NASA’s Curiosity rover discovered some intriguing organic compounds on Mars that could be signs of ancient Mars life, but testing that hypothesis will take a lot more work.
Researchers report in a new study that some of the powdered rock samples collected by Curiosity over the years contain organics rich in a type of carbon that is associated with life on Earth.
However, Mars is very different from our world, and many Martian processes are still unknown. As a result, study team members stressed that it is too early to tell what caused the intriguing chemicals.
“We’re finding things on Mars that are tantalizingly interesting, but we would really need more evidence to say we’ve identified life,” Paul Mahaffy, who served as the principal investigator of Curiosity’s Sample Analysis at Mars (SAM) chemistry lab until retiring from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in December 2021, said in a statement. “So we’re looking at what else could have caused the carbon signature we’re seeing, if not life.”
Almost a decade of sample analysis
Curiosity landed in Mars’ 96-mile-wide (154-kilometer-wide) Gale Crater in August 2012 on a mission to see if the area could have ever supported microbial life. The rover team quickly discovered that Gale’s floor was a potentially habitable environment billions of years ago, with a lake-and-stream system that likely lasted millions of years at a time.
The new study, which will be published in the Proceedings of the National Academy of Sciences on Tuesday (Jan. 18), examined two dozen powdered rock samples collected by Curiosity with its percussive drill from a variety of locations between August 2012 and July 2021. The rover fed this material into SAM, which can identify and characterize organics — carbon-containing molecules that are the building blocks of life on Earth.
The scientists found that nearly half of these samples were enriched in carbon-12, the lighter of the two stable carbon isotopes, compared to previous measurements of Mars meteorites and the Martian atmosphere. (Isotopes are versions of an element that contain different numbers of neutrons in their atomic nuclei. Carbon-12 has six neutrons, and the far less abundant carbon-13 has seven.)
These high-carbon-12 samples were collected from five different locations within Gale Crater, all of which had ancient surfaces that had been preserved over eons.
Because organisms on Earth preferentially use carbon-12 for metabolic processes, enrichment in this isotope in ancient rock samples here is commonly interpreted as a signal of biotic chemistry. Carbon cycles on Mars, however, aren’t well understood enough to make similar assumptions about Red Planet discoveries, according to study team members.
The researchers proposed three explanations for the intriguing carbon signal. The first involves Mars microbes producing methane, which is then converted into more complex organic molecules after interacting with UV light in the Red Planet’s atmosphere. These larger organics then fell to the ground and were incorporated into the rocks sampled by Curiosity.
But similar reactions involving UV light and non-biological carbon dioxide, by far the most abundant gas in Mars’ atmosphere, could have generated the result as well. According to the researchers, it’s also possible that the solar system passed through a massive molecular cloud rich in carbon-12 a long time ago.
“All three explanations fit the data,” study leader Christopher House, a Curiosity scientist based at Penn State University, said in the same statement. “We simply need more data to rule them in or out.”
More data needed
The new discovery is particularly intriguing because of the carbon-12 enrichment, but Curiosity has previously detected organic compounds on Mars. The mission team, for example, previously reported the detection of organics in powdered rock samples. On several occasions, the six-wheeled robot has driven through plumes of methane, the simplest organic molecule.
It’s not clear what’s causing Mars’ gaseous methane to form or how old it is. For example, the compound could be produced today by microbes metabolizing beneath the frigid Martian surface. It could also be produced by underground interactions of rock and hot water without the involvement of life. It could also be ancient material that was trapped underground long ago and occasionally “burps” up onto the surface today, produced either by organisms or abiotically.
The Curiosity team would like to drive through another methane plume and determine its carbon-12 content, in order to learn more about the origins of these organics. However, given that researchers cannot predict when and where such plumes will appear, this would require a lot of luck.
Another Mars rover, Perseverance, a NASA robot that landed inside a different Red Planet crater in February 2021, could provide additional useful data. Perseverance is searching for evidence of ancient Mars life and collecting dozens of samples, which will be returned to Earth for analysis as early as 2031.
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