Instruments aboard India's Chandrayaan-1 spacecraft have detected millions of tons of water ice near the moon's north pole.
NASA's Mini-SAR radar found more than 40 small craters with water ice, ranging from one to nine miles in diameter. Although the total amount of ice depends on its thickness in each crater, it's estimated there could be at least 600 million metric tons of water ice.
Mini-SAR uses the polarization properties of reflected radio waves to characterize surface properties. Most of the Moon has a low circular polarization ratio (CPR), but some targets have high CPR. These include very rough, fresh surfaces - and ice.
Numerous craters near the poles of the Moon have interiors that are in permanent sun shadow. These areas are very cold and water ice is stable there essentially indefinitely. Fresh craters show high degrees of surface roughness (high CPR) both inside and outside the crater rim, caused by sharp rocks and block fields that are distributed over the entire crater area. However, Mini-SAR has found craters near the north pole that have high CPR inside, but not outside their rims.
This suggests that the high CPR is not caused by roughness, but by ice within the interiors of these craters.
The estimated amount of water ice is consistent with the quantity estimated from Lunar Prospector’s neutron data - several hundred million metric tons.
"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," said Paul Spudis, principal investigator of the Mini-SAR experiment at the Lunar and Planetary Institute in Houston.
"The new discoveries show the moon is an even more interesting and attractive scientific, exploration and operational destination than people had previously thought."
The results back up findings from other NASA instruments. The Moon Mineralogy Mapper discovered water molecules in the moon's polar regions, while water vapor was detected by NASA's Lunar Crater Observation and Sensing Satellite, LCROSS.