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Shackleton crater on the Moon’s south pole has been somewhat of an enigma, as its permanently shadowed interior has made it difficult to detect what is inside. But with new observations using the laser altimeter on the Lunar Reconnaissance Orbiter (LRO) spacecraft, a team of researchers has essentially illuminated the crater’s interior with laser light, measuring its albedo, or natural reflectance. The scientists found that the crater’s floor is quite bright, an observation consistent with the presence of ice. In fact, ice may make up 22 percent of the material on the crater floor, with possibly more ice embedded within the crater walls.
“We decided we would study the living daylights out of this crater,” said Maria Zuber from the Massachuesetts Institute of Technology, who lead a team to study Shackleton Crater. “From the incredible density of observations we were able to make an extremely detailed topographic map.”
For laser altimeter observations, elevation maps can be created by measuring the time it takes for laser light to bounce down to the Moon’s surface and back to the instrument. The longer it takes, the lower the terrain’s elevation. Using these measurements, the group mapped the crater’s floor and the slope of its walls.
The team used over 5 million measurements to create their detailed map.
While the crater’s floor was relatively bright, Zuber and her colleagues observed that its walls were even brighter. The finding was at first puzzling. Scientists had thought that if ice were anywhere in a crater, it would be on the floor, where no direct sunlight penetrates. The upper walls of Shackleton crater are occasionally illuminated, which could evaporate any ice that accumulates. A theory offered by the team to explain the puzzle is that “moonquakes”– seismic shaking brought on by meteorite impacts or gravitational tides from Earth — may have caused Shackleton’s walls to slough off older, darker soil, revealing newer, brighter soil underneath. Zuber’s team’s ultra-high-resolution map provides strong evidence for ice on both the crater’s floor and walls.
“There may be multiple explanations for the observed brightness throughout the crater,” said Zuber. “For example, newer material may be exposed along its walls, while ice may be mixed in with its floor.”
The crater, named after the Antarctic explorer Ernest Shackleton, is nearly 20 km (more than 12 miles) wide and over 3 km (2 miles) deep — about as deep as Earth’s oceans. Zuber described the crater’s interior as “extremely rugged … It would not be easy to crawl around in there.”
She added that the new topographic map will help researchers understand crater formation and study other uncharted areas of the moon.
“I will never get over the thrill when I see a new terrain for the first time,” Zuber said. “It’s that sort of motivation that causes people to explore to begin with. Of course, we’re not risking our lives like the early explorers did, but there is a great personal investment in all of this for a lot of people.”
Ben Bussey, staff scientist at Johns Hopkins University’s Applied Physics Laboratory, said the new evidence for ice in Shackleton crater may indeed help determine the course for future lunar missions.
“Ice in the polar regions has been sort of an enigmatic thing for some time … I think this is another piece of evidence for the possibility of ice,” Bussey says. “To truly answer the question, we’ll have to send a lunar lander, and these results will help us select where to send a lander.”
And for any humans explorers, a crater like Shackleton at the lunar poles may well be the best location for a base, as the poles contain regions of near-permanent sunlight needed for power, and regions of near-permanent darkness containing ice — both of which would be essential resources for any lunar colony.
The team’s research was published today in the Journal Nature.
Lead image caption: 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. Credit: NASA/Zuber, M.T. et al., Nature, 2012
Second image caption: This is an elevation map of Shackleton crater made using LRO Lunar Orbiter Laser Altimeter data. The false colors indicate height, with blue lowest and red/white highest. Credit: NASA/Zuber, M.T. et al., Nature, 2012