|
Scientists
using the Mini-RF radar on NASA’s Lunar Reconnaissance Orbiter (LRO)
have estimated the maximum amount of ice likely to be found inside a
permanently shadowed lunar crater located near the moon’s South Pole. As
much as five to ten percent of material, by weight, could be patchy
ice, according to the team of researchers led by Bradley Thomson at
Boston University’s Center for Remote Sensing, in Mass.
“These
terrific results from the Mini-RF team contribute to the evolving story
of water on the moon,” says LRO’s deputy project scientist, John Keller
of NASA’s Goddard Space Flight Center in Greenbelt, Md. “Several of the
instruments on LRO have made unique contributions to this story, but
only the radar penetrates beneath the surface to look for signatures of
blocky ice deposits.”
These
are the first orbital radar measurements of Shackleton crater, a
high-priority target for future exploration. The observations indicate
an enhanced radar polarization signature, which is consistent with the
presence of small amounts of ice in the rough inner wall slopes of the
crater. Thomson and his colleagues reported the findings in a paper
recently published in the journal Geophysical Research Letters.
“The
interior of this crater lies in permanent shadow and is a ‘cold trap’—a
place cold enough to permit ice to accumulate,” says Mini-RF’s
principal investigator, Ben Bussey of the Johns Hopkins University
Applied Physics Laboratory in Laurel, Md. “The radar results are
consistent with the interior of Shackleton containing a few percent ice
mixed into the dry lunar soil.”
These
findings support the long-recognized possibility that areas of
permanent shadow inside polar impact craters are sites of the potential
accumulation of water. Numerous lines of evidence from recent spacecraft
observations have revised the view that the lunar surface is a
completely dry, inhospitable landscape. Thin films of water and hydroxyl
have been detected across the lunar surface using several space-borne
near-infrared spectrometers. Additionally, orbital neutron measurements
indicate elevated levels of near?surface hydrogen in the polar regions;
if in the form of water, this hydrogen would represent an average ice
concentration of about 1.5% by weight in the polar regions.
The
Shackleton findings are also consistent with those of the recent LCROSS
spacecraft’s controlled collision with a nearby permanently shadowed
polar region near the lunar South Pole, which revealed evidence for
water in the plume kicked up by its impact. A radar instrument flown on
India’s Chandrayaan-1 spacecraft in 2009 found evidence for ice deposits
in craters at the lunar North Pole. Measurements of the albedo (surface
reflectance) inside Shackleton crater using LRO’s laser altimeter and
far?ultraviolet detector are also consistent with the presence of a
small amount of ice.
NASA’s Lunar Reconnaissance Orbiter has been studying the moon since June 2009. Credit: NASA |
“Inside
the crater, we don’t see evidence for glaciers like on Earth,” says
Thomson. “Glacial ice has a whopping radar signal, and these
measurements reveal a much weaker signal consistent with rugged terrain
and limited ice.”
The
radar measurements of Shackleton crater were made during three separate
observations between December 2009 and June 2010. Radar illuminates
shadowed regions and can detect deposits of water or ice, which have a
distinctive radar polarization signature compared to the surrounding
material. In addition, radar penetrates the terrain to depths of a meter
or two and can measure water or ice buried beneath the surface. Radar
measurements of Shackleton crater place an upper bound on the ice
content of the uppermost meter of loose material of the crater’s walls
at between five and ten percent ice by weight.
“We
are following up these tantalizing results with additional
observations,” says Bussey. “Mini-RF is currently acquiring new bistatic
radar images of the moon using a signal transmitted by the Arecibo
radio telescope in Puerto Rico. These bistatic images will help us
distinguish between surface roughness and ice, providing further unique
insights into the locations of volatile deposits.”
The
Mini-RF instrument, operated at the Johns Hopkins Applied Physics
Laboratory in Laurel, Md., is one of seven instruments on board NASA’s
LRO spacecraft. NASA Goddard developed and manages the LRO mission.
LRO’s current Science Mission is implemented for NASA’s Science Mission
Directorate. NASA’s Exploration Systems Mission Directorate sponsored
LRO’s initial one-year Exploration Mission that concluded in September
2010.
Source: NASA
