100 and 50 years ago

NATURE | VOL 428 | 8 APRIL 2004 | www.nature.com/nature 611 could be released, the OMEGA results described by Bibring et al. exclude any possibility that the southern perennial cap could appreciably affect the atmospheric pressure. The search for the existence of water on Mars, past and present, will continue. Future reports will often confirm less-direct evidence, as is the case with the information about the widespread existence of water ice at the martian south pole provided by OMEGA. But this does not lessen the importance of these discoveries. Did Mars ever support life? Will Mars support human life in the future? The answers depend on understanding the past and present distribution of both water and CO2.Life, as we know it, requires liquid water. Yet the long-term stability of liquid surface water requires a thicker atmosphere than Mars has at present. OMEGA’s observations show that its past atmosphere, predominantly CO2, is not locked up in the polar caps. We can hope that OMEGA and her sister instruments on Mars Express, Mars Global Surveyor and Mars Odyssey, along with reports from the