1. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
2. U.S. Geological Survey, Flagstaff, AZ 86001, USA.
3. Department of Astronomy, Cornell University, Ithaca, NY 14853, USA.
4. Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
5. Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
6. Physikalisches Institut, University of Bern, Bern, Switzerland.
7. NASA Ames Research Center and SETI Institute, Moffett Field, CA 94035, USA.