1. Institute of Geophysics and Planetary Physics and Department of Earth and Space Science, University of California, Los Angeles, California 90095, USA
2. Laboratoire de Physique de la Matière Condensée, CNRS UMR6622 and Université de Nice-Sophia Antipolis, B.P. 71, Parc Valrose, 06108 Nice Cedex 2, France