The martian surface has been continuously exposed to galactic cosmic radiation. Since organic compounds are degraded by ionizing radiation, knowledge of their decay constants is fundamental to predicting their stability on the martian surface. In this study, we report the radiolysis constant for the destruction of soil organic compounds at a starting concentration of ∼2011 μg C/g_soil from the Mojave Desert. The soils were exposed to gamma irradiation with absorbed doses of up to 19 MGy at room temperature, representing ∼250 million years of exposure to galactic cosmic rays. The destruction of total soil organic carbon and the formation of gases were investigated by a sequential on-line analytical array coupled to gas chromatography-mass spectrometry. Soil inorganic and organic carbon were degraded exponentially with a radiolysis constant 0.3 MGy^-1(30%) producing mostly carbon dioxide (93.2%), carbon monoxide (6.2%), and methane (0.6%). Using the dose rate measured by the Radiation Assessment Detector on board the Curiosity rover, we make predictions on the survival of organic compounds in the cold martian subsurface. It is estimated that soil organic compounds with initial concentrations as those found today at the Mojave Desert would have been destroyed to levels <1 ppb at 0.1 m in depth in ∼2000 Myr. Pristine organic compounds are expected to be present at a depth of ∼1.5 m. These results are relevant for the search of organic compounds in past, present, and future missions to Mars. In particular, we predict that the upcoming ExoMars will encounter pristine organic compounds at this depth.

中文翻译：

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60Co 伽马射线对莫哈韦沙漠土壤碳的辐射降解：宇宙辐射对火星有机化合物生存的影响

火星表面一直受到银河宇宙辐射的影响。由于有机化合物会被电离辐射降解，因此了解它们的衰减常数对于预测它们在火星表面的稳定性至关重要。在这项研究中，我们报告了起始浓度为 ~2011 μg C/g_土壤时破坏土壤有机化合物的辐射分解常数来自莫哈韦沙漠。土壤在室温下暴露于伽马辐射，吸收剂量高达 19 MGy，代表了大约 2.5 亿年的银河宇宙射线暴露。通过与气相色谱-质谱联用的连续在线分析阵列研究了土壤总有机碳的破坏和气体的形成。土壤无机碳和有机碳呈指数级降解，辐解常数为 0.3 MGy ^-1(30%) 主要产生二氧化碳 (93.2%)、一氧化碳 (6.2%) 和甲烷 (0.6%)。使用好奇号火星车上的辐射评估探测器测量的剂量率，我们对寒冷的火星地下有机化合物的存活率进行预测。据估计，最初浓度与今天在莫哈韦沙漠发现的土壤有机化合物一样，在 2000 Myr 深度 0.1 m 处被破坏到 <1 ppb 的水平。原始有机化合物预计存在于约 1.5 m 的深度。这些结果与过去、现在和未来的火星任务中有机化合物的搜索有关。特别是，我们预测即将到来的 ExoMars 将在这个深度遇到原始的有机化合物。