“It’s like nothing on Earth today”: the unusual carbon cycle detected by the Curiosity rover on Mars

The analysis of carbon isotopes in sediment samples taken by the rover Curiosity reveal an unusual carbon cycle in Mars which is nothing like what we see on Earth today.

Samples taken from half a dozen exposed locations in Gale Crater, including a cliff, leave researchers with three plausible explanations for the origin of the carbon: 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 details about the carbon cycle that occurred, even if it happened a 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 systemChristopher H. House, a professor of geosciences at Penn State University, said in a statement. “Both exist in everything, but because carbon-12 reacts faster than carbon-13, looking at the relative amounts of each in samples can reveal the carbon cycle.

Curiosity, which is run by NASA’s Jet Propulsion Laboratory, has spent the past nine years exploring an area of ​​Gale Crater that has exposed layers of ancient rock. The rover drilled into the surface of these layers and retrieved samples of buried sedimentary layers. Curiosity heated the samples in the absence of oxygen to separate the 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 was formed. Some of the carbon was exceptionally carbon-13 depleted while other carbon samples were enriched.

“Extremely carbon-13 depleted samples are a bit like samples of Australia taken from sediments that had 2.7 billion yearsHouse 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 from different materials and processes than Earth”.

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

According to House, every two hundred million years the solar system passes through a galactic molecular cloud. “Does not deposit much dustHouse said. “It’s hard to see any of these depositional events in the Earth’s record.”

To create a layer that Curiosity could sample, the galactic dust cloud would first have lowered the temperature on a Mars that still contained water and created glaciers. The dust would have settled on the ice and then had to stay in place once the glacier melted, leaving behind a layer of grime that included the carbon.

So far, there is limited evidence of past glaciers in Gale Crater on Mars. According to the researchers, “this explanation plausible, but requires investigation additional”. A second possible explanation for lower amounts of carbon 13 is the ultraviolet conversion of carbon dioxide to organic compounds such as formaldehyde.

“There is work predicting that UV rays could cause this type of fractionation,” House said. “But nevertheless, we need more experimental results that show this size fractionation for us to accept or rule out this explanation.”

The third possible method for producing carbon-13 depleted samples has a biological basis.

On Earth, a strongly carbon-13-depleted signature of a paleosurface would indicate that past microbes consumed microbially produced methane. Ancient Mars may have had large plumes of methane released from underground where methane production would have been energetically favorable. The released methane would then be consumed by surface microbes or react with ultraviolet light and be deposited directly on the surface.

However, according to the researchers, currently no sedimentary evidence of surface microbes in the past landscape of Mars, so the biological explanation highlighted in the article relies on ultraviolet light to place the carbon-13 signature in the ground.

“All three possibilities point to an unusual carbon cycle that looks like nothing on earth todayHouse said. “But we need more data to determine which of these is the correct explanation. It would be nice if the rover detected a large plume of methane and measured carbon isotopes from it, but while there are plumes of methane, most are small and no rover has ever sampled one large enough to measure carbon isotopes. “.

House also notes that finding the remains of microbial mats or evidence of glacial deposits could also shed some light on things. “We’re being cautious with our interpretation, which is the best way to go when studying another world,” House said.

Curiosity is still collecting and analyzing samples and will return to the wall where it found some of the samples in this study in about a month.

“This investigation achieved an old goal for the exploration of MarsHouse said. “Measuring different isotopes of carbon — one of the most important geological tools — from sediments on another habitable world, and doing it by looking at 9 years of exploration.”