Meteorites
NASA maintains a catalog of 34 Mars meteorites. These assets are highly valuable since they are the only physical samples available of Mars. Studies conducted by NASA's Johnson Space Center show that at least three of the meteorites contain potential evidence of past life on Mars, in the form of microscopic structures resembling fossilized bacteria (so-called biomorphs). Although the scientific evidence collected is reliable, its interpretation varies. To date, none of the original lines of scientific evidence for the hypothesis that the biomorphs are of exobiological origin (the so-called biogenic hypothesis) have been either discredited or positively ascribed to non-biological explanations.
Over the past few decades, seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:
- Is the geologic context of the sample compatible with past life?
- Is the age of the sample and its stratigraphic location compatible with possible life?
- Does the sample contain evidence of cellular morphology and colonies?
- Is there any evidence of biominerals showing chemical or mineral disequilibria?
- Is there any evidence of stable isotope patterns unique to biology?
- Are there any organic biomarkers present?
- Are the features indigenous to the sample?
For general acceptance of past life in a geologic sample, essentially most or all of these criteria must be met. All seven criteria have not yet been met for any of the Martian samples, but continued investigations are in progress.
As of 2010, reexaminations of the biomorphs found in the three Martian meteorites are underway with more advanced analytical instruments than previously available. The scientists conducting the study at Johnson Space Center believed that before the end of the year they would find in the meteorites definitive evidence for past life on Mars.
Read more about this topic: Life On Mars