Radiolytic Degradation of Soil Carbon from the Mojave Desert by 60Co Gamma Rays: Implications for the Survival of Martian Organic Compounds Due to Cosmic Radiation.
Radiolytic Degradation of Soil Carbon from the Mojave Desert by 60Co Gamma Rays: Implications for the Survival of Martian Organic Compounds Due to Cosmic Radiation.
复制标题
60Co 伽马射线对莫哈韦沙漠土壤碳的辐射分解：宇宙辐射对火星有机化合物生存的影响。
作者：
José Alfredo Rojas Vivas;R. Navarro‐González;José de la Rosa;P. Molina;S. Sedov;C. Mckay
火星表面一直暴露在银河系的宇宙辐射之下。由于有机化合物会被电离辐射降解，因此了解它们的衰变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，我们报告的辐解常数的土壤有机化合物的破坏在2011 μg C/g土壤的起始浓度从莫哈韦沙漠。这些土壤受到伽马辐照，室温下吸收剂量高达19兆戈瑞，相当于2.5亿年的银河宇宙射线辐照。利用气相色谱-质谱联用技术研究了土壤有机碳的破坏和气体的形成。土壤无机碳和有机碳呈指数降解，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Currency探测器上的辐射评估探测器测量的剂量率，我们对寒冷的火星地表下有机化合物的生存进行预测。据估计，如果土壤中的有机化合物的初始浓度与今天在莫哈韦沙漠中发现的相同，那么在2000万年前，在0.1米深处，它们将被破坏到<1 ppb的水平。原始有机化合物预计存在于1.5米深处。这些结果与过去、现在和未来火星任务中的有机化合物搜索有关。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。
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/gsoil 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.