1. National Institute of Polar Research Tachikawa Tokyo 190‐8518 Japan
2. Université Européenne de Bretagne Lorient France
3. CNRS UMR 6538 (Domaines Océaniques) U.B.O.‐I.U.E.M. Place Nicolas Copernic 29280 Plouzané Cedex France
4. Planetary and Space Sciences Department of Physical Sciences The Open University Walton Hall Milton Keynes MK7 6AA UK