Koyama Shungo, Yoshida Tatsuya, Furukawa Yoshihiro, Terada Naoki, Ueno Yuichiro, Nakamura Yuki, Kamada Arihiro, Kuroda Takeshi, Vandaele Ann Carine
Graduate School of Science, Tohoku University, Sendai, Miyagi, 980-8578, Japan.
Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium.
Sci Rep. 2024 Sep 17;14(1):21214. doi: 10.1038/s41598-024-71301-w.
Organic matter in the Martian sediments may provide a key to understanding the prebiotic chemistry and habitability of early Mars. The Curiosity rover has measured highly variable and C-depleted carbon isotopic values in early Martian organic matter whose origin is uncertain. One hypothesis suggests the deposition of simple organic molecules generated from C-depleted CO derived from CO photochemical reduction in the atmosphere. Here, we present a coupled photochemistry-climate evolution model incorporating carbon isotope fractionation processes induced by CO photolysis, carbon escape, and volcanic outgassing in an early Martian atmosphere of 0.5-2 bar, composed mainly of CO, CO, and H to track the evolution of the carbon isotopic composition of C-bearing species. The calculated carbon isotopic ratio in formaldehyde (HCO) can be highly depleted in C due to CO-photolysis-induced fractionation and is variable with changes in atmospheric CO/CO ratio, surface pressure, albedo, and H outgassing rate. Conversely, CO becomes enriched in C, as estimated from the carbonates preserved in ALH84001 meteorite. Complex organic matter formed by the polymerization of such HCO could explain the strong depletion in C observed in the Martian organic matter. Mixing with other sources of organic matter would account for its unique variable carbon isotopic values.
火星沉积物中的有机物可能是理解早期火星的益生元化学和宜居性的关键。好奇号火星车测量了早期火星有机物中高度可变且贫碳的碳同位素值，其来源尚不确定。一种假说认为，这些有机物是由大气中因一氧化碳光化学还原产生的贫碳一氧化碳生成的简单有机分子沉积而成。在此，我们提出了一个光化学 - 气候演化耦合模型，该模型纳入了在早期火星0.5 - 2巴的主要由一氧化碳、二氧化碳和氢气组成的大气中，由一氧化碳光解、碳逃逸和火山排气引发的碳同位素分馏过程，以追踪含碳物种碳同位素组成的演化。由于一氧化碳光解诱导的分馏作用，计算得出的甲醛（H₂CO）中的碳同位素比率可能会高度贫碳，并且会随着大气一氧化碳/二氧化碳比率、表面压力、反照率和氢气排气率的变化而变化。相反，正如从ALH84001陨石中保存的碳酸盐所估计的那样，二氧化碳会富集碳。由这种甲醛聚合形成的复杂有机物可以解释在火星有机物中观察到的强烈的碳贫化现象。与其他有机物质来源混合可以解释其独特的可变碳同位素值。
