a lot more time for possible ice to accumulate,” says Bussey, “and over billions of years it is feasible that you could build up a significant reserve.”
There is still debate about whether ice could have been brought to the Moon by comets, or delivered as the hydrogen-rich solar wind reacted with oxygen in the Moon’s surface rocks to produce thin films of water.
Bussey’s theory that an older crater will have allowed more ice to accumulate will stand up only if the ice came to the Moon on comets, says Manuel Grande of the University of Wales, Aberystwyth, UK, who worked on the SMART-1 mission. A solar wind source could deposit ice, but at such a slow rate that losses would cancel out any large-scale accumulation of ice.
The presence of any water on the Moon still hasn’t been proved conclusively, adds Grande. But evidence of hydrogen was found at both poles by NASA’s Lunar Prospector probe in 1999. “It seems perverse to think there’s hydrogen there without it being water,” Grande says.
Future data will come from the Japanese space agency and its Kaguya mission, launched in September 2007, says Bussey. Other future Moon fact-finding missions include NASA’s Lunar Reconnaissance Orbiter, now expected to launch in early 2009 after a recent delay. The Indian Space agency is launching Chandrayaan-1 in September, with instruments on board to work out the geology and chemistry of the Moon’s surface, especially at the poles. "Between [these missions] we hope they will map out the most promising locations that will have ice," says Bussey.
Spudis, P. D. et al. Geophys. Res. Lett., 35, L14201 (2008)
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Sanderson, K. Age makes Moon crater attractive site for lunar base. Nature (2008). https://doi.org/10.1038/news.2008.987