Frank Keppler of the University of Heidelberg in Germany and his colleagues may have just found a new way to determine whether Mars has indigenous life. The emphasis is on “indigenous,” because one of the long-lasting problems in astrobiology is how to distinguish putative Martian life from organisms that may have stowed away on spacecraft sent from Earth.
Keppler’s approach, as explained in today’s edition of the journal Scientific Reports, goes back to a finding from the Viking mission of the 1970s related to the detection of chloromethane—an organic compound produced naturally on Earth by marine algae and wood-rotting fungi, among other sources. When chloromethane was identified on Mars by one of Viking’s life detection instruments, it was initially dismissed as contamination from Earth. But later work, based on more recent findings by the Phoenix Mars lander and the Curiosity rover, strongly hinted that the chloromethane was indigenous to Mars.
Keppler’s team re-did some of the same analyses done during these missions: heating a soil sample to temperatures as high as 450oC, then watching for the release of volatile organic compounds, including chloromethane. The difference is that they used two different kinds of soil: a terrestrial sample from Hawaii (similar in composition to Martian soil) and a sample from the Murchison meteorite, which is known to contain organic compounds. The release of chloromethane from the Murchison meteorite closely matched the results from NASA’s Mars missions, both in magnitude and isotopic composition. The likely conclusion is that the chloromethane found on these past missions was from meteorites that had impacted Mars in the past—ergo, an extraterrestrial signal, and not a case of terrestrial contamination.
Further, after analyzing the isotopic composition of hydrogen and carbon in the samples, the researchers found a unique signature that can establish whether the chloromethane comes from Earth contamination, a meteorite such as Murchison, or from native biology on Mars. Their proposed isotopic signature for Martian life is based on a few assumptions, but they’re reasonable—including the assumption that life on Earth and Mars would have similar metabolic pathways and chemical processes.
These are valuable insights, because we can put them into practice on future missions. The more “biosignatures” we can establish, the firmer a future detection of life on Mars will be. Of course, the most convincing finding would still be to see (through a microscope on Mars) a Martian microbe wiggling in its surrounding environment—something so different in structure and biochemistry from life as we know it that we’d know for sure it is not a hitchhiker from Earth!