Evidence of habitability and habitation of Mars may be forthcoming by returning samples to Earth. Clear objectives and associated choices of samples is essential to maximize the opportunities presented by returned samples. In the context of the solar system, the relative similarity of Earth and Mars generates an expectation of biochemical harmony for Earth and Mars. We can confidently predict that any biochemical scaffolding on Mars would be based on carbon and any biochemical solvent would be based on water. To expect otherwise would require planetary conditions and chemistries that differ dramatically from those of either Earth or Mars. Reduced carbon is therefore a beacon for the potential discovery of evidence of life in a sample. Any reduced carbon detected in samples from Mars should also have features that provide the ability to discriminate between non-life and life sources and, preferably, between an origin on Earth and Mars. For detecting life, the usefulness of organic carbon to biochemistry is in its ability to form complex and specific organic structures.
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