A US instrument on board Chandrayaan-1 found evidence for water molecules and water-molecule wanna-bes bound in minerals on the moon's surface. They appear across virtually the entire lunar landscape.
It's unclear if the water detected by the orbiter represents a valuable exploration resource or a scientific curiosity. The results don't reveal how deep underground the molecules are found – something that would indicate the volume of water potentially present. Roughly one ton of soil on the moon holds up to a quart of water, say members of the research team.
It's some of the strongest evidence yet for the presence of water on the moon. Other missions have turned up more indirect evidence, for instance, by using radar.
Even if the amount of water turns out to be too small to exploit, the findings could help explain the likely presence of water ice in permanently shadowed bottoms of craters at the lunar poles, which NASA’s Lunar Reconnaissance Orbiter (LRO) discovered recently. It also found evidence for ice at the surface outside crater rims.
"If the water molecules are as mobile as we think they are – even a fraction of them – they provide a mechanism for getting water to those permanently shadowed craters," explains Carle Pieters, a planetary scientist at Brown University in Providence, R.I. and lead scientist for the instrument on Chandrayaan-1 that contributed to the results.
The findings may also open a window on processes on Mercury, where planetary scientists say they've seen hints of water ice in that planet's polar craters.
The results are reported in three research papers published Thursday by the journal Science's online outlet, Sciencexpress. One instrument involved: Chandrayaan’s Moon Minerology Mapper. Designed to pick up the infrared signatures of minerals on the lunar surface, the mapper was originally intended to fly on NASA’s LRO. But since the LRO focuses on the moon's dark and chilly places – that lack the sunlit surfaces required to use the infrared mapper – Dr. Pieters offered it up to the Chandaryaan-1 mission.
Researchers also used data from NASA’s Cassini Orbiter, currently at Saturn, and the Deep Impact Mission to a pair of comets.
Precursor to water
Aside from water molecules, scientists found a precursor to water known as hydroxyl, which has one oxygen atom and one hydrogen atom.
Oxygen is common in a range of minerals found on the moon and elsewhere, says John Mustard, another planetary scientist at Brown and a member of one of the teams reporting the Chandrayaan results. The hydrogen probably comes from the sun, via its solar wind – a constant flow of protons the sun gives off. A lone proton is a hydrogen nucleus.
Hydrogen readily binds with oxygen in highly-reactive minerals, he continues. That forms the hydroxyl. Pummel hydroxyl with more solar-wind protons, and something eventually will stick, giving the molecule the second hydrogen ion it needs to become water.
Moving to the poles
So how might the water or hydroxyl land up in the poles? Environmental processes, Dr. Mustard says, explaining that during the day some of these molecules might get caught up in the moon's tenuous atmosphere and transported elsewhere. Then he stops and says: "I can't even imagine we're talking about environmental processes that involve water on the moon!"
A similar process may be at work on Mercury, he says, where the planet's close proximity to the sun would allow the solar wind to deposit on the surface even more protons than the moon.
Researchers may get some sense of the volume of the lunar cache of water from the LRO. Richard Vondrak, LRO's project scientist at NASA's Goddard Space Flight Institute in Greenbelt, Md., says the craft's neutron spectrometer should be able to tell whether water and hydroxyl occurs only as a thin veneer, or there’s more to it than meets the infrared mapper.
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