Astrobiology Workshop, Macquarie University July 12-13 2001

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Martian Life: Evidence from Martian Meteorites

Everett Gibson (NASA Johnson Space Center)

In August 1996 our research team presented a chain of evidence within the ALH84001 meteorite which could be interpreted as signatures of relic Martian life. Additional research has shown that two more Martian meteorites contain similar evidence. Three SNC meteorites ranging in age from 4.5 Ga. to 1.3 Ga. to 165 m.y. contain features suggestive of past biogenic activity on Mars. Because we do not know what Martian life looks like or its physical or chemical properties, the only tools or criteria which the scientific community have to evaluate the evidence of past life on Mars is to use those used to evaluate the evidence for early life on Earth. Eight criteria established for evaluating evidence of early life on Earth include: geologic context of sample; sample's age and stratigraphic location; cellular morphology; colonies or biofilms; biominerals or evidence of chemical disequilibria; stable isotope patterns unique to biology; presence of organic biomarkers; and proof that the features are indigenous to the sample. For acceptance of past life within terrestrial samples essentially most or all of these criteria must be met.

The features within ALH84001's carbonate globules and the preterrestrial aqueous alteration phases of Nakhla and Shergotty have been interpreted as containing evidence of possible past life on Mars. Studies have shown conclusively that the reduced carbon components in ALH84001 and Nakhla are indigenous to the meteorites and are not terrestrial contaminants. Based on carbon isotopic compositions and mineralogical morphologies, there is no question or disagreement that the carbonate globules are embedded magnetites in ALH84001, and the preterrestrial aqueous alteration products in Nakhla and Shergotty were formed on Mars. The truncated hexa-octahedral magnetite crystals in ALH84001 have been shown to be identical to magnetites produced by magnetotactic bacteria. There are no known inorganic processes to explain the observation of truncated hexa-octahedral magnetite in ALH84001. They appear to best be interpreted as Martian magnetofossils. Possible microfossil structures and some reduced carbon components in the carbonates and preterrestrial aqueous alteration products, are therefore, almost certainly indigenous, but other possible evidence for life (e.g. amino acids) may be a result of terrestrial contamination.

Our hypothesis of possible early life on Mars was presented five years ago and today we believe that it stands stronger than when originally presented. To date, no fatal strikes have been made on any of the original four lines of evidence. While details of the hypothesis are evolving as new data is generated, we believe that our basic premise remains intact: these meteorites contain evidence suggestive for early life on Mars. The truncated hexa-octahedral magnetite crystals in ALH84001 constitute evidence of the oldest life yet found.