February 6, 2007
Looking for Microbial Martians
More than 30 years ago, when NASA's two Viking landers looked for signs of life on Mars, the results were ambiguous. Although no strong evidence has since emerged for life on Mars, the planet now seems considerably more hospitable than it once did "“ especially since the announcement last December that liquid water had flowed on its surface within the last few years.
But it will be the European Space Agency (ESA), with its ExoMars mission, that will deliver the first comprehensive life-detection science package since Viking to the martian surface. Like Viking, ExoMars will consist of an orbiter and a lander, but the lander will include a rover capable of traveling several kilometers. The spacecraft is scheduled for launch in 2013.
One of the critical instruments on the ExoMars lander will be the Urey Mars Organic and Oxidant Detector, funded by NASA. Urey will search for the molecular signatures of proteins, DNA and RNA in the martian regolith. The project will follow in the footsteps of the Viking landers, says Jeffrey Bada, who is directing Urey's development. Bada is a professor of marine chemistry at Scripps Research Institute in La Jolla, California, and director of the NASA Specialized Center of Research and Training in Exobiology.
ExoMars will contain a drill able to extract samples from two meters below the surface. The craft will deliver soil and rock samples to the Urey instrument, named for Nobel-prize-winning chemist Harold Urey, a participant in the famous 1953 Miller-Urey experiment, which showed that organic molecules could form under primitive-Earth conditions. Organic compounds extracted from the samples will be exposed to a fluorescent dye that attaches to molecules that contain an amine (NH2) group. This common biological structure is part of amino acids and some of the nucleic-acid bases in RNA and DNA. The dye, called flourescamine, "is a highly specific reagent," Bada says.
A laser inside the instrument will illuminate the sample, causing amine-containing compounds labeled with flourescamine to appear in a detector even if they are present only in minute amounts, Bada says. "This instrument will be the most sensitive thing that has ever landed on Mars, one million times more sensitive than Viking. If amino acids are there, we stand a very good chance of finding them."
But detecting amino acids is only a first step. Any given amino acid can appear in two forms that look like mirror images of each other -- the so-called left-handed and right-handed forms. This quality is called chirality. Amino acids in some types of meteorites contain a nearly equal mix of left- and right-handed amino acids.
Urey will determine the chirality of amino acids with a "microfabricated capillary electrophoresis instrument." This device, developed at the University of California at Berkeley, is a miniaturized version of a standard biology-lab instrument that can determine sample composition. It will also use an advanced instrument to detect the chirality of amino acids.
"It's the chirality that is key," says Richard Matheis, a UC Berkeley chemist who developed the electrophoresis instrument. "If you have carbon-based life, you have chirality."
If Urey detects a preponderance of left-handed amino acids, that could mean the sample was contaminated with life from Earth, or that the life detected is related to life on Earth. Finding a preponderance of right-handed molecules would be hitting the jackpot, Bada says. Both he and Matheis would view right-handed chirality an almost airtight proof that a novel form of life exists on Mars.
But even if Urey finds a 50-50 mix of left- and right-handed organic compounds, Matheis says, that would be interesting if it occurs at least a meter below the surface "“below the range of possible contamination from meteorite deposition. This could mark the degraded remains of life, he says. On Earth, he notes, biologically derived amino acids decay over millions of years to a 50-50 mix of right- and left-handed chirality. If life was once present on Mars, perhaps millions or billions of years ago, and then died out, a similar process of degradation may have taken place there, leaving behind equal amounts of left- and right-handed amino acids.
Such a finding would not be as dramatic as discovering clear evidence of Martian life, Matheis admits, "but it would tell you that there are organic molecules on Mars, and we don't know that yet."
One of Urey's jobs will be to follow up on an ambiguity in the results from Viking. The spacecraft carried gas chromatograph-mass spectrometers (GC-MS), which are routinely used to detect and analyze chemicals. The GC-MS found no sign of organic molecules.
But Viking also carried an experimental device that released a nutrient compound containing the radioactive isotope carbon-14. The hypothesis was that any life present in the soil would incorporate the radiocarbon during metabolism, and then release radioactive carbon dioxide. Viking indeed picked up a signal of carbon-14. Some scientists saw this as a sign of life, but because the GC-MS detected no organic molecules, many scientists deemed it a false positive. Follow-up experiments on Earth with identical equipment, however, have shown that the GC-MS could have missed organic material if it was present in the Viking samples in minute quantities.
The leading hypothesis for the results of the Viking nutrient experiment is that Mars's soil contains highly oxidized material, and that the carbon-14 release was caused by a chemical reaction involving these oxidants, rather than by biological processes. Urey's oxidant detector will be the first instrument sent to Mars capable of testing its soil for the presence of oxidants.
Urey will have another advantage over Viking, Matheis says. "Rather than try to detect the living organisms, we are trying to detect biomarkers." The challenge of exploring for life in new worlds, he says, is this: "How broadly can you look, while still learning something specific? Urey does a very unique job of looking with a broad brush, while still being able to answer a very specific question."
But he concedes that the Urey experiment rests on an assumption of its own: that life on Mars, even if it had a different chemical basis than life on Earth, would contain amino acids that favor one chirality or the other.
Prototypes of the Urey instruments have been tested in Chile's Atacama Desert, which is perhaps the driest place on Earth and is considered a good stand-in for the Martian surface.
Turning to speculation, Bada says amino acids may exist at the surface of Mars. "Meteorites, asteroids, dust, all this stuff has plowed into the planet," as it has on Earth, "and we know that a significant fraction of meteorites contain amino acids." But oxidizing chemicals and radiation at the surface of Mars would almost certainly destroy any exposed organic compounds. "They would fry just about everything, so it would be very surprising to see anything in the upper meter, but below that, things could be preserved for the whole age of the planet."
Any amino acids present in the deeper samples could have a non-biotic source, Bada says, which is why the chirality detector is so critical. "Our philosophy has always been, you can't just go to Mars and look for amino acids. If you find them, you've got to determine what the source is, and that is what we are going to do."
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