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J. L. Margot, D. B. Campbell (Cornell University), R. F. Jurgens, M. A. Slade (JPL/Caltech)
Earth-based radar interferometry [1] has been used to map the lunar polar regions and Tycho Crater at high spatial (~100~m) and height (~50~m) resolutions. Compared to existing topographic data sets, the radar observations offer digital elevation models with dense horizontal spacing and improved height resolution. Earth-based radars can also provide measurements of the largely unknown topography in the polar regions. Elevation data and radar imagery obtained with the Goldstone X-band system (\lambda = 3.5 cm) are presented for the Tycho Crater area, with a spatial resolution of 200~m and a height resolution of 30~m. A careful comparison of the radar-derived topography with Clementine altimetry points [2] reveals a very good agreement between the two techniques. Rms deviations between the radar-derived heights and 87 Clementine points available over the 200 x 200 km scene are ~100~m. The digital elevation model allows detailed morphometry of the 85 km diameter crater: the floor of Tycho lies 3970~m below a 1738~km radius sphere, and the crater's central peak rises 2400~m above the floor. The average rim crest elevation is 730~m above the 1738~km datum, giving a mean rim to floor depth of 4700~m. The floor has two distinct units with the western section being higher in elevation by ~200~m. This dichotomy is consistent with an asymmetry in the crater shape which reveals that maximum wall slumping occured in the western and southwestern regions of the crater. Digital elevation models of the polar regions are being used to estimate the location of permanently shadowed areas which may harbor ice deposits [3]. The range of illumination conditions over the lunar polar regions could be sampled by an imaging instrument in a polar orbit during a full terrestrial year. Alternatively, topographic maps obtained with Earth-based radar can be used to model the illumination conditions over the entire solar illumination cycle.
[1] I. I. Shapiro {\it et al.} (1972). {\it Science}, {\bf 178}, 939. [2] D. E. Smith {\it et al.} (1997). {\it JGR}, {\bf 102}, 1591. [3] W. C. Feldman {\it et al.} (1998). Submitted to {\it Science}.