1. Planetary Sciences Laboratory, Martin Marietta Aerospace, Denver, Colorado 80201
2. U.S. Geological Survey, Reston, Virginia 22092
3. Department of Geology, Pomona College, Claremont, California 91711
4. Department of Geology and Institute of Meteoritics, University of New Mexico, Albuquerque 87123
5. U.S. Geological Survey Reston, Virginia 22092