1. KVI Atomic Physics, Rijksuniversiteit Groningen, Zernikelaan 25, 9747 AA Groningen, the Netherlands
2. Planetary Exploration Group, Space Department, Johns Hopkins University, Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, USA
3. SRON, Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Landleven 12, 9747 AD Groningen, the Netherlands