1. Head of the Planetary Systems Branch, Code 693, Goddard Space Flight Center, Greenbelt, MD 20771.
2. Associate research scientist at the Center for Astrophysical Sciences, Johns Hopkins University, Baltimore, MD 21218.
3. Members of the Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721.
4. Research Associate at the Steward Observatory, University of Arizona, Tucson, AZ 85721.