An analysis of carbon isotopes in sediment samples taken by NASA’s Curiosity rover from Gale crater, Mars, leave planetary researchers with three plausible explanations for the carbon’s origin: cosmic dust, ultraviolet degradation of carbon dioxide, or ultraviolet degradation of biologically produced methane.
A selfie taken by NASA’s Mars rover Curiosity on Sol 2291 (January 15, 2019) at the ‘Rock Hall’ drill site, located on Vera Rubin Ridge. Image credit: NASA / JPL-Caltech / MSSS.
Carbon has two stable isotopes: carbon-12 and carbon-13.
By looking at the amounts of each in a substance, researchers can determine specifics about the carbon cycle that occurred, even if it happened a very long time ago.
“The amounts of carbon-12 and carbon-13 in our Solar System are the amounts that existed at its formation. Both exist in everything, but because carbon-12 reacts more quickly than carbon-13, looking at the relative amounts of each in samples can reveal the carbon cycle,” said Professor Christopher House, a researcher in the Department of Geosciences at Penn State.
Curiosity drilled into the surface of exposed layers of ancient rock in Gale crater and recovered samples from buried sedimentary layers. The rover heated the samples in the absence of oxygen to separate any chemicals.
Spectrographic analysis of a portion of the reduced carbon produced by this pyrolysis showed a wide range of carbon-12 and carbon-13 amounts depending on where or when the original sample formed.
Some carbon was exceptionally depleted in carbon-13 while other carbon samples where enriched.
“The samples extremely depleted in carbon-13 are a little like samples from Australia taken from sediment that was 2.7 billion years old,” Professor House said.
“Those samples were caused by biological activity when methane was consumed by ancient microbial mats, but we can’t necessarily say that on Mars because it’s a planet that may have formed out of different materials and processes than Earth.”
To explain the exceptionally depleted samples, Professor House and his colleagues suggest three possibilities: a cosmic dust cloud, ultraviolet radiation breaking down carbon dioxide, or ultraviolet degradation of biologically created methane.
“Every couple of hundred million years the Solar System passes through a galactic molecular cloud,” Professor House said.
“It doesn’t deposit a lot of dust. It is hard to see any of these deposition events in the Earth record.”
To create a layer that Curiosity could sample, the galactic dust cloud would have first lowered the temperature on a Mars that still contained water and created glaciers
The dust would have deposited on top of the ice and would then need to remain in place once the glacier melted, leaving behind a layer of dirt that included the carbon. This explanation is plausible, but it requires additional research.
A second possible explanation for lower amounts of carbon-13 is the ultraviolet conversion of carbon dioxide to organic compounds like formaldehyde.
The third possible method of producing carbon-13 depleted samples has a biological basis.
On Earth, a strongly carbon-13 depleted signature from a paleosurface would indicate past microbes consumed microbially produced methane. Ancient Mars may have had large plumes of methane being released from the subsurface where methane production would have been energetically favorable.
Then, the released methane would either be consumed by surface microbes or react with ultraviolet light and be deposited directly on the surface.
However, according to the team, there is currently no sedimentary evidence of surface microbes on the past Mars landscape, and so the biological explanation highlighted in the paper relies on ultraviolet light to place the carbon-13 signal onto the ground.
“All three possibilities point to an unusual carbon cycle unlike anything on Earth today,” Professor House said.
“But we need more data to figure out which of these is the correct explanation. It would be nice if the rover would detect a large methane plume and measure the carbon isotopes from that, but while there are methane plumes, most are small, and no rover has sampled one large enough for the isotopes to be measured.”
The team’s paper will be published in the Proceedings of the National Academy of Sciences.
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Christopher House et al. 2022. Depleted carbon isotope compositions observed at Gale crater, Mars. PNAS, in press
