By Ron Cowen
Don’t bother bringing ice skates to the moon’s Shackleton crater.
The crater sits at the moon’s south pole and never receives direct sunlight. Even though it is chilly enough to contain frozen water, it has no visible patches of ice, new images from a Japanese spacecraft reveal.
Planetary scientists have hotly debated for years whether craters on Earth’s moon contain substantial reserves of frozen water. Although it could be difficult to dig out the ice, especially if it’s mixed with soil, the frozen material could provide hydrogen fuel or drinking water for lunar settlers.
Because the sun makes a shallow angle at the moon’s poles, the bottoms of polar craters there never directly see the light of day and are an ideal place to trap frozen water. In 1994, the Clementine spacecraft bounced radio waves off the moon and found tentative evidence for frozen water inside polar craters, although more recent radar studies from Earth found no signs of ice in the crater.
In 1998, a NASA spacecraft called Lunar Prospector (SN: 3/14/98, p. 166) found a small excess of hydrogen nuclei at the lunar poles, a further indication that some polar craters, including the sunless Shackleton, contain ice.
Prospector lacked a camera, but high-resolution images taken by Japan’s lunar-orbiting Kaguya craft, launched in 2007, show that Shackleton has no obvious deposits of pure water-ice, Junichi Haruyama of the Japan Aerospace Exploration Agency in Sagamihara and his colleagues report online October 23 in Science.
Although data gathered by the craft indicate that Shackleton’s floor has a temperature of less than 90 kelvins (–183° Celsius), cold enough to freeze water, the crater’s bottom has, at most, only a small percentage of frozen water interspersed with soil, the researchers say. The craft’s Terrain Camera, which can discern features as small as 10 meters, was able to image Shackleton’s floor because sunlight scattered from the crater’s inner wall, near the rim, illuminates the floor. Ice would show up as bright, highly reflective patches, and the images show no such features.
It’s possible, the researchers note, that the small excess of hydrogen ions recorded by the Lunar Prospector are merely ions from the solar wind that became trapped in the crater’s lunar soil, rather than evidence of frozen water.
“Who ever claimed there was visible water-ice inside Shackleton?” asks Paul Spudis of the Lunar and Planetary Institute in Houston, a researcher affiliated with the Clementine mission. “If you read our original 1996 Science paper, we advocated patchy, dirty ice in Shackleton to explain our observations. Nothing in these new pictures shows this configuration to be untenable.”
Spudis made his comments from India, where he witnessed the October 22 launch of the robotic mission Chandrayaan-1, India’s first venture to the moon. With two NASA instruments on the mission — a detector that will map the moon’s minerals and a radar instrument that will examine the nature of material about two meters below the surface — “we’ll soon have more data on this area of the moon,” says Spudis. “The search [for ice] continues.”
