The rugged and parched expanses of Western Australia are where many of the oldest signs of ancient life on Earth have been found, embedded in the sedimentary rocks that have been undisturbed there for eons. One particularly significant finding from the Tumbiana Formation contained a substantial and telltale excess of the carbon-12 isotope compared with carbon-13.
Since carbon 12 is used by living organisms, that carbon-12 excess in the rocks was interpreted to mean that some life-form had been present long ago (about 2.7 billion years) and left behind that “signature” of its presence. What was once a microbial mat that could have produced the carbon-12 excess was ultimately found nearby.
After nine years of exploring Gale Crater on Mars, scientists with NASA’s Curiosity rover have collected a substantial number of rock samples that they have similarly drilled, pulverized, gasified and analyzed.
And as explained in an article in the Proceedings of the National Academy of Science (PNAS,) researchers have found quite a few Martian specimen that have the same carbon-12 excesses as those found in Western Australia.
Paul Mahaffy of NASA’s Goddard Space Flight Center, long-time principal investigator for the instrument that found the carbon-12 excess on Mars, called the results “tantalizingly interesting.”
And the lead author of the PNAS paper, Christopher House of Penn State University, said that “On Earth, processes that would produce the carbon signal we’re detecting on Mars are biological.” Like from Western Australia and elsewhere.
So something unusual and important has been discovered. But exactly what it is and how it came to be remains very much a work in progress.
Perhaps biology did play a role, the team writes. If so, it would involve ancient bacteria in the Martian surface that would have produced a unique carbon signature when they released methane into the atmosphere. Ultraviolet light would have then converted that gas into larger, more complex molecules that would rain down and become part of Martian rocks.
But so, too, might unique Martian atmospheric conditions coupled with incoming ultraviolet light be the source of the carbon-12 excesses. Or maybe they came from a very-long-ago Martian trip through a molecular cloud rich in carbon-12 dust.
“The hardest thing is letting go of Earth and letting go of that bias that we have and really trying to get into the fundamentals of the chemistry, physics, and environmental processes on Mars,” said Goddard astrobiologist Jennifer L. Eigenbrode, who participated in the carbon study. Previously, Eigenbrode led an international team of Curiosity scientists in the detection of myriad organic molecules – ones that contain carbon – on the Martian surface.
“We need to open our minds and think outside the box,” Eigenbrode said in a release, “and that’s what this paper does.”
Christopher House, a member of the science team working with Curiosity, said that determining and then understanding carbon-12 to carbon-13 ratios on Mars has been a long-term goal of NASA and Mars scientists.
The samples were collected during drilling campaigns at six Gale Crater sites over nine years — from mudstone, from sandstone, from a dune and from the side of a rock wall. The powdered samples were placed in the rover’s Tunable Laser Spectrometer (TLS) and heated to 1,500 degrees Fahrenheit.
The instrument then measured the isotopes from some of the reduced carbon that was set free in the heating process. Isotopes are atoms of an element with different masses due to their distinct number of neutrons — in this case six for carbon-12 compared with seven for carbon-13 — and they are used to understand better the chemical and biological evolution of planets.
Carbon is particularly important in astrobiology and planetary and life sciences since it is an element found in all life on Earth. It flows continuously through the air, water, and ground in a cycle that’s well understood thanks to isotope measurements.
Living creatures on Earth use the smaller, lighter carbon-12 atom to metabolize food or for photosynthesis versus the heavier carbon-13 atom. As a result, there is significantly more carbon-12 than carbon-13 in some ancient rocks and, along with other evidence, this suggests to scientists they’re looking at signatures of life-related chemistry.
Looking at the ratio of these two carbon isotopes helps Earth scientists tell what type of life they’re looking at and the environment it lived in. But as House put it, “Mars is very different.”
