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T. H. McConnochie (Cornell), B. J. Buratti, J. K. Hillier, K. A. Tryka (JPL)
We describe the results of an analysis of permanently shadowed regions of the Moon for optical and near-IR signatures of water ice on the lunar surface. Significant amounts of multiply scattered radiation partly illuminate primary shadows cast by craters and other features. If there is water ice in the permanently shadowed regions of the surface of the Moon, its spectral signature should appear in this multiply scattered light. Images obtained by Clementine are particularly well suited to this task because the spacecraft's polar orbit allowed images of the poles to be acquired on nearly every orbit, resulting in literally thousands of images taken within a few degrees of each pole, and because the filters on the Ultraviolet-Visual camera (UVVIS) and the Near Infrared camera (NIR) were placed at major absorption bands or within important continuum features of water ice (Nozette et al. 1994, Science 266, 1835-1893).
Approximately 5800 images obtained by the UVVIS camera and 1800 images obtained by the NIR camera were calibrated and combined into co-added mosaics to create multispectral maps of the lunar poles with the highest possible signal-to-noise. Unfortunately, analysis of our UVVIS mosaics indicates that any possible signal from multiply scattered light in primary shadows was overwhelmed by instrumental stray light. For the NIR camera, we were able to determine the normalized reflectance of several regions that were identified by Margot et al. (1999, Science 284, 1658-1660) as permanent shadows. We have identified one permanently shadowed crater with a 1.5 micron band spectral signature indicative of between 2.5 and 21 percent fractional coverage of H2O frost. However, the same region shows a 2.0 micron spectral signature that is inconsistent with the presence of any water.