[1] WASHINGTON UNIV, DEPT EARTH & PLANETARY SCI, MCDONNELL CTR SPACE SCI, ST LOUIS, MO 63160 USA
[2] UNIV NEWCASTLE UPON TYNE, SCH PHYS, INST LUNAR & PLANETARY SCI, NEWCASTLE UPON TYNE NE1 7RU, TYNE & WEAR, ENGLAND
[3] CALTECH, JET PROP LAB, PASADENA, CA 91103 USA