NASA’s Curiosity rover landed on Mars on August 6, 2012, and has since been scouring Gale Crater to take samples and send the results home for researchers to interpret. Analyzing carbon isotopes in sediment samples taken from six exposed sites, including an exposed cliff, leaves researchers with three plausible explanations for the carbon’s origin: cosmic dust, ultraviolet decay of carbon dioxide or ultraviolet decay of biologically produced methane.
Researchers noted today (January 17) in Proceedings of the National Academy of Sciences That “these three scenarios are unconventional, in contrast to common processes on the ground.”
Carbon has two stable isotopes, 12 and 13. By looking at the amounts of each in a substance, researchers can work out the details of the carbon cycle that took place, 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 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, examining the relative amounts of both in samples can reveal the carbon cycle. »
Curiosity, operated 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 drilled on the surface of these layers and samples from the buried sedimentary layers. Curiosity heated the samples in the absence of oxygen to separate the chemicals. Spectroscopic analysis of some of the 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. Carbon 13 was exceptionally depleted while some other carbon samples were enriched.
“The carbon-13 depleted samples are very similar to the Australian samples from 2.7 billion-year-old sediments,” House said. “These samples were the result of biological activity when methane was consumed by ancient microbial mats, but we cannot necessarily say this about Mars because it is a planet that may have formed from materials and processes different from those on Earth.”
To explain the exceptionally depleted samples, the researchers suggested three possibilities: a cloud of cosmic dust, ultraviolet radiation that breaks down carbon dioxide, or ultraviolet decomposition of biologically produced methane.
According to House, every two hundred million years the solar system passes through a galactic molecular cloud.
“It doesn’t deposit a lot of dust,” House said. “It is difficult to see any of these deposition events in the Earth’s records.”
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. The dust would have settled on top of the ice, and then stayed 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 further 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 articles that predict that UV rays can cause this kind of splitting,” House said. “However, we need more experimental results that demonstrate this volume splitting 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, a very ancient surface signature of carbon-13 indicates that earlier microbes consumed the methane generated by the microbes. Ancient Mars probably had large plumes of methane spewing out from under the earth where methane production was energetically favourable. The released methane is then either consumed by surface microbes or reacts with ultraviolet light and is deposited directly on the surface.
However, according to the researchers, there is currently no sedimentary evidence of surface microbes on past Martian landscapes, so the biological explanation highlighted in the paper relies on ultraviolet radiation to position 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 determine which one is the correct interpretation. It would be great if the rover could detect a large methane plume and measure carbon isotopes from that, but while there are methane plumes, most are small, and no rover has ever sampled large enough to measure isotopes. »
House also points out that the discovery of microbial mat remains or evidence of ice deposits could also shed some light.
“We are keen on our interpretation, which is the best way forward to study another world,” House said.
Curiosity continues to collect and analyze samples and will be back in a sweat as it found some samples for this study in about a month.
“This research has fulfilled the long-term goal of exploring Mars,” House said. “Measuring different isotopes of carbon – one of the most important geological tools – from sediments in another habitable world, done by examining 9 years of exploration.”
Gregory M. Wong also works on the Penn State Project, and recently holds a Ph.D. in Earth Sciences.
Other research participants at NASA’s Jet Propulsion Laboratory were Christopher R. Webster, Research Associate and Principal Investigator. Gregory J. Fleisch, Applications of Science software engineer; Amy E. Hoffman, Research Scientist; To the Solar System Exploration Division, NASA Goddard Space Flight Center: Heather B. Franz, researcher; 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 student.
NASA supported this project.
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