Radar Images Reveal Tons of Water Likely at the Lunar Poles

Radar has been used since the 1960s to map the lunar surface, but until recently it has been difficult to get a good look at the Moon's poles. In 2009, the Mini-SAR radar instrument on the Chandrayaan-1 spacecraft was able to map more than 95% of both poles at 150 meter radar resolution, and now the Mini-RF instrument on the Lunar Reconnaissance Orbiter — which has 10 times the resolution of the Mini-SAR — is about halfway through its first high-resolution mapping campaign of the poles. The two instruments are revealing there are likely massive amounts of water in the permanently shadowed craters at the poles, with over 600 million metric tons at the north pole alone. "If that was turned into rocket fuel, it would be enough to launch the equivalent of one Space Shuttle per day for over 2,000 years," said Paul Spudis, principal investigator for the Mini-SAR, speaking at the annual Lunar Forum at the Ames Research Center in July.

Both Spudis and Ben Bussey, principal investigator for LRO's Mini-RF shared images from their respective instruments at the Forum, highlighting polar craters that exhibit unusual radar properties consistent with the presence of ice.
They have found over 40 craters on the Moon's north pole that exhibit these properties.
Both instruments provide details of the interior of shadowed craters, not able to be seen in visible light. In particular, a measurement called the circular polarization ratio (CPR) shows the characteristics of the radar echoes, which give clues to the nature of the surface materials in dark areas. The instruments send pulses of left-polarized radio waves 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 instruments measure the ratio of left to right circular polarized power sent back, which is the CPR.
Few places – even in our solar system — have a CPR greater than 1 but such places have thick deposits of ice, such as Martian polar caps, or the icy Galilean satellites. They are also seen in rough, rocky ejecta around fresh, young craters, but there, scientists also observe high CPR outside the crater rim such as in this image, below of the Main L crater on the Moon.

The fresh impact crater Main L (14 km diameter), which shows high CPR inside and outside its rim. The histograms at right show that the high CPR values within (red line) and outside the crater rim (green line) are nearly identical. Credit: NASA

Most of the Moon has low CPR, but dozens of anomalous north pole craters, such as a small 8 km crater within the larger Rozhdestvensky crater, had a high CPR on the inside, with a low CPR on the rims. That suggests some material within the craters, rather than surface roughness, caused the high CPR signal.
"Geologically, we don't expect rough, fresh surfaces to be present inside a crater rim but absent outside of it," Spudis said. "This confirms the high CPR in these anomalous craters is not caused by surface roughness, and we interpret this to mean that water ice is present in these craters."

An “anomalous” crater on the floor of Rozhdestvensky, near the north pole of the Moon. The histogram of CPR values clearly shows that interior points (red line) have higher CPR values than those outside the crater rim (green line). Credit: NASA

Additionally, the ice would have to be several meters thick to give this signature. "To see this elevated CPR effect, the ice must have a thickness on the order of tens of wavelengths of the radar used," he said. "Our radar wavelength is 12.6 cm, therefore we think that the ice must be at least two meters thick and relatively pure."Recent Mini-SAR images (top image) from LRO confirm the Chandrayaan-1 data, with even better resolution. The Mini-RF, Bussey said, is equivalent to a combination of the Arecibo Observatory and the Greenbank Radio telescope in looking at the Moon. "Our polar campaign will map from 70 degrees to the poles and so far we are very pleased with the coverage and quality of the data," Bussey said.
Spudis said they are seeing less anamolous craters on the Moon's south pole, but both he and Bussey are looking forward to comparing more data between the two radar instruments to learn more about the permanently shadowed craters on the Moon.
Additionally, other instruments on LRO will also provide insights into the makeup of these anomalous craters