Curiosity, led by NASA’s Jet Propulsion Laboratory in Southern California, has spent the past nine years exploring Gale Crater that has revealed layers of ancient rock. The probe dug into the surface of these layers and retrieved samples from the buried sedimentary layers. Curiosity heated the samples in the absence of oxygen to separate any chemicals. Spectroscopic analysis of the fraction of reduced carbon from this pyrolysis showed a wide range of amounts of carbon 12 and carbon 13 depending on where or when the original sample was formed. Some carbon was exceptionally depleted in carbon 13 while other carbon samples were enriched.
“The samples that are very carbon-13 depleted are somewhat similar to samples from Australia taken from sediments that were 2.7 billion years old,” House said. “These samples were the result of biological activity when methane was consumed by ancient microbial mats, but we can’t necessarily say that on Mars because it is a planet that may have formed from different materials and processes than Earth.”
To explain the exceptionally depleted samples, the researchers suggested three possibilities – a cosmic dust cloud, ultraviolet radiation that breaks down carbon dioxide, or ultraviolet decomposition of biologically produced methane.
According to House, every few hundred million years the solar system passes through a galactic molecular cloud.
“It doesn’t 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 can sample, the galactic dust cloud first lowered the temperature on Mars that still contains water and created glaciers. Dust would have deposited on top of the ice and would then need to stay in place once the glacier melted, leaving behind a layer of dirt that included carbon.
So far, there is limited evidence of past glaciers at Gale Crater on Mars. According to the researchers, “This explanation is reasonable, but requires additional research.”
A second possible explanation for the lower amounts of carbon 13 is the ultraviolet conversion of carbon dioxide into organic compounds such as formaldehyde.
“There are papers that predict that UV rays can cause this kind of fragmentation,” House said. “However, we need more experimental results that demonstrate this fragmentation size so that we can rule out or rule out this explanation.”
The third possible method for producing carbon-13 depleted samples has a biological basis.
On Earth, the strong signature of carbon-13 depleted from the ancient surface indicates earlier microbes that consumed microbe-produced methane. Ancient Mars may have had large plumes of methane that were released from the Earth’s interior where methane production was energetically favourable. Then, the released methane can be consumed by surface microbes or it can react with ultraviolet light and be deposited directly on the surface.
However, according to the researchers, there is currently no sedimentary evidence of surface microbes on the past Martian landscape, so the biological explanation highlighted in the paper relies on ultraviolet radiation to place the carbon-13 signal on Earth.
“All three possibilities point to an unusual carbon cycle unlike anything on Earth today,” House said. But we need more data to know which one is the correct interpretation. It would be great if the rover detected a large plume of methane and measured isotopes of carbon from it, but while there are methane plumes, most of them are small, and no rover has ever sampled a large enough sample for isotope measurements.”
House also points out that finding remnants of microbial mats or evidence of icy deposits can also clear things up a bit.
“We are cautious in our interpretation, which is the best course of action when studying another world,” House said.
Curiosity is still collecting and analyzing samples and will return to the experiment where it found some samples in this study in about a month.
“This research has fulfilled the long-term goal of exploring Mars,” House said. “To measure different carbon isotopes – one of geology’s most important tools – from sediments in another habitable world, done by looking at 9 years of exploration.”
Reference: “Depleted Carbon Isotope Compositions Observed at Gale Crater, Mars” Jan. 17, 2022, Available here. Proceedings of the National Academy of Sciences.
Also working on the project was Penn State’s Gregory M. Wong, a recent Ph.D. in Earth sciences.
Other research participants at NASA’s Jet Propulsion Laboratory were Christopher R. Webster, Fellow and Senior Research Scientist; Gregory C. Fleisch, Scientific Applications Software Engineer; Amy E. Hoffman, Research Scientist; In the Solar System Exploration Division, NASA’s Goddard Space Flight Center: Heather B. Franz, research scientist; Jennifer C. Stern, Research Assistant; Alex Pavlov, astronomer; Jennifer L. Eigenbrod, research assistant; Daniel B. Glavin, associate director for strategic sciences; Charles A. Malspin, Head of the Planetary Environments Laboratory; Mahaffey, retired director of the Solar System Exploration Division; At the University of Michigan: Sushil K. Atria, professor of climate and space science and engineering and director of the Planetary Science Laboratory; At the Carnegie Institution for Science: Andrew Steele, scientist; And at Georgetown University and NASA’s Goddard Space Flight Center: Maeva Milan, Postdoctoral Fellow.
NASA supported this project.