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Possible Evidence of Martian Bacteria
half-seas-over writes "NASA issued a very interesting press release today. It highlights a recent study that compared tiny magnetite crystals in the Allan Hill meteorite to similar magnetite crystals that are created here on Earth by bacteria (who use the magnetite as a compass). The study (abstract available here (PDF) from this site) uses fairly strict criteria to determine that 25% of the magnetite content of the meteorite was created by ancient (>3.9Gyr ago) martian bacteria... either that or there is some strange natural process that makes very pure, isolated magnetite crystals that we haven't imagined or seen on Earth which is present on Mars. We'll have to wait and see what happens next, 'extraordinary claims require extraordinary evidence' -Carl Sagan."
The rock is from Mars. Atmosphere isotope ratios are unambiguous in such cases. Every planet has it's own "signature" that, with the right equipment, is possible to detect. This was demonstrated ages ago by Monica Grady and others in the UK.
You're correct that it's easier to get a meteorite from a smaller body, and we do get loads of them (positively identified Martian meteorites number in the dozens rather than the thousans). However, Mars is massive and has had many massive impacts. The amount of material ejected from it's surface means that it would be amazing if none had reached the Earth.
Also, regarding life, it's unlikely that life would have evolved on any body that did not have liquid water. Liquid water has always been unstable on asteroids, whereas there have significant periods of Martian history (likely when the impact occurred) where liquid water was thought to be stable, possibly over hundreds of millions of years. In fact, there are even points on present day Mars where, for a limited period during the year, water can be stable on the surface. Of course, if you believe Fred Hoyle, life could be everywhere, but, based on an Earth model, life seems far more likely to have evolved on Earth and/or Mars.
The evidence for this being a result of biological activity is still highly ambiguous however, which is why we need to get samples back from Mars.
-Karl
Dr Karl Mitchell
Planetary Science Research Group
Environmental Science Department
Lancaster University
UK