Radar data analysed from India’s Chandrayaan-1 spacecraft reveals ice deposits in craters dotted around the Moon’s north pole that could contain as much as 600 million tons of ice.

NASA's Mini-SAR instrument detected more than 40 small craters containing water ice. The craters range in size from 2 to 15 kilometres in diameter and lie in permanent shadow, preventing the ice from evaporating into space. Although the Moon is drier than any desert on Earth, the north polar craters could contain as much as 600 million tons of ice.

Mini-SAR uses the polarization properties of reflected radio waves to characterise surface properties. The instrument sends out pulses that are left-circular polarized; typical planetary surfaces reverse the polarization so that “normal” echoes are right-circular polarized. The ratio of received power in the same sense transmitted (left circular) to the opposite sense (right circular) is called the circular polarization ratio (CPR). Most of the Moon has a low CPR, but regions displaying high CPR values include rough terrain associated with ejected material from fresh craters. Ice, although transparent to radio energy scatters the pulses, thus giving an enhancement in same sense – left circular – reflections and an elevated CPR value.

Many craters have high CPR values inside and outside of their crater rims, which is typical of the rough surfaces associated with relatively fresh, young craters. But a number of the north polar craters corresponding to sites of permanent shadow have high CPR values inside their rims but not outside. In these cases, the high CPR value cannot be associated with surface roughness and must be caused by a material that is restricted to the interiors of the basin.

"After analysing the data, our science team determined a strong indication of water ice, a finding which will give future missions a new target to further explore and exploit," says Jason Crusan, program executive for the Mini-RF Program for NASA's Space Operations Mission Directorate. The science team estimate that, depending on the size of the crater, the ice deposits could be several metres thick.

The water inventory at the north pole is derived from two processes: by delivery from comet impacts billions of years ago, and through a recently confirmed process that suggests water molecules are being produced all over the Moon's surface every day. During the daytime the solar wind, which includes hydrogen ions, interacts with oxygen in the lunar soil to form and accumulate hydroxyl and water molecules. Water is lost at noon when it is hotter, but when it cools down in the evening it can accumulate water again, and over time the water molecules naturally migrate to the poles. Confirmation of this process came from three different missions: Chandrayaan-1, Deep Impact and Cassini, and was announced six months ago.

Around the same time NASA's Lunar Reconnaissance Orbiter (LRO) also found evidence for widespread hydrogen all over the Moon, and not just at permanently shadowed polar locations. Then, the Lunar Crater Observation and Sensing Satellite (LCROSS) that plummeted into a south pole lunar crater in October, threw up a plume of debris that contained the signature of water.

"The emerging picture from the multiple measurements and resulting data of the instruments on lunar missions indicates that water creation, migration, deposition and retention are occurring on the Moon," says Paul Spudis, principal investigator of the Mini-SAR experiment. "The new discoveries show the Moon is an even more interesting and attractive scientific, exploration and operational destination than people had previously thought."

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