If you want to detect life on another planet, look for biomarkers—spectroscopic signatures of chemicals that betray the activity of living things. And in fact we may have already found a biomarker. In 2003 Earth-based astronomers caught glimpses of methane in the Martian atmosphere. The discovery was initially controversial, so much so that the discoverers themselves held back from publishing it. But the two of us and our colleagues recently confirmed the presence of methane using NASA’s Curiosity rover. It is the most tangible evidence we have ever collected that we may not be alone in the universe.
Almost no matter where the methane comes from, it’s an intriguing discovery. If you dropped a molecule of methane into the atmosphere of Mars, it would survive about 300 years—that’s how long, on average, it would take for solar ultraviolet radiation and other Martian gases to destroy the molecule. By rights, the Martian atmosphere should have been scrubbed of its methane eons ago. So, the methane we see must come either from a source that is producing methane today or from a subsurface reservoir that is venting methane produced sometime in the past. On Earth, 95 percent of methane is biological in origin. The class of bacteria known as methanogens feeds on organic matter and excretes methane. They populate our planet’s wetlands, which account for nearly a quarter of the methane present in the Earth’s atmosphere globally. Cows’ gut bacteria are the second largest producers. It is the possibility of microbial life that has propelled the search for methane on Mars.
But even if the methane there comes from geologic processes, it would give us a profound new respect for what looks outwardly like a geologically dead world. Methane can be produced by the geochemical process of serpentinization, which is widespread in Earth’s crust, especially at warm and hot hydrothermal vents on the ocean floor known as Lost City and Black Smokers. This process requires a source of geologic heat as well as liquid water. Those happen to be two main ingredients of life, as well.
The mystery isn’t just that we see methane when we shouldn’t. It’s also that, in a sense, we see too much of it. The Mars methane abundance varies dramatically in location and time, implying not only an unknown source, but also an unknown sink. The variation was evident in the very first detections from telescopes in Hawaii and Chile, reported by NASA astronomer Michael Mumma at a meeting of the Division of Planetary Sciences in 2003. The following year, Vittorio Formisano of the Institute for Interplanetary Space Physics in Rome and his team (including one of us, Atreya) published findings from the European Space Agency’s Mars Express orbiter. Like Mumma, Formisano’s team observed variations in methane abundance, although the values measured from Mars Express were much lower, about 15 parts per billion by volume (ppbv) global average. By comparison, the methane abundance on Earth is 1875 ppbv. (Gas concentrations are commonly measured by the volume a gas occupies, as opposed to its mass.)