(CN) — The Mars rover Curiosity, which has spent nearly a decade on the red planet gathering samples, is leading scientists closer to answers about the planet’s history thanks to analyses of carbon isotopes found in its sediment.
In a study published Monday in the Proceedings of the National Academy of Sciences, a team of researchers posit three possible explanations for the presence of carbon taken from six samples — either ultraviolet degradation of carbon dioxide, ultraviolet degradation of methane or cosmic dust.
“All three of these scenarios are unconventional, unlike processes common on Earth,” the researchers said in the study.
Christopher House, professor of geosciences at Penn State, said the answer could help researchers better understand Mars’ history. He noted that scientists can learn much by studying carbon’s two stable isotopes, 12 and 13.
"The amounts of carbon 12 and carbon 13 in our solar system are the amounts that existed at the formation of the solar system," he said in a statement. "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.”
The NASA Curiosity rover touched down on Mars on Aug. 6, 2012 and has been collecting samples in the Gale Crater ever since. The crater was ideal for research since it contains exposed layers of rock that would be otherwise difficult for the rover to reach.
"The samples extremely depleted in carbon 13 are a little like samples from Australia taken from sediment that was 2.7 billion years old," House 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 out of different materials and processes than Earth."
The scientists proposed three possibilities to explain the depleted samples, “a cosmic dust cloud, ultraviolet radiation breaking down carbon dioxide, or ultraviolet degradation of biologically created methane.”
House said our solar system passes through a galactic molecular cloud once in about 200 million years, but “it doesn’t deposit a lot of dust.”
“It is hard to see any of these deposition events in the Earth record,” he said.
The researchers explained the galactic dust cloud would have needed to lower Mars’ temperature to a point where glaciers formed and dust settled on top. But they said “there is limited evidence of past glaciers at Gale Crater.
“This explanation is plausible, but it requires additional research,” the scientists said.
Another possibility is the ultraviolet conversion of carbon dioxide to organic compounds.
"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 third possibility would involve microbial life, something the researchers said there is very little evidence of.
"All three possibilities point to an unusual carbon cycle unlike anything on Earth today," House said. "But we need more data to figure out which of these is the correct explanation. It would be nice if the rover would detect a large methane plume and measure the carbon isotopes from that, but while there are methane plumes, most are small, and no rover has sampled one large enough for the isotopes to be measured.”
House said the research will continue as Curiosity continues to collect samples.
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