[1] DLR Micrograv User Support Ctr, D-51147 Cologne, Germany
[2] DLR Inst Planetary Res, Rutherfordstr 2, D-12489 Berlin, Germany
[3] Univ Cologne, Dept Nucl Chem, Otto Fischer Str 12-14, D-50674 Cologne, Germany
[4] Univ Saarland, Dept Mat & Mat Technol, D-66123 Saarbrucken, Germany
[5] Univ Gottingen, Inst Phys 1, D-37077 Gottingen, Germany
[6] Finnish Meteorol Inst, Earth Observat Div, POB 503, FI-00101 Helsinki, Finland