Since 2012, NASA’s Curiosity rover has trundled across Mars, drilling into rocks and running the grit through a sophisticated onboard chemistry lab, aiming to tease out evidence for life. Today, a team of rover scientists announced an intriguing signal, one that may or may not be evidence of past life, but is, at the very least, surprisingly weird. The team found that the carbon trapped in a handful of rocks probed by the rover is dramatically enriched in light isotopes of carbon. On Earth, the signal would be seen as strong evidence for ancient microbial life.
Given that this is Mars, however, the researchers are reluctant to make any grand claims, and they have worked hard to concoct alternative, nonbiological explanations involving ultraviolet (UV) light and stardust. But those alternatives are at least as far-fetched as a scenario in which subterranean microbes emitted the enriched carbon as methane gas. The team concludes the study does “inch up the plausibility” that microbes once existed on the planet—and could still today, says Christopher House, a biogeochemist at Pennsylvania State University, University Park, and lead author of the study, which was published today in the Proceedings of the National Academy of Sciences.
Mark Harrison, a planetary scientist at the University of California, Los Angeles, who is unaffiliated with the rover team, agrees that the carbon enrichment is a tantalizing hint at ancient life. But, “The authors are appropriately conservative,” he says, noting that such signatures are debated even on Earth and that nonbiological explanations can’t be ruled out.
The new study takes advantage of a time-honored insight: Life is lazy. Carbon exists in two stable isotopic forms: “light” carbon-12, which makes up the vast majority of carbon, and carbon-13, which is weighed down by an extra neutron. Because of this extra neutron, carbon-13 tends to make molecules with slightly tougher bonds. As a result, life has evolved mechanisms that favor the easier to divide carbon-12, and most organic molecules created by life are enriched in carbon-12. Methane from rice paddies, for example, is enriched in light carbon, as compared with the nonbiological methane from hydrothermal seafloor vents.
The team looked at 24 different rock samples drilled during Curiosity’s journey across Gale crater, which contains the mudstones of an ancient lake. The pulverized rock was baked in an oven in the rover’s belly, which converted trace amounts of carbon trapped in the rock into methane gas. The gas was then probed by a laser, which revealed the methane’s isotopic makeup. The results varied widely, but at six sites, the amount of carbon-12 to carbon-13 was more than 70 parts per thousand higher than an Earth-based reference standard. “These are dramatic signals,” House says. Because the strongest signals came from rocks at the top of ridges and other topographic highs in the crater, the team believes the enriched carbon was somehow deposited out of the atmosphere billions of years ago, rather than left by lake sediments.
Concentrating light carbon to such high levels might have taken multiple steps. The researchers envision deep subsurface microbes, feeding on the slightly light carbon found in martian magma, and emitting methane gas. (The martian atmosphere is deficient in light carbon, so the researchers consider it an unlikely microbial feedstock.) Then, other microbes at the surface would feed on the emitted methane, further ratcheting up the levels of light carbon, and fixing it in the fossil record when they died.
Still, the rover has seen no physical traces of ancient microbes, so the researchers say it’s also possible deep microbes might have jump-started the enrichment, with UV light driving it the rest of the way. The UV light might have broken apart the microbial methane, further enriching its light carbon while creating daughter products like formaldehyde that would eventually settle on the surface.
Or perhaps the young Solar System, including early Mars, passed through an interstellar cloud of gas and dust, which is believed to happen every 100 million years or so. The carbon in such dust is light, matching the levels seen by Curiosity, to judge by samples trapped in meteorites. The cloud might have blocked sunlight and plunged Mars into a deep freeze, causing widespread glaciation and preventing the light carbon in the rain of cosmic dust from being diluted by other carbon sources. House concedes that the scenario requires an incredible coincidence of events, and there’s no evidence of glaciation at Gale crater. But he says it can’t be ruled out.
More prosaically, a few studies suggest UV rays can generate the signal without help from biology at all. UV can react with carbon dioxide—which makes up 96% of the martian atmosphere—to produce carbon monoxide that is enriched in carbon-12. Yuichiro Ueno, a planetary scientist at the Tokyo Institute of Technology, says he has recently confirmed the process can occur in unpublished lab results. “The reported carbon isotope ratios are exactly what I have expected,” he says.
Ueno says early Mars may have had a different atmosphere, perhaps rich in hydrogen, that reacted with the carbon monoxide to form a host of organic molecules. Those would eventually fall out of the air, depositing the signature Curiosity detected.
All these scenarios would play out in the ancient past. But Curiosity is also sniffing for carbon in today’s martian air. It has detected methane, but at concentrations far too low for the rover to directly measure carbon isotope levels. (Confoundingly, sensitive instruments in orbit see no methane.) Should light carbon ever be detected in a thicker plume of methane, it would open an even more exciting possibility, House says. “Even though we’re looking at a potentially ancient process, the methane today could be from the same biosphere sustained till now.”