Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London, UK.
Astrobiology. 2021 Feb;21(2):199-218. doi: 10.1089/ast.2019.2211. Epub 2020 Nov 23.
Acidic iron- and sulfur-rich streams are appropriate analogues for the late Noachian and early Hesperian periods of martian history, when Mars exhibited extensive habitable environments. Any past life on Mars may have left behind diagnostic evidence of life that could be detected at the present day. For effective preservation, these remains must have avoided the harsh radiation flux at the martian surface, survived geological storage for billions of years, and remained detectable within their geochemical environment by analytical instrument suites used on Mars today, such as thermal extraction techniques. We investigated the detectability of organic matter within sulfur stream sediments that had been subjected to artificial maturation by hydrous pyrolysis. After maturation, the samples were analyzed by pyrolysis-gas chromatography-mass spectrometry (py-GC-MS) to determine whether organic matter could be detected with this commonly used technique. We find that macromolecular organic matter can survive the artificial maturation process in the presence of iron- and sulfur-rich minerals but cannot be unambiguously distinguished from abiotic organic matter. However, if jarosite and goethite are present in the sulfur stream environment, they interfere with the py-GC-MS detection of organic compounds in these samples. Clay reduces the obfuscating effect of the oxidizing minerals by providing nondeleterious adsorption sites. We also find that after a simple alkali and acid leaching process that removes oxidizing minerals such as iron sulfates, oxides, and oxyhydroxides, the sulfur stream samples exhibit much greater organic responses during py-GC-MS in terms of both abundance and diversity of organic compounds, such as the detection of hopanes in all leached samples. Our results suggest that insoluble organic matter can be preserved over billions of years of geological storage while still retaining diagnostic organic information, but sample selection strategies must either avoid jarosite- and goethite-rich outcrops or conduct preparative chemistry steps to remove these oxidants prior to analysis by thermal extraction techniques.
富铁和富硫的酸性溪流是火星历史上晚诺亚纪和早赫斯珀里安纪时期的合适模拟物，当时火星表现出广泛的宜居环境。火星上任何过去的生命都可能留下生命的诊断证据，这些证据在今天可能被探测到。为了有效保存，这些残留物必须避免火星表面的恶劣辐射通量，在数十亿年的地质储存中幸存下来，并在当今火星上使用的分析仪器套件（如热提取技术）所检测到的地球化学环境中保持可检测性。我们研究了在水热热解作用下人工成熟的硫流沉积物中有机物的可检测性。成熟后，通过热解-气相色谱-质谱法（py-GC-MS）对样品进行分析，以确定该常用技术是否可以检测到有机物。我们发现，在富铁和富硫矿物存在的情况下，大分子有机物可以在人工成熟过程中存活，但不能与非生物有机物明确区分。然而，如果黄钾铁矾和针铁矿存在于硫流环境中，它们会干扰这些样品中有机化合物的 py-GC-MS 检测。粘土通过提供非有害的吸附位来减少氧化矿物的干扰作用。我们还发现，在一种简单的碱酸浸出过程中，去除了铁的硫酸盐、氧化物和氢氧化物等氧化矿物后，硫流样品在 py-GC-MS 中表现出更高的有机响应，无论是有机化合物的丰度还是多样性，如所有浸出样品中都检测到藿烷。我们的研究结果表明，不溶性有机物可以在数十亿年的地质储存中保存下来，同时仍保留有诊断意义的有机信息，但样品选择策略必须避免黄钾铁矾和针铁矿含量丰富的露头，或在通过热提取技术进行分析之前进行制备化学步骤以去除这些氧化剂。
