NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft hasreturned data that indicate ice may make up as much as 22 percent ofthe surface material in a crater located on the moon's south pole.
The team of NASA and university scientists using laser light fromLRO's laser altimeter examined the floor of Shackleton crater. Theyfound the crater's floor is brighter than those of other nearbycraters, which is consistent with the presence of small amounts ofice. This information will help researchers understand craterformation and study other uncharted areas of the moon. The findingsare published in Thursday's edition of the journal Nature.
"The brightness measurements have been puzzling us since two summersago," said Gregory Neumann of NASA's Goddard Space Flight Center inGreenbelt, Md., a co-author on the paper. "While the distribution ofbrightness was not exactly what we had expected, practically everymeasurement related to ice and other volatile compounds on the moonis surprising, given the cosmically cold temperatures inside itspolar craters."
The spacecraft mapped Shackleton crater with unprecedented detail,using a laser to illuminate the crater's interior and measure itsalbedo or natural reflectance. The laser light measures to a depthcomparable to its wavelength, or about a micron. That represents amillionth of a meter, or less than one ten-thousandth of an inch. Theteam also used the instrument to map the relief of the crater'sterrain based on the time it took for laser light to bounce back fromthe moon's surface. The longer it took, the lower the terrain'selevation.
In addition to the possible evidence of ice, the group's map ofShackleton revealed a remarkably preserved crater that has remainedrelatively unscathed since its formation more than three billionyears ago. The crater's floor is itself pocked with several smallcraters, which may have formed as part of the collision that createdShackleton.
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.
(Photo Credit: : NASA/Zuber, M.T. et al., Nature, 2012)
The crater, named after the Antarctic explorer Ernest Shackleton, istwo miles deep and more than 12 miles wide. Like several craters atthe moon's south pole, the small tilt of the lunar spin axis meansShackleton crater's interior is permanently dark and thereforeextremely cold.
"The crater's interior is extremely rugged," said Maria Zuber, theteam's lead investigator from the Massachusetts Institute ofTechnology in Cambridge in Mass. "It would not be easy to crawlaround in there."
While the crater's floor was relatively bright, Zuber and hercolleagues observed that its walls were even brighter. The findingwas at first puzzling. Scientists had thought that if ice wereanywhere in a crater, it would be on the floor, where no directsunlight penetrates. The upper walls of Shackleton crater areoccasionally illuminated, which could evaporate any ice thataccumulates. A theory offered by the team to explain the puzzle isthat "moonquakes"-- seismic shaking brought on by meteorite impactsor gravitational tides from Earth -- may have caused Shackleton'swalls to slough off older, darker soil, revealing newer, brightersoil underneath. Zuber's team's ultra-high-resolution map providesstrong evidence for ice on both the crater's floor and walls.
"There may be multiple explanations for the observed brightnessthroughout the crater," said Zuber. "For example, newer material maybe exposed along its walls, while ice may be mixed in with itsfloor."
The initial primary objective of LRO was to conduct investigationsthat prepare for future lunar exploration. Launched in June 2009, LROcompleted its primary exploration mission and is now in its primaryscience mission. LRO was built and is managed by Goddard. Thisresearch was supported by NASA's Human Exploration and OperationsMission Directorate and Science Mission Directorate at the agency'sheadquarters in Washington.
Source: NASA/Goddard Space Flight Center