Preservation of organic matter by reactive iron species on Mars relevant to Mars Sample Return

背景：NASA的火星2020毅力漫游车和ESA的ExoMars火星车Rosalind Franklin都将在火星上寻找潜在生命的迹象。毅力号于2020年7月发射升空，将于2021年2月在杰零陨石坑着陆。Jeero陨石坑曾经拥有一个湖泊，并在流入通道的入海口拥有类似三角洲的沉积，作为其调查的优先区域。毅力号将通过收集和缓存样品以供后续任务返回地球，从而启动火星样品回收工作。罗莎琳德·富兰克林将于2022年发射升空，并在火星上的奥夏平面着陆。该区以广泛分布的粘土矿藏为特征，还包括隐蔽的河道和类似三角洲的沉积。罗莎琳德·富兰克林配备了一台钻头，可以从2米深的地方收集样本，以寻找火星表面从HARS辐射环境中保存下来的有机物质。铁矿物在封存和保存碳方面发挥着关键作用。地球沉积物中超过20%的有机碳直接与活性铁相结合(Lalonde等人。2012)，也被称为生锈的碳汇。有机物-铁矿物复合体相互稳定，防止有机物降解和矿物转化，由此涉及的铁物种往往以纳米颗粒和X射线非晶态形式出现(Schröder等人)。2016)。然而，在成岩作用中，铁的氧化还原特性促进了有机物的氧化(Posth等人。2013年；Schröder等人。2016年；Tan等人。2020)。在此基础上，寻找潜在有机生物特征的一个很好的目标将是富含纳米铁相和X射线非晶态矿物。在古瑟夫陨石坑和火星子午线平原上调查的水蚀变岩石和土壤含有丰富的纳米相铁氧化物(例如Morris等人。2019年)，盖尔陨石坑河湖沉积物中20-60wt%的矿物为X射线非晶态，这种非晶态相富含铁(例如Rampe等人)。2017、2019年)。类似的矿藏应该瞄准杰零陨石坑和奥夏平原，以寻找有机生物特征。然而，识别富含活性铁相的矿床(因此缺乏成岩或变质叠加)是困难的。分别在古瑟夫陨石坑和子午线平原着陆的火星探测探测器勇气号和机遇号使用的主要矿物学工具是穆斯堡尔光谱仪，而盖尔陨石坑的MSL好奇号火星车使用的是X射线衍射仪。Persistance和Rosalind Franklin将使用拉曼光谱仪进行矿物学分析。目标：-使用X射线衍射、穆斯堡尔和拉曼光谱研究不同年龄的含有活性铁矿物相(即缺乏成岩或变质叠加)的沉积物和沉积岩-发展穆斯堡尔光谱作为分析返回样品的工具方法：我们将识别和收集一系列不同年龄范围的富含活性铁矿物相的沉积物和沉积岩。使用X射线衍射仪、穆斯堡尔谱和拉曼光谱仪对这些矿物进行矿物学研究，并用XRF进行元素分析，用扫描电子显微镜进行空间特征分析。我们将测量有机碳含量并量化特定的碳化合物。不同的穆斯堡尔光谱学应用--透射式和低温测量；无损后向散射应用；基于同步加速器的应用--将被使用和优化，以期将其应用于退回样品的调查。

Background: Both NASA's Mars 2020 Perseverance rover and ESA's ExoMars rover Rosalind Franklin will be looking for signs of potential life on Mars. Perseverance launched in July 2020 and will land at Jezero crater in February 2021. Jezero crater once held a lake and has delta-like deposits at the mouth of inflow channels as prioriy area for its investigations. Perseverance will kick-off the Mars Sample Return effort by collecting and caching samples for return to Earth by subsequent missions. Rosalind Franklin will launch in 2022 and land at Oxia Planum on Mars. This area is characterized by wide-spread clay mineral deposits and includes subtle fluvial channels and delta-like deposits as well. Rosalind Franklin id equipped with a drill to collect samples from up to 2 m depth to look for organic material preserved from the hars radiation environment on Mars' surface.Iron minerals play a key role in the sequestration and preservation of carbon. Over 20% of organic carbon in sediments on Earth is directly bound to reactive iron phases (Lalonde et al. 2012), also dubbed the Rusty Carbon Sink. The organic matter-iron mineral complexes mutually stabilize each other against organic matter degradation and mineral transformation, whereby the iron species involved often occur in nanoparticulate and X-ray amorphous forms (Schröder et al. 2016). Upon diagenesis, however, the redox properties of iron facilitate the oxidation of organic matter (Posth et al. 2013; Schröder et al. 2016; Tan et al. 2020). On that basis, a good target for the search for potential organic biosignatures would be rich in nanophase iron phases and X-ray amorphous minerals. Aqueously altered rocks and soils investigated in Gusev crater and at Meridiani Planum on Mars contain abundant nanophase iron oxides (e.g. Morris et al. 2019), and 20-60 wt% of minerals in fluvio-lacustrine deposits in Gale crater are X-ray amorphous and this amorphous phase is rich in iron (e.g. Rampe et al. 2017, 2019). Similar deposits should be targeted in Jezero crater and at Oxia Planum to look for organic biosignatures. However, the difficulty is to recognize deposits rich in reactive iron phases (and therefore lacking a diagenetic or metamorphic overprint). The Mars Exploration Rovers Spirit and Opportunity who landed at Gusev crater and Meridiani Planum, respectively, had Mössbauer spectrometers as their principle mineralogical tool, while the MSL Curiosity rover at Gale crater used XRD. Perseverance and Rosalind Franklin will use Raman spectrometers for mineralogical analyses.Objectives:- Investigate sediments and sedimentary rocks of different ages, which contain reactive iron mineral phases (i.e. lack diagenetic or metamorphic overprint) using XRD, Mössbauer and Raman spectroscopy- Develop Mössbauer spectroscopy as a tool to be used for the analysis of returned samplesMethodology: We will identify and collect a series of sediments and sedimentary rocks rich in reactive iron mineral phases with a range of different ages. These will be investigated for their mineralogy using XRD, Mössbauer and Raman Spectroscopy, plus XRF for elemental analysis and SEM for spatial charactiersation. We will measure organic carbon content and quantify specific carbon compounds. Different Mössbauer spectroscopy applications - transmission and low-temperature measurements; non-destructive backscattering applications; synchrotron-based applications - will be used and optimized with a view to apply the to the investigation of returned samples.