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 ¹³C 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 ¹³C depletion is required. Here we show from CO₂ photolysis experiments and theoretical considerations that solar ultraviolet photolysis of CO₂ in a reducing atmosphere can yield strongly ¹³C-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 ¹³C enrichment of early Martian CO₂ 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 CO₂ 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.

在早期火星沉积物中发现的有机物可能为了解该行星的环境条件、生命起源前的化学和可居住性提供线索，但其起源仍不清楚。好奇号火星车最近检测到盖尔陨石坑沉积有机物中¹³ C严重消耗。尽管这种神秘的消耗仍然存在争议，但如果正确的话，就需要一种引起如此强烈的¹³ C 消耗的机制。在这里，我们通过CO ₂光解实验和理论考虑表明，在还原性气氛中CO ₂的太阳紫外光解可以强烈产生¹³ C贫化的CO。我们认为，从光解产生的CO中合成有机化合物是一种合理的机制，可以解释早期火星沉积物中同位素贫乏有机物的来源。此外，这种机制可以解释早期火星CO _2的¹³ C 富集，而不需要长期碳逃逸到太空。使用我们估计的同位素分馏因子进行的质量平衡模型计算表明，早期火星上约 20% 的火山 CO ₂排放通过 CO转化为有机物，这与碳酸盐碳同位素的可用数据一致。尽管已经提出了有机化合物生产的替代途径，但我们的研究结果表明，大量的有机物可能是在还原性的早期火星大气中由二氧化碳合成的，并沉积在沉积物中。