Mars can be both beautiful and terrifying for those looking to find 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 its first billion years, the planet was submerged in seas and oceans. It was then covered by thick layers of air. However, the magnetic field eventually stopped working, and it was allowed to be purged by the solar wind, which then wiped out the atmosphere.
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. The results of a new study have been published by NASAPublished Jan. 18, 2008 in The Proceedings of 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). The following is an example of a rover’s laboratory oven. 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. Surprise! It wasn’t which kind.
Two principal carbon isotopes exist: carbon-13 (with six protons & seven neutrons) and carbon-12 (with six protons & 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. Curiosity discovered plenty: Nearly all of the Martian samples Curiosity studied contained significantly higher levels than what scientists usually detect in Martian meteorites and in the Martian atmosphere.
House and colleagues propose an interesting biological explanation for House’s findings. The ancient Martian microbes that grew in and beneath the soil would have preferred to grab the carbon-12 than the carbon-13. They would then metabolize the isotope producing methane. House and his colleagues believe that methane would rise into the atmosphere where it would be broken down by ultraviolet radiation. Carbon-12 would then precipitate back to 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. One possibility is that ultraviolet energy from sun could have caused changes to the Martian atmospheric molecular structure, resulting in excess carbon dioxide and carbon-12 which may have fallen on the surface, just like in biological processes.
“There are papers that predict that UV could cause this type of fractionation,” said House in a Statement releasedPenn State. “However, we need more experimental results showing this … fractionation so we can rule in or rule out this explanation.”
Or else, more alarmingly, meteorites suggest that, every 100,000,000 years, the sun passes through an interstellar storm rich in several elements. This carbon could theoretically have been rained on Mars by the sun, and this could be explained by new discoveries. 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.
Curiosity will be continuing its investigations, analysing more Martian surfaces, while also looking out for methane plumes, which are released regularly from Mars’s surface. They will then check them for trace 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.”