Mars is both a wonderful and a terrible place to go looking for life. On the one hand, the planet is a wasteland, where wintertime temperatures plunge to -153º C (-225º F), and the atmosphere—such as it is—is just 1% the density of Earth’s and composed principally of carbon dioxide. On the other hand, the Red Planet wasn’t always such a wreck. For the first billion or so years of its 4.5 billion year life span, it was awash in oceans and seas and protected by a thick blanket of air. Eventually, however, its magnetic field shut down, allowing the solar wind to claw away the atmosphere and the water to vanish into space.
But that first billion years offered Mars plenty of time to cook up at least microbial life, some of which may have died and left chemical traces on the surface—or even have retreated underground to continue thriving in deep, warm aquifers. Now, a new study, announced by NASA and published on Jan. 18 in the Proceedings of the National Academy of Sciences, suggests that some of those lingering surface markers of ancient life may have been found—lying in plain sight, in fact.
The new research, led by geoscientist Christopher House of Pennsylvania State University, was based on work conducted by NASA’s Curiosity rover, which has spent the last nine and a half years in Mars’s Gale Crater, a one-time lake, studying its rocks and surface sediments in search of clues to the planet’s geologic—and biologic—history. In the first part of House’s study, the rover used its on-board drill to collect rock and soil samples at 24 different sites around Gale Crater. The samples were then transferred to a laboratory oven within the body of the rover and heated to about 850º C (1,500º F). A laser spectrometer then went to work, analyzing the chemistry of the vaporized samples—looking especially for carbon, the elemental backbone of all life as we know it. Plenty of carbon was indeed detected—which was pretty much as expected. The surprise was just which type.
Carbon comes in two principal isotopes: carbon-13, with six protons and seven neutrons; and carbon-12, with six protons and six neutrons. Carbon-13 doesn’t play well with biology; its heavier structure makes for tougher molecular bonds that don’t allow for the nimble coupling, decoupling and recombining that make biological processes possible, and that carbon-12 performs so easily. The more carbon-12 you find in a Martian sample, the greater the possibility that you’re looking at an artifact of early life. And Curiosity found plenty of it: Nearly half of the samples the rover studied had significantly higher levels of carbon-12 than scientists typically detect in Martian meteorites or in the Martian atmosphere.
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House and his colleagues posit an intriguing biological explanation for their findings: Ancient Martian microbes growing in and under the soil would have preferentially grabbed the available carbon-12 over carbon-13, metabolizing the isotope and producing methane as a byproduct. The methane would have risen into the atmosphere, where it would have been broken down by ultraviolet light, and the carbon-12 would then have precipitated back down as a dusting on the surface. Adding support to that idea was that the samples were collected in the relative highlands and cliffs of Gale Crater—which would have been above the ancient water level and been particularly exposed to the precipitating carbon-12.
“The large carbon-12 amounts observed [on Mars] are found on Earth in biological methane or when biological methane is consumed by microbes,” wrote House in an email to TIME. “In some ways, the Martian samples resemble Earth rocks from Australia from 2.7 billion years ago, when our atmosphere was rich in biological methane.”
NASA is no less sanguine about the findings—even if cautiously so. “We’re finding things on Mars that are tantalizingly interesting,” said Paul Mahaffy, a recently-retired member of the Curiosity science team, in a statement. “But we would really need more evidence to say we’ve identified life.”
Mahaffy’s caution is well-placed, because even House admits there are other, non-biological phenomena that could explain the new findings. For one thing, ultraviolet energy from the sun might have caused changes in the molecular makeup of the Martian atmosphere, producing excess amounts of carbon dioxide and carbon-12, which would then have rained down on the surface just as it would in biological processes.
“There are papers that predict that UV could cause this type of fractionation,” said House in a statement released by Penn State. “However, we need more experimental results showing this … fractionation so we can rule in or rule out this explanation.”
Alternatively, and more dramatically, evidence from meteorites indicates that every 100 million years or so, the solar system passes through an interstellar cloud that is rich in multiple elements, including lighter carbon-12. In theory, that carbon could have rained down on Mars and could explain the new findings. The problem with that scenario is that the cloud would have led to global cooling that would in turn have resulted in glaciation in Gale Crater—signs of which have not been detected. “We have not seen significant evidence for a glacier at Gale Crater yet,” House says.
The Curiosity rover, meantime, will continue its explorations, not only analyzing more of the Martian surface, but also sniffing for methane plumes that are known to be released from beneath the surface of Mars periodically, and checking them for the telltale carbon. That would be something of a gold standard finding—possibly indicating not just ancient, but extant life.
“If we were to [discover a large enough plume],” House says, “the result might match the carbon on the ancient surface, suggesting that the same microbes still inhabit the subsurface.” Short of that case-closed discovery, House is reserving his judgment. “All three of the explanations proposed fit the data that we have,” he says. “We are being cautious with our interpretations here, but that is the right approach when studying another world such as Mars.”