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
WASHINGTON -- NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft has
returned data that indicate ice may make up as much as 22 percent of
the surface material in a crater located on the moon's south pole.
The team of NASA and university scientists using laser light from
LRO's laser altimeter examined the floor of Shackleton crater. They
found the crater's floor is brighter than those of other nearby
craters, which is consistent with the presence of small amounts of
ice. This information will help researchers understand crater
formation and study other uncharted areas of the moon. The findings
are published in Thursday's edition of the journal Nature.
"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."
The spacecraft mapped Shackleton crater with unprecedented detail,
using a laser to illuminate the crater's interior and measure its
albedo or natural reflectance. 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
team also used the instrument to map the relief of the crater's
terrain based on the time it took for laser light to bounce back from
the moon's surface. The longer it took, the lower the terrain's
elevation.
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.
The crater, named after the Antarctic explorer Ernest Shackleton, is
two miles deep and more than 12 miles wide. Like several craters at
the moon's south pole, the small tilt of the lunar spin axis means
Shackleton crater's interior is permanently dark and therefore
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, 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 initial primary objective of LRO was to conduct investigations
that prepare for future lunar exploration. Launched in June 2009, LRO
completed its primary exploration mission and 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.
