NASA’s Curiosity rover landed on Mars on August 6, 2012 and has been roaming Gale Crater ever since, taking samples and sending the results home for researchers to interpret. Analysis of carbon isotopes in sediment samples taken from half a dozen exposed sites, including an exposed cliff, leaves researchers with three plausible explanations for the carbon’s origin — cosmic dust, ultraviolet degradation of carbon dioxide, or ultraviolet degradation of biologically produced methane.
The researchers note that today Proceedings of the National Academy of Sciences that “all three of these scenarios are unconventional, in contrast to processes that are common on Earth.”
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 happened, even if it was a long time ago.
“The amounts of carbon 12 and carbon 13 in our solar system are the amounts that were present when the solar system formed,” said Christopher H. House, professor of earth sciences, Penn State. “Both are present 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, managed 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 took samples from buried layers of sediment. Curiosity heated the samples in the absence of oxygen to separate all chemicals. Spectrographic analysis of a portion of the reduced carbon produced by this pyrolysis revealed a wide range of carbon-12 and carbon-13 amounts, depending on where or when the original sample formed. Some of the carbon was exceptionally depleted in carbon 13 while other carbon samples were enriched.
“The extremely carbon-13-depleted samples are a bit like samples from Australia taken from sediments that were 2.7 billion years old,” House said. “These samples were caused by biological activity as 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 propose three possibilities – a cosmic dust Cloud, ultraviolet radiation breaking down carbon dioxide, or ultraviolet breaking down biologically produced methane.
According to House, the solar system passes through a galactic molecular cloud every few hundred million years.
“It doesn’t collect much dust,” House said. “It’s hard to see any of these depositional events on Earth’s record.”
To create a layer that Curiosity could Probethe galactic dust cloud would have lowered the temperature and created glaciers first on a Mars that still contained water. The dust would have settled on the ice and would then have to stay in place as the glacier melts, leaving behind a layer of dirt that contains the carbon.
So far, there is only limited evidence of former glaciers at Gale Crater on Mars. “This explanation is plausible, but requires further research,” the researchers said.
A second possible explanation for lower levels of carbon 13 is the ultraviolet conversion of carbon dioxide into organic compounds such as formaldehyde.
“There are studies that predict UV could cause this type of fractionation,” House said. “However, we need more experimental results showing this size fractionation so that we can include or rule out this explanation.”
The third possible method for preparing carbon-13-depleted samples has a biological basis.
On Earth, a heavily carbon-13 depleted paleosurface signature would indicate that past microbes consumed microbially produced methane. On ancient Mars, large clouds of methane may have been released from the subsurface where methane production would have been energetically favorable. Then the released methane would either be consumed by surface microbes or react with ultraviolet light and be deposited directly on the surface.
According to the researchers, however, there is currently no sedimentary evidence of surface microbes in the past Martian landscape, and thus the biological explanation highlighted in the publication relies on it ultraviolet light to put the carbon-13 signal on the ground.
“All three possibilities point to an unusual carbon cycle that still exists 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 could detect a large methane plume and measure the carbon isotopes from that, but as long as there are any methane Plumes, most are small, and no rover has taken a sample large enough to measure the isotopes.
House also notes that finding microbial mat remnants or evidence of glacial deposits might clear things up a bit as well.
“We’re cautious about our interpretation that the best course is in 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 has achieved a long-standing goal for the exploration of Mars,” House said. “Measure differently carbon Isotopes – one of geology’s most important tools – from sediments on another habitable world, and this with a look at 9 years of exploration.”
Also working on the Penn State project was Gregory M. Wong, a recent graduate geoscientist.
Depleted carbon isotopic compositions observed at Gale Crater on Mars Proceedings of the National Academy of Sciences (2022). doi.org/10.1073/pnas.2115651119
Pennsylvania State University
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