1. Space Research Institute, 117810 Moscow, U.S.S.R.
2. NASA Ames Research Center, Moffett Field, CA 94035.
3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena 91125.
4. Jet Propulsion Laboratory, California Institute of Technology, Pasadena 91109.
5. Centre National d'Etudes Spatiales, 75039 Paris Cedex 01, France.