1. School of Physical Science; University of Kent; Canterbury CT2 7NH UK
2. Department of Earth Sciences; Natural History Museum; London SW7 5BD UK
3. Dunholme, Raven Hall Road Ravenscar North Yorkshire YO13 0NA UK
4. Imaging and Analysis Centre; Natural History Museum; London SW7 5BD UK
5. Hawai'i Institute of Geophysics and Planetology; University of Hawai'i at Manoa; 1680 East-West Road, POST 602 Honolulu Hawai'i 96822 USA