The Martian atmosphere, for instance, is made up largely of carbon dioxide and is quite thin, as opposed to our thick atmosphere with nitrogen and oxygen. The nature of the Martian atmosphere in turn affects the amount of radiation that reaches the lower atmosphere and the surface and potentially leads to very different kinds of chemical and photo-chemical reactions.
House said the team theorized and examined numerous non-biological explanations for the carbon-13 to carbon-12 ratio they had uncovered, and came up with two main possibilities.
The first is that the carbon signature could have resulted from the interaction of ultraviolet light from the Sun, or more distant sources, with carbon dioxide gas in the Martian atmosphere, producing new carbon-containing molecules that would have settled to the surface. The ubiquity of carbon dioxide in the Martian atmosphere adds to the plausibility.
And the other possibility suggests that the carbon-12 excess could have been left behind from a rare event hundreds of millions of years ago when the solar system might have passed through a giant molecular cloud rich in the type of carbon detected. Such clouds high in carbon-12 are known to be formed near the galactic center.
“As a team, we’re pretty much in agreement that all three hypotheses fit the data,” House said.
He also said that he expected many laboratories to begin testing those theories to see if they can potentially explain, or rule out, any of the three possibilities for what is definitively a carbon-12 excess on parts of Mars.
The emphasis on providing both a biological explanation and abiotic explanations is very much in keeping with evolving NASA efforts to rein in claims that extraterrestrial life, or signs of past extraterrestrial life, had been found.
Late last year, NASA chief Scientist James Green (now retired) was the lead author on a paper in the journal Nature that argued such claims have too often been made without taking the planetary context into consideration and without vigorously examining non-biological explanations. The paper, co-authored by several other senior NASA astrobiologists, called for the creation of a scale to assess any claim of extraterrestrial life before it went public.
Last summer, two groups associated with the NASA Astrobiology Program hosted a community workshop on biosignature standards of evidence. It developed a generalized framework for biosignature detection and assessment, and wrote a draft white paper summarizing the workshop discussions and findings on best practices.
House said that his team was certainly cognizant of those efforts as the paper was being written.
As explained by House, the carbon-12 excess — which on Earth is associated with the release of the gas methane — is closely tied to other efforts to detect and measure that gas on Mars.
More than a decade ago, NASA scientists reported finding two large plumes of methane on Mars via ground-based telescopes. The finding was exciting because biology could have been the source of the methane and it was also controversial because the authors, and others, have not found similarly large methane plumes since.
The Curiosity rover has detected several methane plumes during its travels, supporting that earlier detection by NASA’s Michael Mumma and Geronimo Villanueva. But the Gale Crater plumes were not nearly as large as the ones earlier reported.
Also, the joint European and Russian Trace Gas Orbiter, which circles Mars, has never detected any methane in the planet’s atmosphere. A possible explanation why: It studies the upper atmosphere rather than the lower atmosphere where methane is likely to appear and quickly disperse.
But methane is a potential key to understanding whether life once existed on Mars. And that, House said, is part of why Paul Mahaffy — the NASA principal investigator of the instrument that detected both the carbon isotope excesses and the smaller methane plumes at Gale — found the excess carbon-12 excess “tantalizingly interesting.” More evidence is needed before any claims of a biological origin, he said, but the plot line is so intriguing.
It’s all part of a vast, confusing but emerging story about Mars and especially about the early period when the planet was warmer and wetter and more habitable than it is today.
On the subject of Mars, this remarkable image below is from the United Arab Emirates’ Hope spacecraft, launched to the planet in 2021.
The photo shows Tharsis Montes—three shield volcanoes in a row with about 430 miles between them. The left-most volcano of the three is Ascraeus Mons, with a peak that to reaches 59,000 feet. Olympus Mons, the big circle below the other three, has a summit at 72,000 ft.—about two and a half times Mount Everest’s height above sea level. It is the largest mountain or volcano in the solar system. (UAESA/MBRSC/HopeMarsMission/EXI/Andrea Luck)