1. Planetary Sciences Unit, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom
2. Department of Earth Sciences, Oxford University, Oxford OX1 3PR, United Kingdom
3. Department of Physics, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, United Kingdom
4. Max Planck Institut für Chemie, Saarstrasse 23, Mainz, Federal Republic of Germany