Reduced carbon released from powder from this drill hole was strongly depleted in carbon 13, the surprising carbon isotopic signature reported by the team.

Analysis of carbon isotopes in sediment samples taken from half a dozen exposed locations, including an exposed cliff, leave researchers with three plausible explanations for the carbon’s origin — cosmic dust, ultraviolet degradation of carbon dioxide, or ultraviolet degradation of biologically produced methane.

Carbon has two stable isotopes, 12 and 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 the formation of the solar system,” said Christopher H.

“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.”.

Drill powder from this hole showed carbon isotope values indicating a carbon cycle that includes either subsurface life, intense UV radiation penetrating the atmosphere, or Interstellar dust.

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,” said House.

To explain the exceptionally depleted samples, the researchers suggest three possibilities — a cosmic dust cloud, ultraviolet radiation breaking down carbon dioxide, or ultraviolet degradation of biologically created methane.

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.

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.

However, according to the researchers, 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,” said House.

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.”.

“To measure different carbon isotopes — one of the most important geology tools — from sediment on another habitable world, and it does so by looking at 9 years of exploration.”.

Reference: “Depleted carbon isotope compositions observed at Gale crater, Mars” 17 January 2022, Proceedings of the National Academy of Sciences.

It is an interesting question, but consider this: if, after much careful study and consideration we determine that the carbon detected was (most likely) biological in origin, we still haven’t found the life-form responsible, and a likely reason for that is that Mars is no longer suitable for hosting life, something we strongly suspected in the sixties