1. Department of Planetary Sciences. University of Arizona, Tucson 85721
2. U.S. Geological Survey, Flagstaff, Arizona 86001
3. Department of Astronomy, New Mexico State University, Las Cruces 88003
4. NASA Headquarters, Washington, D.C. 20546
5. U.S. Geological Survey, Menlo Park, California 94025
6. Jet Propulsion Laboratory, Pasadena, California 91103
7. Smithsonian Astrophysical Observatory, Cambridge, Massachusetts 02138
8. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena 91125
9. Rand Corporation, Santa Monica, California 90406
10. University College London, London WC 1E 6BT, England
11. Division of Geological and Planetary Sciences, California Institute of Technology
12. Jet Propulsion Laboratory
13. U.S. Geological Survey, Flagstaff
14. Institute for Astronomy, University of Hawaii, Honolulu 96822
15. Department of Earth and Space Sciences, State University of New York, Stony Brook 11790
16. Cornell University, Ithaca, New York 14853
17. U.S. Geological Survey, Flafstaff
18. Lunar and Planetary Laboratory, University of Arizona
19. Space Science and Engineering Center, University of Wisconsin, Madison 53706
20. Cornell University