Nasa estimates ice content on Moon's shackleton crater
Elevation (left) and shaded relief (right) image of Shackleton, a 21-km-diameter (12.5-mile-diameter) permanently shadowed crater adjacent to the lunar south pole. The structure of the crater's interior was revealed by a digital elevation model constructed from over 5 million elevation measurements from the Lunar Orbiter Laser Altimeter. Photo Credit: NASA/Zuber, M.T. et al., Nature, 2012

Nasa's scientists have discovered the presence of ice on the floor and walls of the Shackleton crater, which is present at the South pole of the Moon, according to a study published in Nature journal.

A team of Nasa and university scientists studied the data collected by Nasa's Lunar Reconnaissance Orbiter (LRO) spacecraft and found that ice makes up to 22 per cent of the Shackleton crater's surface which is present on the moon's South pole.

The scientists used laser light from LRO to illuminate the surface and inside walls of the crater. They measured the natural reflectance to a depth of about a micron.

Shackleton crater, named after Antarctic Explorer Ernest Shackleton, is two miles deep and more than 12 miles wide. Similar to other craters which are located on the Moon's South Pole, the interior of Shackleton crater is dark owing to the small tilt of the lunar spin axis.

When the scientists observed the crater, they noticed brightness on the crater's floor compared to other craters, which prevails due to the presence of ice on it. The findings may help in the study of crater formation.

"The brightness measurements have been puzzling us since two summers ago," said Gregory Neumann of NASA's Goddard Space Flight Center in Greenbelt, Md., a co-author of the paper.

"While the distribution of brightness was not exactly what we had expected, practically every measurement related to ice and other volatile compounds on the moon is surprising, given the cosmically cold temperatures inside its polar craters," he said.

Nasa scientists were also surprised when they found ice on the interior walls of the crater. While a crater's floor will be dark, the walls get illuminated occasionally as direct sunlight penetrates. This may evaporate the ice that gets accumulated on the walls.  But the scientists noticed that Shackleton crater's walls were brighter than the surface.

They believe that it is possibly because of moonquakes, caused by meteor collision or pull of the earth that may have blown off the older and darker soil on the walls and expose new brighter soil.

"There may be multiple explanations for the observed brightness throughout the crater," said Maria Zuber, the team's lead investigator from the Massachusetts Institute of Technology in Cambridge.

"For example, newer material may be exposed along its walls, while ice may be mixed in with its floor," she said.

The scientists also made a remarkable revelation of the presence of a preserved crater unscathed since it was formed three billion years ago on the Moon. The crater's floor itself has several craters which may have formed when the Shackleton was formed due to some collision.