1. Department of Astronomy, Cornell University, Ithaca, NY 14853, USA.
2. Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
3. Abteilung Kosmochemie, Max-Planck-Institut für Chemie, Postfach 3060, D-55128 Mainz, Germany.
4. NASA Ames/SETI Institute, Moffett Field, CA 94035, USA.
5. Department of Geological Sciences, University of Nevada, Reno, NV 89557, USA.