Fresh evidence for life on Mars

Evidence that Mars once fostered life, at least in microbial form, is supported by two papers published in the journal Science today. The findings are likely to spark intense debate in Houston, Texas, next week, when an international gathering of geochemists and biologists have set aside two days of a week-long conference programme solely to debate results from examination of samples from the meteorite ALH84001.

The first paper, by a team from the California Institute of Technology, found that magnetic grains inside the meteorite cooled in the presence of a strong magnetic field on Mars about 4 billion years ago, but were not reheated. This would back up the theory put forward last August by Nasa scientists, who suggested that carbonaceous deposits were formed by living organisms, rather than high-temperature reactions.

The second paper, which examines the ratios of carbon isotopes within the carbonate deposits, also suggests they were formed at low temperatures. "Everything we see is consistent with biological activity," said John Valley, a geochemist at the University of Wisconsin-Madison. "Though I still wouldn't rule out low-temperature inorganic processes. We have not proven that this represents life on Mars, but we have disproven the high- temperature hypothesis." Charles Arthur