1. D. J. Lawrence, W. C. Feldman, B. L. Barraclough, R. C. Elphic, D. R. Thomsen, Space and Atmospheric Sciences, Mail Stop D466, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Telephone: 505-667-0945, Fax: 505-665-7395,. A. B. Binder, Lunar Research Institute, Gilroy, CA, 95020, USA. S. Maurice, Observatoire Midi-Pyrénées, Toulouse, France.