The Martian surface shows ample evidence of its warm, watery past. Deltas, ancient lakebeds, and dry river channels are plentiful. When the Curiosity rover found organic matter in ancient sediments in the Jezero Crater paleolake, it was tempting to conclude that life created the matter.<\/p>\n
However, new research suggests that non-living processes are responsible.<\/p>\n
<\/p>\n There are three carbon isotopes on Earth: carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C). Earth\u2019s carbon is almost entirely carbon-12. It makes up 99% of the carbon on Earth, with carbon-13 making up the other 1%. (14C is extremely rare and unstable, so it decays into nitrogen-14.) <\/p>\n In 2022, MSL Curiosity took an inventory of organic carbon in sediments at Gale Crater. Organic carbon is usually described as carbon atoms bonded covalently to hydrogen atoms and is the basis for organic molecules. The carbon in organic carbon can be either carbon-12 or carbon-13, and the amounts are important. At Gale Crater, Curiosity found about 200 to 273 parts per million of organic carbon. \u201cThis is comparable to or even more than the amount found in rocks in very low-life places on Earth, such as parts of the Atacama Desert in South America, and more than has been detected in Mars meteorites,\u201d said Jennifer Stern, a Space Scientist at NASA\u2019s Goddard Space Flight Center when the results came in.<\/p>\n This carbon is important evidence in understanding Mars\u2019 history. It can tell scientists about the planet\u2019s atmospheric processes and environmental conditions and even shed light on potential life. In fact, understanding Martian carbon can aid our understanding of habitability and prebiotic chemistry on distant exoplanets. The isotope ratio in this carbon is different than on Earth. It has a lower amount of carbon-13 relative to carbon-12 compared to Earth. Why the discrepancy?<\/p>\n In recent research in Nature Geoscience, a team of researchers tried to understand the difference between Earth\u2019s and Mars\u2019s carbon isotope ratios. The work is titled \u201cSynthesis of\u00a013<\/sup>C-depleted organic matter from CO in a reducing early Martian atmosphere.\u201d The lead author is Yuichiro Ueno, a biogeochemist in the Department of Earth and Planetary Sciences at the Tokyo Institute of Technology.<\/p>\n \u201cStrong\u00a013<\/sup>C depletion in sedimentary organic matter at Gale crater was recently detected by the Curiosity rover,\u201d the authors write. \u201cAlthough this enigmatic depletion remains debated, if correct, a mechanism to cause such strong\u00a013<\/sup>C depletion is required.\u201d\u00a0<\/p>\n The amount of carbon-13 in the Martian sediments is far lower than in Earth\u2019s sediments. <\/p>\n \u201cOn measuring the stable isotope ratio between 13C and 12C, the Martian organic matter has a 13C abundance of 0.92% to 0.99% of the carbon that makes it up,\u201d lead author Ueno explained in a press release. \u201cThis is extremely low compared to Earth\u2019s sedimentary organic matter, which is about 1.04%, and atmospheric CO2, around 1.07%, both of which are biological remnants and are not similar to the organic matter in meteorites, which is about 1.05%.\u201d <\/p>\n The meteorite data is important because a four billion-year-old Martian meteorite named ALH 84001 is enriched in carbon-13, adding to the enigma of Mars\u2019 carbon. Somehow, carbon-13 became depleted in the intervening billions of years. Solar escape is one possible reason for the carbon-13 depletion, but the authors discount that. There likely wasn\u2019t enough time for enough carbon-13 to escape. \u201cFurthermore, based on geomagnetic observations, early Mars probably had a geomagnetic field before 4?Ga,\u201d the authors write. That field would\u2019ve prevented solar escape.<\/p>\n To determine what\u2019s behind this discrepancy, Ueno and his co-researchers simulated different Martian atmospheric conditions to see what would happen. <\/p>\n Their results show that isotope fractionation by solar UV light is responsible for Mars\u2019 13C depletion. <\/p>\n Carbon-12 and carbon-13 respond differently to UV light. Carbon-12 preferentially absorbs UV, which dissociates it into carbon monoxide that\u2019s depleted in carbon-12. What\u2019s left behind is CO2 enriched with carbon-13. <\/p>\n Scientists have observed this process in the upper atmospheres of Earth and Mars. In Mars\u2019 reducing atmosphere, where oxygen was depleted, the CO2 enriched with carbon-13 would\u2019ve transformed into formaldehyde and possibly methanol. But those compounds didn\u2019t remain stable. In Mars\u2019 early days, the surface temperature was close to the freezing point of water, and it never exceeded about 27 Celsius (80 F.) In that temperature range, the formaldehyde and other compounds could\u2019ve dissolved in water. From there, they gathered in sediments.<\/p>\n But that\u2019s not the end of Mars\u2019 carbon isotope story. <\/p>\n The researchers used models to show that in a Mars atmosphere with a CO2 to CO ratio of 90:10, 20% of the CO2 would have converted to CO, leading to the sedimentary carbon isotope ratio we see today. The remaining atmospheric CO2 would be higher in C-13, and both values are in line with what Curiosity found, and with the ancient Martian meteorite ALH 84001.<\/p>\n This is a plausible scenario that can explain Curiosity\u2019s curious carbon findings. <\/p>\n The team\u2019s study also includes some other important details. For instance, atmospheric CO may not have come solely from photolysis by UV light. Some could have come from volcanic eruptions. And atmospheric CO may not have been the sole source of organics that found their way into the sediments. But either way, the results tell scientists something about Mars\u2019 carbon cycle. <\/p>\n It also tells us to expect to find more organics in Martian sediments in the future. <\/p>\n \u201cIf the estimation in this research is correct, there may be an unexpected amount of organic material present in Martian sediments. This suggests that future explorations of Mars might uncover large quantities of organic matter,\u201d said Ueno.<\/p>\n While the research shows us that life needn\u2019t be present to produce these organics, it can\u2019t rule life out. Nobody can, at least not yet. <\/p>\n The research also shows how complex atmospheric chemistry can be and how difficult it can be to draw conclusions from atmospheric studies of exoplanets. The JWST has examined several exoplanet atmospheres and found some interesting results. But there\u2019s so much we don\u2019t know. This research is a reminder that any conclusions are likely premature. <\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/06\/news-34811-p1.jpg\")
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