In 2013, a chemical detector called Urey will travel to Mars with the European Space Agency’s ExoMars mission. Developed by researchers at NASA, the University of California, San Diego, and the University of California, Berkeley, Urey will search for chemical evidence of past or present life in the form of protein building blocks called amino acids. Urey, which will travel on a solar-powered rover, will also assess how long organic compounds can survive on the planet’s harsh, dusty surface.
The two things necessary for life as we know it are water and organic compounds like amino acids and the nucleic acids in DNA and RNA. “We now have clear evidence that there was, and still is, water on Mars,” says Jeffrey Bada, principal investigator on the Urey project. A recent study in the journal Science demonstrated that if the water on Mars’s south pole melted, it would cover the entire planet to a depth of 36 feet. If there are also organic compounds on Mars, says Bada, it’s possible that some form of life has existed or continues to exist on the planet.
So far no evidence of organic compounds on Mars has been found. The 1976 Viking missions tested for them using mass spectrometry. But, Bada says, “in that mission, they only scratched the surface of Mars.” Viking’s two landers probed 10 centimeters below the planet’s surface. Bada, a professor of marine chemistry at the University of California, San Diego, says researchers now believe that intense radiation and oxidizing conditions on the Martian surface would have destroyed any evidence of organic compounds at this depth.
A drill on the ExoMars rover will penetrate two meters into the planet and bring up samples for analysis. Urey processes samples as an espresso machine processes coffee grounds: using high-temperature, high-pressure water brought from Earth. The resulting concentrated brew is heated in an oven to evaporate the water, leaving behind a residue of potential organic compounds. The residue is heated under tremendous pressure until it vaporizes, then is passed over a “cool finger”–a cold rod covered with fluorescent tags. Any organic compounds present condense onto the tags and are detected by a laser.
Urey then strips the tagged organic compounds from the cool finger and puts them into solution. The solution is run through a tiny channel. An electrical field applied across the channel then causes the compounds to separate according to their molecular mass. After the compounds separate, the position of each tagged compound is read using laser light and sent to remote researchers on Earth who can use this information to identify the compounds.
Urey’s main target is amino acids, which come in two forms: left-handed and right-handed. Like human hands, the two are mirror images of each other. When researchers make amino acids from raw ingredients like water and ammonia in the lab, a 50-50 distribution of left and right results. But life on Earth makes and uses only left-handed amino acids. There’s no particular reason for this, says Bada, but it is a defining characteristic of Earth life. “The selection of handedness is a matter of chance,” he says.
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.
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