1. Department of Geology; Lund University; Sölvegatan 12 22362 Lund Sweden
2. Centro de Astrobiología (CSIC/INTA); Instituto Nacional de Técnica Aeroespacial; Ctra de Torrejón a Ajalvir km 4 Torrejón de Ardoz Madrid 28850 Spain
3. Department of Earth Sciences; University of Gothenburg; PO Box 460 SE-405 30 Göteborg Sweden