1. Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275
2. McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130
3. Solar System Exploration Division, National Aeronautics and Space Administration, Washington, DC 20546
4. Department of Astronomy, Cornell University, Ithaca, NY 14853
5. University of London Observatory, London, NW7 2QS England
6. U.S. Geological Survey, Flagstaff, AZ 86001