1. Departamento de Astronomia, Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS 91509-900, Brazil
2. Centro de Astrobiología, CSIC-INTA, Torrejón de Ardoz, E-28850 Madrid, Spain
3. School of Physics and Astronomy, The University of Nottingham, University Park, Nottingham NG7 2RD, UK