1. Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
2. Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, UK
3. School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, UK
4. School of Physics, University of Bristol, Bristol BS8 1TL, UK
5. Institute de Planétiology, Universityé Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France
6. SRON Netherlands Institute for Space Research, PO Box 800, NL-9700 AV Groningen, the Netherlands