1. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
2. Planetary and Space Sciences, School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
3. Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK
4. Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussée Boulevard, BG-1784 Sofia, Bulgaria