Much More Water on Moon than Previously Thought
by Phillip Ball
Doubling of cold, dark lunar craters raises human colonisation hopes.
Nature — The Moon may harbour five times more water than we thought, reckon researchers in the United States who have doubled previous estimates of how much of the lunar surface is permanently dark. This is encouraging news for those in favour of human colonization of the Moon.
Any ice that accumulated in sunless pits on the Moon billions of years ago would still be frozen there, having never sublimed and floated free of the tenuous gravity, explain Ben Bussey, of the University of Hawaii, and his colleagues.
A colony supported by lunar ice reserves would re-energize the US space programme and could mine for precious minerals, argues team member Paul Spudis. Lunar ice might be melted to provide water for colonists, he speculates, and it could be split using solar power into hydrogen and oxygen to make rocket fuel.
NASA’s Lunar Prospector spacecraft first spotted signs of huge quantities of ice – perhaps hundreds of millions of tonnes – in 1998, in dark craters at the Moon’s poles. This finding seemed to confirm the tentative indications of a layer of ice in the deep crater basin of the South Pole reported by the Clementine lunar mission in 1996.
Bussey’s group suggests that there could be closer to a billion tonnes of water on the Moon.
Dark arts
The average temperature of the Moon’s surface is -23 ºC. It gets much warmer in direct sunlight. But in permanently shadowed regions, temperatures never top -230 ºC. Here water, delivered on comets and meteorites in the Moon’s early history, could stay frozen indefinitely.
Because the Moon rotates on an axis that is almost vertical with respect to the plane of the Earth’s orbit around the sun, sunlight at its poles is always steeply slanted, casting long shadows. So the floors of some craters close to the poles are always dark.
Bussey’s team has calculated the area of crater floor that stays in shadow as the lunar seasons change. This isn’t easy, because we don’t have complete, detailed maps of the Moon’s surface. So the researchers estimate the shrouded area by studying a few typical craters of various sizes at different latitudes.
They find that craters between 1 and 20 kilometres across create about 7,500 square kilometres of permanent shadow near the North Pole and about 6,500 square kilometres around the South Pole. And this is just a lower limit – other larger or more complex craters are likely to add even more permanent shadow.
Previous maximum estimates were 2,650 and 5,100 square kilometres of darkness for the North and South Poles, respectively.
References
Bussey, D. B. J. et al. Permanent shadow in simple craters near the lunar poles. Geophysical Research Letters, 30, 1278, published online, doi:10.1029/2002GL016180 (2003). |Article|
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