DOI: 10.1126/science.284.5420.1658 , 1658 (1999); 284Science et al.J. L. Margot,Interferometry: A Survey of Cold Trap LocationsTopography of the Lunar Poles from Radar www.sciencemag.org (this information is current as of September 1, 2008 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/284/5420/1658version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/284/5420/1658#otherarticles, 7 of which can be accessed for free: cites 16 articlesThis article 33 article(s) on the ISI Web of Science. cited byThis article has been http://www.sciencemag.org/cgi/content/full/284/5420/1658#otherarticles 1 articles hosted by HighWire Press; see: cited byThis article has been http://www.sciencemag.org/cgi/collection/planet_sciPlanetary Science : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. is aScience1999 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on September 1, 2008 www.sciencemag.orgDownloaded fromTopography of the Lunar Polesfrom Radar Interferometry: ASurvey of Cold Trap LocationsJ. L. Margot,1*† D. B. Campbell,1† R. F. Jurgens,2M. A. Slade2Detailed topographic maps of the lunar poles have been obtained by Earth-based radar interferometry with the 3.5-centimeter wavelength GoldstoneSolar System Radar. The interferometer provided maps 300 kilometers by 1000kilometers of both polar regions at 150-meter spatial resolution and 50-meterheight resolution. Using ray tracing, these digital elevation models were usedto locate regions that are in permanent shadow from solar illumination and mayharbor ice deposits. Estimates of the total extent of shadowed areas polewardof 87.5 degrees latitude are 1030 and 2550 square kilometers for the north andsouth poles, respectively.Topographic depressions near the lunar polesprovide a low-temperature environment thatis a potential reservoir of water ice deposits(1, 2). The case for lunar ice was recentlystrengthened by the detection of a hydrogensignature near the polar regions by the neu-tron spectrometer aboard the Lunar Prospec-tor (LP) spacecraft (3). However, it is stillunknown whether the excess hydrogen mea-sured by LP correlates with regions where thethermal conditions are compatible with thepresence of water ice. Here, radar-derivedtopographic maps of the lunar poles are usedto locate shadowed regions that may act ascold traps for ice deposits.Studies of the behavior of volatiles onthe moon have suggested that water icedeposits might survive for billions of yearsin cold traps near the lunar poles (1, 2). Thepossible source mechanisms for that waterinclude cometary and meteroidal impacts,reduction of FeO by hydrogen derived fromthe solar wind, and degassing of the interior(1, 2). Water molecules liberated on themoon assume ballistic trajectories (1, 2, 4 )until they are destroyed in flight or trappedin low-temperature regions (5 ). Ice on thelunar surface is subject to destructive pro-cesses, including solar photon–induced de-sorption (2, 6 ), erosion by sputtering (2, 6,7 ), and photodissociation by interstellarhydrogen Lyman-a radiation (6 ). Thesesurface loss processes imply that any icedeposits would have to be protected by athin regolith layer.The first attempts to provide observationalevidence of lunar ice deposits used radartechniques (8, 9). Icy bodies in the solar sys-tem exhibit abnormal radar signatures, highbackscatter cross sections, and a ratio of samesense–to–opposite sense circular polarizationthat is larger than unity. These characteristicshave been observed unambiguously in Earth-based radar echoes from permanently shad-owed craters near the poles of Mercury (10),but the lunar observations were much lessconclusive. A study of Clementine radar dataresulted in a report of a possible detection ofwater ice on the moon (8), but a separateanalysis of the same data did not find adistinctive ice signature (11). Earth-based ra-dar observations of the lunar poles did notshow evidence of ice deposits (9).Because the lunar poles have never beenobserved over the entire possible range ofsolar illumination conditions (12), establish-ing the correlation between the hydrogen sig-nature detected by LP (3) and the locations ofterrain in permanent shadow has been diffi-cult. We used Earth-based radar interferom-etry to obtain topographic maps of the lunarpolar regions at 150-m spatial and 50-mheight resolutions. These digital elevationmodels (DEMs) were used to simulate solarillumination conditions to determine the lo-cations of the lunar cold traps, which arepossible reservoirs of water ice. Unlike pre-vious estimates of the extent of shadowedareas at the lunar poles (8, 13), this surveytook into account the entire solar illuminationcycle.Detailed topographic maps of the lunarsurface can be acquired with an Earth-basedradar interferometer (14, 15 ). The measure-ments rely on range differences betweenpoints on the lunar surface and two receiverslocated on Earth. Changes in elevation areobservable because they affect the relativephase between the two radar echoes (14).The antennas of the Deep Space Network(DSN) at Goldstone, California, were used toobserve the lunar poles in October 1997. The70-m parabolic dish was used to transmit apulsed binary-coded waveform at a wave-length of 3.5 cm; two 34-m antennas separat-ed by 20 km formed the receiving interferom-eter. Separate maps of the (complex) radarbackscatter were produced from each receiv-ing site by means of the conventional delay-Doppler technique (16 ). The interferometricphase was then extracted on a per pixel basisand translated to topographic changes.The viewing geometry at the time of ourobservations was such that the radar line ofsight rose 6° to 7° above the horizon at thelunar poles, which is close to the maximumallowed by the Earth-moon geometry. Atthese angles, a fraction of the lunar landscapein the