1. S. F. Hviid, M. B. Madsen, H. P. Gunnlaugsson, W. Goetz, J. M. Knudsen, A. R. Dinesen, C. T. Mogensen, M. Olsen, C. T. Pedersen, L. Vistisen, Oersted Laboratory, Niels Bohr Institute for Astronomy, Physics, and Geophysics, University of Copenhagen, Copenhagen, Denmark.
2. R. B. Hargraves, Department of Geosciences, Princeton University, Princeton, NJ 08544, USA.
3. P. Smith and D. Britt, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.