All science begins in a star trek mode: go where no one has gone before and discover new things without knowing in advance what they might be. As researchers complete their initial surveys and accumulate a long list of questions, they shift to a Sherlock Holmes mode: formulate specific hypotheses and develop ways to test them. The exploration of Mars is now about to make this transition. Orbiters have made global maps of geographic features and composition, and landers have pieced together the broad outlines of the planet's geologic history. It is time to get more sophisticated.

Our team has built the Mars Science Laboratory, also known as the Curiosity rover, on the hypothesis that Mars was once a habitable planet. The rover carries an analytic laboratory to test that hypothesis and find out what happened to the early clement environment we believe the planet had. Loosely defined, a habitable environment has water, energy and carbon. Past missions have focused on the first requirement and confirmed that Mars had—and occasionally still has—liquid water [see “The Red Planet's Watery Past,” by Jim Bell; Scientific American, December 2006]. Those missions have also seen hints of geochemical gradients that would provide energy for metabolism. But none has seen carbon in a form potentially suitable for life.

Like the twin Viking landers of the mid-1970s, Curiosity carries a gas chromatograph/mass spectrometer capable of sensing organic compounds, whether biological or abiological in origin. Unlike Viking, however, Curiosity is mobile and is touching down in a far more promising site. More important than finding carbon itself, the mission aims to discover how to conduct the search. Even on Earth, we are not entirely sure how to trawl the deep geologic record for preserved biosignatures. Paradoxically, the very characteristics that make so many environments habitable—water, oxidants, and chemical and temperature gradients—also tend to destroy organic compounds. Paleontologists have learned to seek the rare circumstances that facilitate preservation, such as geochemical conditions that favor very early mineralization. Silica, phosphate, clay, sulfate and, less commonly, carbonate are all known to entomb organics as they precipitate. Orbiters have made maps of some of these minerals at Curiosity's landing site, which will guide its perambulations.