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.
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We thank the staff of the Arecibo and Green Bank Observatories for their assistance with the observations, and J. Chandler for providing the lunar ephemeris observing files. This work was supported in part by grants from the NASA Planetary Astronomy and Planetary Geology and Geophysics Programs. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the US National Science Foundation. The Green Bank Telescope is part of the National Radio Astronomy Observatory, a facility of the NSF operated under a cooperative agreement by Associated Universities, Inc. Author Contributions The work was initiated by D.B.C., who also wrote the initial draft of the paper. D.B.C., B.A.C., L.M.C. and J.-L.M. planned and made the observations. Software development and data reduction were done by B.A.C, L.M.C.and J.-L.M. All authors participated in the interpretation of the results.
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Earth, Moon, and Planets (2010)