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The photochemical stability of carbonates on Mars

机译：碳酸盐在火星上的光化学稳定性

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Carbonates, predominately MgCO3, have been spectroscopically identified at a level of 2-5% in martian dust. However, in spite of this observation, and a large number of climate studies that suggest 1 to several bars of CO2 should be sequestered in carbonate rocks, no outcrop-scale exposures of carbonate have been detected anywhere on Mars to date. To address one hypothesis for this long-standing puzzle, the effect of ultraviolet (UV) light on the stability of calcium carbonate in a simulated martian atmosphere was experimentally investigated. Using C-13-labeled calcite, we found no experimental evidence of the UV photodecomposition of calcium carbonate in a simulated martian atmosphere. Extrapolating the lower limit of detection of our experimental system to an upper limit of carbonate decomposition on Mars yields a quantum efficiency of 3.5 x 10(-8) molecules/photon over the wavelength interval of 190-390 nm and a maximum UV photodecomposition rate of 1.2 x 10(-13) kg m(-2) s(-1) from a calcite surface. The maximum loss of bulk calcite due to this process would be 2.5 nm year(-1) (Mars year). However, calcite is expected to be thermodynamically stable on the surface of Mars, and potential UV photodecomposition reaction mechanisms indicate that, though calcium carbonate may decompose under vacuum, it would be stable in a CO2 atmosphere. Given the expected stability of carbonate on Mars and our inability to detect carbonate decomposition, we conclude that it is unlikely that the apparent absence of extensive carbonate deposits on the martian surface is due to UV photodecomposition in the current environment.
机译：经光谱鉴定，火星尘埃中的碳酸盐含量主要为MgCO3。然而，尽管进行了这一观察，并且大量的气候研究表明应将1至几巴的CO2封存在碳酸盐岩中，但迄今为止在火星上的任何地方都未发现露头规模的碳酸盐暴露。为了解决这一长期存在的难题的一个假设，我们通过实验研究了紫外线（UV）对模拟火星大气中碳酸钙稳定性的影响。使用C-13标记的方解石，我们没有发现在模拟火星大气中碳酸钙的紫外光分解的实验证据。将我们的实验系统的检测下限外推至火星上碳酸盐分解的上限，在190-390 nm的波长范围内产生的量子效率为3.5 x 10（-8）分子/光子，最大UV光分解速率为从方解石表面1.2 x 10（-13）kg m（-2）s（-1）由于该过程，最大的方解石损失为2.5 nm year（-1）（火星年）。然而，预计方解石在火星表面上是热力学稳定的，潜在的紫外线光分解反应机理表明，尽管碳酸钙可能在真空下分解，但在CO2气氛中会稳定。考虑到碳酸盐在火星上的预期稳定性以及我们无法检测到碳酸盐分解，我们得出的结论是，在火星表面上明显不存在大量碳酸盐沉积的可能性不大可能是由于当前环境中的UV光分解所致。

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