Department of Earth Science and Engineering, Impacts and Astromaterials Research Centre, Imperial College London, London, United Kingdom.
Astrobiology. 2021 Jun;21(6):638-654. doi: 10.1089/ast.2020.2344. Epub 2021 Apr 8.
Comparisons between the preservation potential of Mars-analog environments have historically been qualitative rather than quantitative. Recently, however, laboratory-based artificial maturation combined with kinetic modeling techniques have emerged as a potential means by which the preservation potential of solvent-soluble organic matter can be quantified in various Mars-analog environments. These methods consider how elevated temperatures, pressures, and organic-inorganic interactions influence the degradation of organic biomarkers post-burial. We used these techniques to investigate the preservation potential of deposits from a circumneutral iron-rich groundwater system. These deposits are composed of ferrihydrite (FeHO · 4HO), an amorphous iron hydroxide mineral that is a common constituent of rocks found in ancient lacustrine environments on Mars, such as those observed in Gale Crater. Both natural and synthetic ferrihydrite samples were subjected to hydrous pyrolysis to observe the effects of long-term burial on the mineralogy and organic content of the samples. Our experiments revealed that organic-inorganic interactions in the samples are dominated by the transformation of iron minerals. As amorphous ferrihydrite transforms into more crystalline species, the decrease in surface area results in the desorption of organic matter, potentially rendering them more susceptible to degradation. We also find that circumneutral iron-rich deposits provide unfavorable conditions for the preservation of solvent-soluble organic matter. Quantitative comparisons between preservation potentials as calculated when using kinetic parameters show that circumneutral iron-rich deposits are ∼25 times less likely to preserve solvent-soluble organic matter compared with acidic, iron-rich environments. Our results suggest that circumneutral iron-rich deposits should be deprioritized in favor of acidic iron- and sulfur-rich deposits when searching for evidence of life with solvent extraction techniques.
火星类似环境的保存潜力的比较，历史上一直是定性的，而不是定量的。然而，最近基于实验室的人工成熟度结合动力学建模技术已经成为一种潜在的方法，可以定量评估各种火星类似环境中溶剂可溶性有机物的保存潜力。这些方法考虑了高温、高压和有机-无机相互作用如何影响埋藏后有机生物标志物的降解。我们使用这些技术来研究来自中性到富铁地下水中的沉积物的保存潜力。这些沉积物由水铁矿（FeHO·4HO）组成，这是一种无定形的氢氧化铁矿物，是火星古代湖泊环境中发现的岩石的常见成分，如在盖尔陨石坑中观察到的岩石。天然和合成水铁矿样品都进行了水热解，以观察长期埋藏对样品的矿物学和有机含量的影响。我们的实验表明，样品中的有机-无机相互作用主要由铁矿物的转化所控制。随着无定形水铁矿转化为更结晶的物种，表面积的减少导致有机物的解吸，从而使它们更容易降解。我们还发现，中性到富铁沉积物为溶剂可溶性有机物的保存提供了不利条件。使用动力学参数计算的保存潜力的定量比较表明，与酸性富铁环境相比，中性到富铁沉积物保存溶剂可溶性有机物的可能性要低 25 倍。我们的结果表明，在用溶剂提取技术寻找生命证据时，应该优先考虑酸性富铁和硫沉积物，而不是中性富铁沉积物。
