(ORDO NEWS) — Since 2012, NASA’s Curiosity rover has been scouring Mars, drilling through rock and blasting sand through a complex onboard chemistry lab in an attempt to find evidence of life.
Today, a team of scientists studying the rover announced an intriguing signal that may or may not be evidence of past life, but is at least surprisingly strange. The team found that the carbon in the handful of rocks examined by the rover is significantly enriched in light carbon isotopes. On Earth, this signal would be taken as conclusive evidence for the existence of ancient microbial life.
However, given that we are talking about Mars, the researchers do not want to make grandiose claims, and they have made every effort to come up with alternative non-biological explanations related to ultraviolet radiation and star dust. But these alternatives are at least as far-fetched as the scenario in which underground microbes release enriched carbon as methane gas.
According to Christopher House, a biogeochemist at Pennsylvania State University, University Park and lead author of the study, published today in the journal Proceedings of the National Academy of Sciences, the study “increases the credibility” that microbes once existed on the planet and may exist. and today.
Mark Harrison, a UCLA planetary scientist not associated with the rover team, agrees that carbon enrichment is a tantalizing nod to ancient life. But “the authors are quite conservative,” he says, noting that such signatures are discussed even on Earth and that non-biological explanations cannot be ruled out.
A new study uses a time-tested 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 weighted down by an extra neutron. Because of this extra neutron, carbon-13 tends to create molecules with slightly stronger bonds.
As a result, life has evolved mechanisms that prefer the easier-to-break down carbon-12, and most of the organic molecules created by life are enriched in carbon-12. Methane from paddy fields, for example, is enriched in light carbon compared to non-biological methane from hydrothermal vents on the seafloor.
The team studied 24 different rock samples drilled during Curiosity’s tour of Gale Crater, which contains the mudstones of an ancient lake. The crushed rock was baked in a furnace in the belly of the rover, causing trace amounts of carbon in the rock to turn into methane gas.
The gas was then examined with a laser, which showed the isotopic composition of methane. The results varied greatly, but in six places the amount of carbon-12 in carbon-13 was more than 70 parts per thousand higher than the Earth’s reference.
“These are dramatic signals,” says House. Because the strongest signals came from rocks on ridgetops and other topographical elevations in the crater, the team believes that the enriched carbon somehow fell out of the atmosphere billions of years ago, rather than being left behind by lake sediments.
The concentration of light carbon to such high levels may have occurred in several steps. The researchers suggest that microbes live deep underground, feeding on light carbon contained in Martian magma and emitting methane gas.
(The Martian atmosphere lacks light carbon, which is why the researchers consider it an unlikely raw material for microbes.) Other microbes on the surface would then feed on the emitted methane, further raising the level of light carbon and fixing it in fossils after their death.
However, the rover did not find any physical traces of ancient microbes, so the researchers say it is possible that deep microbes started the enrichment process, and ultraviolet light brought it to an end. Ultraviolet light could break down microbial methane, further enriching it with light carbon and creating progeny such as formaldehyde that would settle on surfaces over time.
Or perhaps the young solar system, including early Mars, passed through an interstellar cloud of gas and dust, which is thought to happen every 100 million years or so. The carbon in this dust is light in color, consistent with levels observed by Curiosity from samples found in meteorites.
The cloud could have blocked sunlight and plunged Mars into deep frost, causing widespread glaciation and preventing other sources of carbon from diluting the light carbon in the cosmic dust rain. House admits that this scenario requires an incredible coincidence of events, and there is no evidence of glaciation in Gale Crater. But he says that it cannot be ruled out.
More prosaically, several studies show that ultraviolet rays can generate a signal without the help of biology at all. Ultraviolet can react with carbon dioxide, which makes up 96% of the Martian atmosphere, producing carbon monoxide enriched in carbon-12.
Yuichiro Ueno, a planetary scientist at the Tokyo Institute of Technology, says he recently confirmed the possibility of this process in unpublished laboratory results. “The resulting carbon isotope ratios are exactly what I expected,” he says.
Ueno says that early Mars may have had a different atmosphere, possibly rich in hydrogen, which reacted with carbon monoxide to form many organic molecules. They would eventually fall out of thin air, leaving trails that Curiosity found.
All of these scenarios may have played out in the ancient past. But Curiosity is also looking for carbon in today’s Martian air. It detected methane, but at too low a concentration for the rover to directly measure carbon isotope levels.
(If light carbon is found in the thicker methane plume, that would open up even more exciting possibilities, says House. “Even if we’re looking at a potentially ancient process, methane today could be from the same biosphere that is still maintained today.”
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