A NASA radar aboard India’s Chandrayaan-I lunar orbiter found 40 craters, ranging in size from 1 to 9 miles across, with pockets of ice. Scientists estimate at least 600 million tons of ice could be entombed in these craters.
The radar, called the Mini-SAR, sends pulses of left-polarized radio waves out to measure the surface roughness of the moon. While smooth surfaces send back a reversed, right-polarized wave, rough areas return left-polarized waves.
Ice, which is transparent to radio waves, also sends back left-polarized waves. The Mini-SAR measures the ratio of left to right circular polarized power sent back, or the circular polarized ratio (CPR). However, a high CPR alone can’t distinguish between rough patches and regions with ice.
The north pole craters had a high CPR on the inside, with a low CPR on the edges. That suggests a material enclosed in the craters, rather than surface roughness, caused the high CPR signal. According to NASA, the ice would have to be relatively pure ice and at least several feet thick to give this signature.
In November, NASA crashed a probe into the Cabeus crater near the moon’s south pole and also found evidence of water.