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Mars exhumes methane on a seasonal cycle, Curiosity reveals; rover also detects ancient organic matter

June 07, 2018

Data from the Curiosity rover, part of two separate studies, furthers scientists' understanding of methane on Mars - suggesting some of it may be trapped in water-based crystals - and identifies additional carbon-bearing molecules, central to understanding processes and conditions on the planet. In the first study, Christopher Webster et al. describe detailed, in situ measurements of atmospheric methane on Mars, which show a seasonal variation in its abundance. Small concentrations of methane have previously been detected in Mars' atmosphere, but its origins have been the subject of great debate. On Earth most methane is produced by biological sources, but numerous abiotic processes have been proposed to explain the Martian methane. Here, the researchers analyzed three Martian years' (55 Earth months') worth of atmospheric measurements collected by the Curiosity rover. The data reveal that background methane levels on the Red Planet have a strong seasonal cycle, ranging between 0.24 to 0.65 parts per billion, peaking near the end of summer in the Northern hemisphere (end of winter in the Southern hemisphere). The authors rule out numerous potential sources of the methane, ultimately suggesting that large amounts of the gas may be stored in the cold Martian subsurface in water-based crystals called clathrates. They propose that seasonal changes in temperature could cause the fluctuating release of methane observed by the rover. In a separate study, Jennifer L. Eigenbrode and colleagues analyze drill samples of soil, also taken by the Curiosity rover, which reveal a number of different organic compounds. Limited organic compounds have previously been identified at the Sheepbed mudstone site in Gale crater. Even so, researchers' understanding of ancient organic matter in Martian sediments is lacking. Here, the researchers analyzed new samples from two sites in the Gale crater: Mojave and Confidence Hills, which harbor mudstones that are approximately three billion years old. Tools on board the Curiosity rover extracted the new samples and heated them, analyzing the molecules that were released. The data reveal the presence of several organic molecules and volatiles reminiscent of organic-rich sedimentary rock found on Earth, including: thiophene, 2- and 3-methylthiophenes, methanethiol, and dimethylsulfide. The authors note that, because many of the molecules analyzed here differ by a single carbon sidechain, they may be fragments from larger molecules. Indeed, a comparison of these samples to organic traces in Martian meteorites also suggests that the former are derived from larger organic molecules. The Gale crater samples retain exceptionally high levels of sulfur, which the author propose helped preserve the organic matter. The implications of these two studies are highlighted in a Perspective by Inge Loes ten Kate.
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American Association for the Advancement of Science

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