[1] Inst Phys Globe, CNRS INSU, UMR 7154, F-75251 Paris 05, France
[2] UPD, CNRS INSU, UMR 7154, F-75251 Paris, France
[3] CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA
[4] Univ Nantes, Lab Planetol & Geodynam, CNRS, UMR 6112, F-44322 Nantes 3, France
[5] Univ Reading, Sch Human & Environm Sci, Reading RG6 6AB, Berks, England
[6] CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA