1. Department of Mineral Sciences; Smithsonian Institution; Washington District of Columbia 20560-0112 USA
2. Department of Astronomy; University of Maryland; College Park Maryland 20742-2421 USA
3. Johns Hopkins University Applied Physics Laboratory; Laurel Maryland 20723 USA
4. Department of Geological Sciences; Brown University; Providence Rhode Island 02912 USA
5. Department of Geology; Texas Christian University; Fort Worth Texas 76129 USA