NASA’s Curiosity rover landed on Mars and has since toured Gale Crater, collecting samples and relaying the data back to Earth for experts to evaluate. Carbon isotope analysis in sediment samples taken from a half-dozen exposed locations, including an exposed cliff, provides 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.
“All three of these situations are unorthodox, unlike processes occurring on Earth,” the researchers write in Proceedings of the National Academy of Sciences. Carbon is made up of 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. House, professor of geosciences, Penn State. “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.”
Curiosity, which is managed by NASA’s Jet Propulsion Laboratory in Southern California, has spent the last nine years studying an area of Gale Crater that has exposed layers of ancient rock. The rover bore through the surface of these layers and recovered materials from buried sedimentary layers. Curiosity heated the materials in the absence of oxygen to separate any compounds. Spectrographic study of a part of the reduced carbon produced by this pyrolysis revealed a wide range of carbon 12 and carbon 13 levels depending on where or when the initial sample was obtained. Some carbon samples were extremely deficient in carbon 13, whilst others were abundant.
This research achieved a long-standing goal for Mars exploration,” House stated. “It looks at 9 years of investigation to determine distinct carbon isotopes – one of the most significant geology tools – from sediment on another livable globe.
Gregory M. Wong
“The samples that are significantly depleted in carbon 13 are similar to samples from Australia taken from sediment that is 2.7 billion years old,” House added. “Those samples were created by biological activity when methane was consumed by ancient microbial mats, but we can’t claim the same for Mars because it may have formed from other materials and processes than Earth.”
The researchers propose three explanations for the severely depleted samples: a cosmic dust cloud, UV radiation breaking down carbon dioxide, or ultraviolet destruction of biologically generated methane.
According to House, every couple of hundred million years the solar system passes through a galactic molecular cloud. “It doesn’t deposit a lot of dust,” said House. “It is hard to see any of these deposition events in the Earth record.”
The cosmic dust cloud would have first decreased the temperature of a Mars that still had water and generated glaciers to form a layer that Curiosity could study. The dust would have accumulated on top of the ice and would have had to remain in place once the glacier melted, leaving behind a layer of dirt containing carbon.
So far, there is little evidence of former glaciers in Mars’ Gale Crater. “This theory is feasible,” the researchers write, “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. “There are papers that predict that UV could cause this type of fractionation,” said House. “However, we need more experimental results showing this size fractionation so we can rule in or rule out this explanation.”
The biological underpinning underpins the third feasible approach of obtaining carbon 13 deficient samples. A paleosurface with a highly carbon 13 depleted signature would indicate that past organisms devoured microbially generated methane. Large plumes of methane may have been emitted from the subsurface of ancient Mars, where methane production would have been energetically favorable. The methane emitted would then be eaten by surface bacteria or would react with ultraviolet radiation and be deposited directly on the surface.
However, because there is no sedimentary evidence of surface bacteria on the previous Mars landscape, the biological explanation highlighted in the report relies on UV light to deposit the carbon 13 signature on the ground, according to the researchers.
“All three hypotheses point to an extraordinary carbon cycle unlike anything seen on Earth today,” House explained. “However, more evidence is required to determine whether of these explanations is right. It would be ideal if the rover could detect a massive methane plume and quantify the carbon isotopes in it, but most methane plumes are small, and no rover has sampled one large enough to measure the isotopes.”
House also mentions that locating the remains of microbial mats or evidence of glacial deposits could help to clarify things. “We’re being cautious in our interpretation, which is the appropriate course of action when investigating another world,” House explained.
Curiosity is continuing to gather and analyze samples, and it will return to the pediment where some of the materials in this study were discovered in roughly a month. “This research achieved a long-standing goal for Mars exploration,” House stated. “It looks at 9 years of investigation to determine distinct carbon isotopes – one of the most significant geology tools – from sediment on another livable globe.” Gregory M. Wong, a recent Penn State doctoral recipient in geosciences, was also involved in the experiment.
