1. Department of Geosciences; Virginia Tech; Blacksburg Virginia 24061 USA
2. Jet Propulsion Laboratory; California Institute of Technology; Pasadena California 91109 USA
3. Institut de Recherche d'Astrophysique et Planétologie; University of Toulouse; Toulouse France
4. US Geological Survey; Astrogeology Science Center; Flagstaff Arizona 86001 USA
5. Earth, Planetary, and Space Sciences; University of California; Los Angeles California 90095 USA