NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft has found indications that water ice may make up as much as 22 percent of the material on the surface of a crater located at the moon’s south pole.
The research team used laser light from LRO’s laser altimeter to examine the floor of Shackleton crater. They found that the crater’s floor is brighter than those of other nearby craters, which they think is consistent with the presence of limited amounts of ice. This data will allow researchers to better understand crater formation and study other uncharted areas of the moon.
“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 on 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.”
Shackleton crater was also mapped with unprecedented detail, by using LRO’s laser to illuminate the crater’s interior and measure its natural reflectance, or albedo. “The laser light measures to a depth comparable to its wavelength, or about a micron. That represents a millionth of a meter, or less than one ten-thousandth of an inch.” The researchers also used the laser to map the crater’s terrain based on the time it took for laser light to bounce back from the moon’s surface. The length of time it takes to bounce back tracks with the terrain’s elevation.
This mapping of Shackleton Crater gave the researchers their best-yet views of the crater’s permanently-shadowed interior.
“In addition to the possible evidence of ice, the group’s map of Shackleton revealed a remarkably preserved crater that has remained relatively unscathed since its formation more than three billion years ago. The crater’s floor is itself pocked with several small craters, which may have formed as part of the collision that created Shackleton.”
Shackleton crater is named after the Antarctic explorer Ernest Shackleton. It’s two miles deep and more than 12 miles wide. Like many other craters at the moon’s south pole, the slight tilt of the lunar spin axis leaves Shackleton crater’s interior permanently dark and also extremely cold.
“The crater’s interior is extremely rugged,” said Maria Zuber, the team’s lead investigator from the Massachusetts Institute of Technology in Cambridge in Mass. “It would not be easy to crawl around in there.”
While the crater’s floor was relatively bright, the researchers observed that the 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.” The ultra-high-resolution map made by the research team provides strong evidence that there is 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.”
LRO’s first primary objective was to conduct investigations that will prepare the way for future lunar explorations. After completing its primary exploration mission some time ago, it is now in its primary science mission. LRO was built and is managed by Goddard. This research was supported by NASA’s Human Exploration and Operations Mission Directorate and Science Mission Directorate at the agency’s headquarters in Washington.
The research has just been published in the journal Nature.
Image Credits: NASA/Zuber, M.T. et al., Nature, 2012