No ice sheet on lunar south pole

Analyzing the highest resolution radar-signal images ever made on the moon, planetary astronomers on Wednesday excluded the possibility of ice existing in craters at the lunar south pole.

Ice may exist only as disseminated grains in the lunar soil, the scientists from Australia and the United States reported in the Oct. 19 edition of the Journal Nature.

Since the 1960s, theories have suggested that ice may exist deep inside impact craters in permanent shadow from the sun.

The theory was bolstered in 1992 when Earth-based radar telescopes located "ice deposits" inside impact craters at the poles of the planet Mercury. Suggestions of lunar ice dated from 1996 when the Clementine spacecraft of the United States gave some signals of ice on crater walls at the south pole.

In 1998, NASA's Lunar Prospector orbiter discovered concentrations of hydrogen at the moon's poles. If this hydrogen were in the form of water molecules, some scientists argued, it would correspond to an average of 1 to 2 percent of water ice in the lunar soil in the shadowed terrain.

Water ice would be a significant resource for any future lunar base, thereby many of the instruments on NASA's 2008 Lunar Reconnaissance Orbiter/Lunar Crater Observation and Sensing Satellite mission seek to learn if water ice is present in permanently shadowed craters.

However, Earth-based radar measurements since the 1990s have consistently failed to detect ice deposits similar to those on Mercury.

In the latest study, a team of planetary scientists acquired new radar images of the lunar south pole at a resolution of 20 meters. There are no evidence for concentrated deposits of water ice at the south pole, the researchers said.

"These new results do not preclude ice being present as small grains in the lunar soil based on the Lunar Prospector's discovery of enhanced hydrogen concentrations at the lunar poles," said Donald Campbell, principal investigator and a professor of the Cornell University.

"There is always the possibility that concentrated deposits exist in a few of the shadowed locations not visible to radars on Earth, but any current planning for landers or bases at the lunar poles should not count on this."

Even in the lunar summer at the south pole, the sun barely edges above the horizon, thus the bottom impact craters never see the sun. Although telescopes on Earth can see some of the shadowed area, only radar can image that terrain permanently in shadow.

In the latest study, the researchers used the giant Arecibo telescope in Puerto Rico to beam radar wave onto lunar south pole, and received the echoed wave with the Green Bank telescope in West Virginia. The echoed wave allowed the researchers to create images that offer the best view ever of the shadowed terrain at the lunar south pole.

The radar scattering parameter, which was previously thought as indication of thick ice deposits, has been found also to match radar echoes from the rock-strewn walls and ejecta of young impact craters at all lunar latitudes, the researchers found.

"If the hydrogen enhancement observed by the Lunar Prospector orbiter indicates the presence of water ice, then our data are consistent with the ice being present only as disseminated grains in the lunar regolith," they wrote in the Nature paper.

Source: Xinhua