1. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
2. Jet Propulsion Laboratory, MS 183-501, California Institute of Technology, Pasadena, CA 91109
3. National Aeronautics and Space Administration, Ames Research Center, MS 245-6, Moffett Field, CA 94035
4. Institute for Astronomy, University of Hawaii, Honolulu, HI 96822