Synthesis of 13C-depleted organic matter from CO in a reducing early Martian atmosphere
Yuichiro Ueno, Johan A. Schmidt, Matthew S. Johnson, Xiaofeng Zang, Alexis Gilbert, Hiroyuki Kurokawa, Tomohiro Usui, Shohei Aoki
Abstract
Abstract Organic matter found in early Martian sediment may yield clues to the planet’s environmental conditions, prebiotic chemistry and habitability, but its origin remains unclear. Strong 13 C depletion in sedimentary organic matter at Gale crater was recently detected by the Curiosity rover. Although this enigmatic depletion remains debated, if correct, a mechanism to cause such strong 13 C depletion is required. Here we show from CO 2 photolysis experiments and theoretical considerations that solar ultraviolet photolysis of CO 2 in a reducing atmosphere can yield strongly 13 C-depleted CO. We suggest that atmospheric synthesis of organic compounds from photolysis-produced CO is a plausible mechanism to explain the source of isotopically depleted organic matter in early Martian sediments. Furthermore, this mechanism could explain 13 C enrichment of early Martian CO 2 without requiring long-term carbon escape into space. A mass balance model calculation using our estimated isotopic fractionation factor indicates the conversion of approximately 20% of volcanic CO 2 emissions on early Mars into organics via CO, consistent with the available data for carbon isotopes of carbonate. Although alternative pathways for organic compound production have been proposed, our findings suggest that considerable amounts of organic matter may have been synthesized from CO in a reducing early Martian atmosphere and deposited in sediments.