Department of Earth Science and Engineering, Impacts and Astromaterials Research Centre, Imperial College London, London, United Kingdom.
Astrobiology. 2021 Jun;21(6):673-691. doi: 10.1089/ast.2020.2226. Epub 2021 Feb 25.
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 -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 -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.
火星上有机物的寻找和特性研究是确定宜居环境、在火星表面和近表面探测生命迹象的核心。氧化铁在火星风化层中普遍存在，并且已知与某些地球环境中有机物的沉积和保存有关，因此氧化铁丰富的沉积物是生命探测任务的潜在目标。最常用于火星有机物特性描述的方法（也计划用于 ExoMars）是热提取，将有机物转移到气相色谱-质谱（GC-MS）检测器。为了有效地在火星样本中使用热提取，有必要探索在氧化铁存在的情况下，潜在生物标志物有机分子在这个过程中是如何演变的。我们已经同时对（Z）-十八碳-9-烯酸和十八烷酸进行了氧化铁的热分解，并通过热解 GC-MS 分析了产物。我们发现，氧化铁的热驱动脱水、还原和再结晶改变了脂肪酸。产物的总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。氧化铁的还原电位和推断自由基形成增加，这种影响的严重程度也随之增加。在所测试的氧化铁中，赤铁矿显示的转化效果最小，其次是磁铁矿、针铁矿，然后是水铁矿。通过热解产物中-烷基苯的分布，可以在高浓度（0.5wt%）下识别母体羧酸的饱和状态。在火星上选择生命探测目标时，如果赤铁矿是主要的氧化铁，可以优先选择这些地点，否则羧酸或类似生物标志物的热分析将导致聚合、芳构化和分解增强，这反过来又会降低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。
