NASA's Curiosity rover landed on March on August 6, 2012, and has since roamed Gale Crater, taking samples and sending the results home so researchers can interpret them. Analysis of carbon isotopes in sediment samples taken from half a dozen exposed sites, including an exposed rock, leaves scientists with three plausible explanations for the origin of carbon - cosmic dust, ultraviolet decomposition of carbon dioxide or ultraviolet decomposition of biologically produced methane.
Researchers note today (January 17, 2022) in Proceedings of the National Academy of Sciences that "All three of these scenarios are unconventional, as opposed to processes common on Earth."
Carbon has two stable isotopes, 12 and 13. By looking at the amount of each in a substance, scientists can determine details of the carbon cycle that occurred, even though 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 geoscience, Penn State. "Both exist in total, but because carbon 12 reacts faster than carbon 13, looking at the relative amounts of each in samples can reveal the carbon cycle."
Curiosity, led by NASA's Jet Propulsion Laboratory in Southern California, 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 found samples from buried sedimentary layers. Curiosity 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 was formed. Some carbon was unusually depleted in carbon 13, while other carbon samples were enriched.
"The samples extremely depleted in carbon 13 are a bit like samples from Australia taken from sediment that was 2.7 billion years old," House said. "These samples were caused by biological activity when methane was consumed by ancient microbial mats, but we can not necessarily say that on Mars because it is a planet that may have been formed by materials and processes other than Earth."
To explain the unusually depleted samples, the researchers suggest three possibilities - a cosmic dust cloud, ultraviolet radiation that decomposes carbon dioxide, or ultraviolet decomposition of biologically created methane.
According to House, the solar system passes through a galactic molecular cloud every few hundred million years.
"It does not deposit a lot of dust," House said. "It's hard to see any of these deposition events in the Earth's record."
To create a layer that Curiosity could try, the galactic dust cloud would first have lowered the temperature of a Mars still containing water, creating glaciers. The dust would have deposited on top of the ice and would then have to remain in place when the glacier melted, leaving a layer of dirt that included the carbon.
So far, there is limited evidence for former glaciers at Gale Crater on Mars. According to the researchers, "this explanation is plausible, but it requires further research."
Another possible explanation for lower amounts of carbon 13 is the ultraviolet conversion of carbon dioxide to organic compounds such as formaldehyde.
"There are papers that predict that UV can cause this type of fractionation," House said. "But we need more experimental results that show this size fractionation, so we can exclude or exclude this explanation."
The third possible method for producing carbon 13-depleted samples has a biological basis.
On Earth, a highly carbon-13 depleted signature from a paleo surface would indicate earlier microbes consuming microbially produced methane. Ancient Mars may have had large tabs of methane released from the subsoil where methane production would have been energetically favorable. Thereafter, 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 researchers, there is currently no sedimentary evidence for surface microbes in the former Mars landscape, and therefore the biological explanation is highlighted in the paper depending on ultraviolet light to place the carbon 13 signal on the ground.
"All three possibilities point to an unusual carbon cycle unlike anything on Earth today," House said. "But we need more data to find out which of these is the correct explanation. It would be nice if the rover would detect a large methane fan and measure the carbon isotopes from it, but while there are methane fans, they are most small and no rover has tried one that is large enough for the isotopes to be measured. "
House also notes that finding remnants of microbial mats or evidence of glacial deposits could also clear things up a bit.
"We are careful with our interpretation, which is the best course when studying another world," House said.
Curiosity is still collecting and analyzing samples and will return to the pediment where it found some of the samples in this study in about a month.
"This research achieved a long-term goal for Mars exploration," House said. "Measuring various carbon isotopes - one of the most important geological tools - from sediment on another habitable world, and doing so by looking at 9 years of exploration."
Reference: "Depleted Carbon Isotope Compositions Observed at Gale Crater, Mars" January 17, 2022, Proceedings of the National Academy of Sciences.
Also, Gregory M. Wong, a recent doctoral student in geoscience, worked on the project from Penn State.
Other participants in the research were at the NASA Jet Propulsion Laboratory: Christopher R. Webster, Fellow and Senior Researcher; Gregory J. Flesch, scientific application software engineer; and Amy E. Hofmann, researcher; at the Solar System Exploration Division, NASA Goddard Space Flight Center: Heather B. Franz, scientist; Jennifer C. Stern, research assistant; Alex Pavlov, space scientist; Jennifer L. Eigenbrode, research assistant; Daniel P. Glavin, Associate Director of Strategic Science; Charles A. Malespin, chief, Planetary Environments Laboratory; and Paul R. Mahaffy, Retired Director of the Solar System Exploration Division; at the University of Michigan: Sushil K. Atreya, Professor of Climate and Space Science and Engineering and Director of the Planetary Science Laboratory; at the Carnegie Institution of Science: Andrew Steele, scientist; and at Georgetown University and NASA Goddard Space Flight Center: Maëva Milan, postdoc.
NASA supported this project.