1. R. Rieder, H. Wänke, J. Brückner, G. Dreibus, Max-Planck-Institut für Chemie, P.O. Box 3060, D-55020 Mainz, Germany.
2. T. Economou and A. Turkevich, Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA.
3. J. Crisp, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
4. H. Y. McSween Jr., Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996, USA.