The search for, and characterization of, organic matter on Mars is central to efforts in identifying habitable environments and detecting evidence of life in the martian surface and near surface. Iron oxides are ubiquitous in the martian regolith and are known to be associated with the deposition and preservation of organic matter in certain terrestrial environments, thus iron oxide-rich sediments are potential targets for life-detection missions. The most frequently used protocol for martian organic matter characterization (also planned for use on ExoMars) has been thermal extraction for the transfer of organic matter to gas chromatography-mass spectrometry (GC-MS) detectors. For the effective use of thermal extraction for martian samples, it is necessary to explore how potential biomarker organic molecules evolve during this process in the presence of iron oxides. We have thermally decomposed iron oxides simultaneously with (z)-octadec-9-enoic and n-octadecanoic acids and analyzed the products through pyrolysis-GC-MS. We found that the thermally driven dehydration, reduction, and recrystallization of iron oxides transformed fatty acids. Overall detectability of products greatly reduced, molecular diversity decreased, unsaturated products decreased, and aromatization increased. The severity of this effect increased as reduction potential of the iron oxide and inferred free radical formation increased. Of the iron oxides tested hematite showed the least transformative effects, followed by magnetite, goethite, then ferrihydrite. It was possible to identify the saturation state of the parent carboxylic acid at high (0.5 wt %) concentrations by the distribution of n-alkylbenzenes in the pyrolysis products. When selecting life-detection targets on Mars, localities where hematite is the dominant iron oxide could be targeted preferentially, otherwise thermal analysis of carboxylic acids, or similar biomarker molecules, will lead to enhanced polymerization, aromatization, and breakdown, which will in turn reduce the fidelity of the original biomarker, similar to changes normally observed during thermal maturation.

中文翻译：

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存在氧化铁时羧酸的热解：对火星任务中生命探测的影响

寻找和表征火星上的有机物质是识别宜居环境和探测火星表面和近地表生命证据的工作的核心。氧化铁在火星风化层中无处不在，并且已知与某些陆地环境中有机物质的沉积和保存有关，因此富含氧化铁的沉积物是生命探测任务的潜在目标。火星有机物表征（也计划在 ExoMars 上使用）最常用的协议是将有机物转移到气相色谱-质谱 (GC-MS) 检测器的热萃取。为了有效利用火星样品的热提取，有必要探索在氧化铁存在的情况下，潜在的生物标志物有机分子在此过程中如何演变。我们同时热分解氧化铁和 (z)-octadec-9-enoic 和正十八烷酸并通过热解-GC-MS 分析产物。我们发现氧化铁的热驱动脱水、还原和重结晶转化了脂肪酸。产物的整体可检测性大大降低，分子多样性降低，不饱和产物减少，芳构化增加。这种影响的严重程度随着氧化铁的还原电位和推断的自由基形成的增加而增加。在所测试的氧化铁中，赤铁矿的转化效果最小，其次是磁铁矿、针铁矿，然后是水铁矿。可以通过n的分布来确定高 (0.5 wt %) 浓度下母体羧酸的饱和状态- 热解产物中的烷基苯。在火星上选择生命探测目标时，可以优先选择赤铁矿为主要氧化铁的区域，否则对羧酸或类似生物标志物分子的热分析将导致聚合、芳构化和分解增强，进而减少原始生物标志物的保真度，类似于通常在热成熟过程中观察到的变化。