Topography of the Lunar Poles from Radar Interferometry A Survey of Cold Trap Locations J L Margot et al Science 284 1658 1999 DOI 10 1126 science 284 5420 1658 The following resources related to this article are available online at www sciencemag org this information is current as of September 1 2008 This article cites 16 articles 7 of which can be accessed for free http www sciencemag org cgi content full 284 5420 1658 otherarticles This article has been cited by 33 article s on the ISI Web of Science This article has been cited by 1 articles hosted by HighWire Press see http www sciencemag org cgi content full 284 5420 1658 otherarticles This article appears in the following subject collections Planetary Science http www sciencemag org cgi collection planet sci Information about obtaining reprints of this article or about obtaining permission to reproduce this article in whole or in part can be found at http www sciencemag org about permissions dtl Science print ISSN 0036 8075 online ISSN 1095 9203 is published weekly except the last week in December by the American Association for the Advancement of Science 1200 New York Avenue NW Washington DC 20005 Copyright 1999 by the American Association for the Advancement of Science all rights reserved The title Science is a registered trademark of AAAS Downloaded from www sciencemag org on September 1 2008 Updated information and services including high resolution figures can be found in the online version of this article at http www sciencemag org cgi content full 284 5420 1658 Topography of the Lunar Poles from Radar Interferometry A Survey of Cold Trap Locations J L Margot 1 D B Campbell 1 R F Jurgens 2 M A Slade2 Detailed topographic maps of the lunar poles have been obtained by Earthbased radar interferometry with the 3 5 centimeter wavelength Goldstone Solar System Radar The interferometer provided maps 300 kilometers by 1000 kilometers of both polar regions at 150 meter spatial resolution and 50 meter height resolution Using ray tracing these digital elevation models were used to locate regions that are in permanent shadow from solar illumination and may harbor ice deposits Estimates of the total extent of shadowed areas poleward of 87 5 degrees latitude are 1030 and 2550 square kilometers for the north and south poles respectively Topographic depressions near the lunar poles provide a low temperature environment that is a potential reservoir of water ice deposits 1 2 The case for lunar ice was recently strengthened by the detection of a hydrogen signature near the polar regions by the neutron spectrometer aboard the Lunar Prospector LP spacecraft 3 However it is still unknown whether the excess hydrogen measured by LP correlates with regions where the thermal conditions are compatible with the presence of water ice Here radar derived topographic maps of the lunar poles are used to locate shadowed regions that may act as cold traps for ice deposits Studies of the behavior of volatiles on the moon have suggested that water ice deposits might survive for billions of years in cold traps near the lunar poles 1 2 The possible source mechanisms for that water include cometary and meteroidal impacts reduction of FeO by hydrogen derived from the solar wind and degassing of the interior 1 2 Water molecules liberated on the moon assume ballistic trajectories 1 2 4 until they are destroyed in flight or trapped in low temperature regions 5 Ice on the lunar surface is subject to destructive processes including solar photon induced desorption 2 6 erosion by sputtering 2 6 7 and photodissociation by interstellar hydrogen Lyman a radiation 6 These surface loss processes imply that any ice deposits would have to be protected by a thin regolith layer The first attempts to provide observational Department of Astronomy Space Sciences Building Cornell University Ithaca NY 14853 USA 2Jet Propulsion Laboratory MS 238 420 4800 Oak Grove Drive Pasadena CA 91109 USA 1 Present address Arecibo Observatory HC3 Box 53995 Arecibo PR 00612 Puerto Rico To whom correspondence should be addressed Email margot naic edu J L M campbell astrosun tn cornell edu D B C 1658 evidence of lunar ice deposits used radar techniques 8 9 Icy bodies in the solar system exhibit abnormal radar signatures high backscatter cross sections and a ratio of same sense to opposite sense circular polarization that is larger than unity These characteristics have been observed unambiguously in Earthbased radar echoes from permanently shadowed craters near the poles of Mercury 10 but the lunar observations were much less conclusive A study of Clementine radar data resulted in a report of a possible detection of water ice on the moon 8 but a separate analysis of the same data did not find a distinctive ice signature 11 Earth based radar observations of the lunar poles did not show evidence of ice deposits 9 Because the lunar poles have never been observed over the entire possible range of solar illumination conditions 12 establishing the correlation between the hydrogen signature detected by LP 3 and the locations of terrain in permanent shadow has been difficult We used Earth based radar interferometry to obtain topographic maps of the lunar polar regions at 150 m spatial and 50 m height resolutions These digital elevation models DEMs were used to simulate solar illumination conditions to determine the locations of the lunar cold traps which are possible reservoirs of water ice Unlike previous estimates of the extent of shadowed areas at the lunar poles 8 13 this survey took into account the entire solar illumination cycle Detailed topographic maps of the lunar surface can be acquired with an Earth based radar interferometer 14 15 The measurements rely on range differences between points on the lunar surface and two receivers located on Earth Changes in elevation are observable because they affect the relative phase between the two radar echoes 14 The antennas of the Deep Space Network DSN at Goldstone California were used to observe the lunar poles in October 1997 The 70 m parabolic dish was used to transmit a pulsed binary coded waveform at a wavelength of 3 5 cm two 34 m antennas separated by 20 km formed the receiving interferometer Separate maps of the complex radar backscatter were produced from each receiving site by means of the conventional delayDoppler technique 16 The interferometric phase was then extracted on a per pixel basis and translated to topographic changes The viewing