1. Earth Sciences Directorate, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
2. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
4. Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
5. Department of Geological Sciences, Brown University, Providence, RI 02912, USA.
6. Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
7. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.