Look to the nitrogen.
I wish to comment on Emily Lakdawalla's article on NASA's new Mars rover Curiosity and the search for life (S&T: December 2011, page 22). The very first thing that investigators should look for, on Mars or any other planet, is a nitrogen cycle. Nitrogen is an essential component of all DNA, RNA, and proteins. Its unique properties provide the glue--or, perhaps more aptly, "Velcro"--that holds DNA in a spiral helix yet allows the helix to partially unzip for duplication or RNA transcription.
On Earth, a major abiotic system for nitrogen fixation is the combination of an oxygen-nitrogen atmosphere and lightning. This combination produces nitric acid, which on the ground is neutralized by metal oxides and carbonates to form nitrate ions that plants can use as a nitrogen source. Some microorganisms also produce nitrate directly in an oxygen-nitrogen atmosphere without the need for abiotic sources. For example, the bacterium Pseudomonas radicicola forms nodules on the roots of some plants, particularly legumes. This is a symbiotic relationship: Pseudomonas supplies nitrate, and the plant returns sugar.
If there is no life on Mars today, it may mean there's no nitrogen cycle. And if there never was a stable, long-lived nitrogen cycle--long enough for life to emerge--there probably never has been life on Mars.