[1] Planetary Sci Inst, Tucson, AZ 85719 USA
[2] Inst Astrofis Canarias, Tenerife 38200, Spain
[3] Univ La Laguna, Dept Astrofis, Tenerife 38205, Spain
[4] Univ Porto, Ctr Astrofis, P-4150762 Oporto, Portugal
[5] Univ Porto, Dept Fis & Astron, Fac Ciencias, P-4150762 Oporto, Portugal