A 50-50 mix of the two forms of amino acids on Mars would suggest that although the planet meets the necessary conditions, there is not currently life on Mars, says Allen Farrington, Urey project manager at NASA's Jet Propulsion Laboratory, in Pasadena, CA. A strong deviation from a 50-50 mix, however, would be evidence of current or recently extinct life on Mars. Bada says the most unambiguous result would be the discovery of a preponderance of right-handed amino acids. This would suggest the presence of life on Mars completely independent of life on Earth. A preponderance of left-handed amino acids would be more mysterious. It could mean that life on Mars also selected the left-handed form by chance. "Or it could imply either that we're Martians or that the Martian bugs are from Earth," says Bada. The two planets swap meteorites--and it's possible that past Mars missions have contaminated the planet with Earth microbes.

Urey will also test how long organic compounds can persist on the Martian surface. President Bush has declared manned missions to Mars a long-term goal. But before human beings or more robots are sent to the planet, it's crucial to have a better understanding of the Martian environment, says Bada. "If the oxidants on Mars are so potent that [organic compounds] get immediately destroyed, astronauts would have to be in a sheltered environment all the time," he says. "Space suits would immediately fall apart."

Passive sensors on the ExoMars rover's deck will test how corrosive the Martian environment is. As an array of about 45 postage-stamp-size thin films of organic compounds, which have been coated on cells on a plate, degrade, the electrical potential across the cells will change measurably. Researchers on Earth will track the degradation in real time: slower degradation means more-hospitable conditions.

Urey prototypes have uncovered organic compounds in the deserts of California and Peru. Technicians at the Jet Propulsion Laboratory are building a sterile, space-worthy iteration of the detector. ExoMars is currently scheduled to launch in 2013. The orbiter will take a two-and-a-half-year route to the planet; after landing, the rover carrying Urey will explore the planet's surface for about 180 days.