No evidence for thick deposits of ice at the lunar south pole

The rim of the Shackleton crater at the lunar south pole is a candidate crash site for NASA's LCROSS probe (Lunar CRater Observation and Sensing Satellite), due to launch in 2008. The plume of debris kicked up by the crash is to be analysed in the hope that it will reveal the water thought to be there. Suggestions of lunar ice date from 1996 when data from the Clementine spacecraft gave some indications of the presence of ice on crater walls at the south pole. Now using high-resolution radar imagery, the radar scattering parameter thought indicative of thick ice deposits has been found also to match radar echoes from the rock-strewn walls and ejecta of young impact craters at all lunar latitudes. There is no evidence for thick ice, though there could be grains of water ice spread more thinly through the lunar soil. Shackleton crater at the Moon’s south pole has been suggested as a possible site of concentrated deposits of water ice, on the basis of modelling of bi-static radar polarization properties and interpretations of earlier Earth-based radar images1,2. This suggestion, and parallel assumptions about other topographic cold traps, is a significant element in planning for future lunar landings. Hydrogen enhancements have been identified in the polar regions3, but these data do not identify the host species or its local distribution. The earlier Earth-based radar data lack the resolution and coverage for detailed studies of the relationship between radar scattering properties, cold traps in permanently shadowed areas, and local terrain features such as the walls and ejecta of small craters. Here we present new 20-m resolution, 13-cm-wavelength radar images that show no evidence for concentrated deposits of water ice in Shackleton crater or elsewhere at the south pole. The polarization properties normally associated with reflections from icy surfaces in the Solar System4,5,6 were found at all the observed latitudes and are strongly correlated with the rock-strewn walls and ejecta of young craters, including the inner wall of Shackleton. There is no correlation between the polarization properties and the degree of solar illumination. If the hydrogen enhancement observed by the Lunar Prospector orbiter3 indicates the presence of water ice, then our data are consistent with the ice being present only as disseminated grains in the lunar regolith.