1. R. H. Brown, Lunar and Planetary Laboratory and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA.
2. D. P. Cruikshank and Y. Pendleton, NASA Ames Research Center, Moffett Field, CA 94035, USA.
3. G. J. Veeder, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.