Constraints on the volatile distribution within Shackleton crater at the lunar south pole

Observations from the Lunar Orbiter Laser Altimeter reveal the Moon’s Shackleton crater to be an ancient, unusually well-preserved simple crater whose interior walls are younger than its floor and rim; the relative brightness of the floor at 1,064 nanometres is most readily explained by minimal volatile accumulation since crater formation and decreased space weathering due to permanent shadow. Little sign of water at lunar south pole The Shackleton crater, which is situated close to the Moon's south pole in almost permanent shadow, has been the target of numerous Earth- and spacecraft-based analyses because of the possibility that it contains water ice. So far, however, results have been inconclusive. Here, Maria Zuber and co-authors present detailed maps of the Shackleton crater based on observations from the Lunar Orbiter Laser Altimeter. The data confirm the crater as one of ancient origin, an unusually well preserved, simple crater with interior walls fresher than the floor or rim. Deposits on the crater floor are nearly the same age as the rim, suggesting little floor deposition since the crater's formation more than 3 billion years ago. The authors conclude that their observations are best explained by downslope movement of regolith on the walls exposing fresher underlying material, rather than by significant amounts of water ice within the crater. The relatively bright crater floor may be a product of decreased space weathering because to shadowing — although a one-micrometre-thick layer containing about 20% surficial ice remains a possibility. Shackleton crater is nearly coincident with the Moon’s south pole. Its interior receives almost no direct sunlight and is a perennial cold trap 1 , 2 , making Shackleton a promising candidate location in which to seek sequestered volatiles 3 . However, previous orbital and Earth-based radar mapping 4 , 5 , 6 , 7 , 8 and orbital optical imaging 9 have yielded conflicting interpretations about the existence of volatiles. Here we present observations from the Lunar Orbiter Laser Altimeter on board the Lunar Reconnaissance Orbiter, revealing Shackleton to be an ancient, unusually well-preserved simple crater whose interior walls are fresher than its floor and rim. Shackleton floor deposits are nearly the same age as the rim, suggesting that little floor deposition has occurred since the crater formed more than three billion years ago. At a wavelength of 1,064 nanometres, the floor of Shackleton is brighter than t