Organic Geochemistry's Looming Test

If the Mars Sample Return program goes as planned, 11 years from now, a canister containing 40 some samples of lake sediments and other rocks from the surface of Mars will land in Utah, and, after a (hopefully brief!) period of worrying about the imagined dangers of extraterrestrial microbes, these samples will be released for study in Earth’s laboratories. All sorts of scientists will approach these samples with all sorts of goals, but the question with arguably the biggest stakes, the broadest interest, and the greatest potential consequences will be whether these rocks contain incontrovertible evidence of present or past life on Mars.

Over the last 26 years, several studies of Martian meteorites have tried to answer this question, with results that depend on what you think counts as ‘evidence’, and what it takes for you to rank that evidence as ‘incontrovertible’. Personally, I am unconvinced by any of the putative biosignatures found so far in the fragments of Mars impact ejecta that have landed on Earth. But returned samples will include sediments collected from well described locations, and just might hold a discovery even a fussy reader like myself would accept: a living organism, a fossil eye ball, or clearly non terrestrial proteins, nucleic acids, or similarly complex biomolecules.