1. Earth and Planetary Sciences Department; Planetary Geosciences Institute; University of Tennessee; Knoxville Tennessee 37996 USA
2. Department of Geological Sciences; University of Cape Town; Rondebosch 7701 South Africa
3. Jet Propulsion Laboratory; California Institute of Technology; Pasadena California 91109 USA
4. Division of Geology and Planetary Science; California Institute of Technology; Pasadena California 91125 USA