Low-mass nitrogen-, oxygen-bearing, and aromatic compounds in Enceladean ice grains
Low-mass nitrogen-, oxygen-bearing, and aromatic compounds in Enceladean ice grains
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DOI：
10.1093/mnras/stz2280
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发表时间：
2019-11-01
影响因子：
4.8
通讯作者：
Srama, R.
中科院分区：
文献类型：
--
作者：
Khawaja, N.;Postberg, F.;Srama, R.
土星的卫星土卫二正从其南极喷发出一股气体和冰粒。羽状物质直接连接到月球的次表层全球海洋，穿过冰壳的裂缝，被抛入太空。据信，次表层海洋与岩石核心接触，存在持续的热液活动。卡西尼号航天器的离子和中性质谱仪(INMS)检测到羽流中的挥发性、气相和有机物质，宇宙尘埃分析仪(CDA)在一小部分冰粒中发现了高质量的复杂有机物质。在这里，我们提出了一个更广泛的成分分析的CDA质谱图的有机冰粒。通过模拟实验，我们发现了土卫二冰粒中低质量有机化合物的光谱特征：含氮、含氧和芳香。通过与INMS结果的比较，我们确定低质量的胺[特别是(二)甲胺和/或乙胺]和羰基(最适合的是醋酸和/或乙醛)分别是含N和O的化合物的最佳候选者。据推测，单个冰粒中的有机浓度各不相同，但可能达到数十毫摩尔的水平。低质量的含氮含氧化合物溶解在海洋中，在海洋表面高效蒸发，通过蒸汽吸附进入冰粒。潜在的部分可溶于水的低质量芳香族化合物也可以通过气雾化进入冰粒。在土卫二温暖的海洋深处，这些胺、羰基和芳香族化合物可能是矿物催化的Friedel-Craft水热合成生物相关有机化合物的理想前体。
Saturn's moon Enceladus is erupting a plume of gas and ice grains from its south pole. Linked directly to the moon's subsurface global ocean, plume material travels through cracks in the icy crust and is ejected into space. The subsurface ocean is believed to be in contact with the rocky core, with ongoing hydrothermal activity present. The Cassini spacecraft's Ion and Neutral Mass Spectrometer (INMS) detected volatile, gas phase, organic species in the plume and the Cosmic Dust Analyser (CDA) discovered high-mass, complex organic material in a small fraction of ice grains. Here, we present a broader compositional analysis of CDA mass spectra from organic-bearing ice grains. Through analogue experiments, we find spectral characteristics attributable to low-mass organic compounds in the Enceladean ice grains: nitrogen-bearing, oxygen-bearing, and aromatic. By comparison with INMS results, we identify low-mass amines [particularly (di)methylamine and/or ethylamine] and carbonyls (with acetic acid and/or acetaldehyde most suitable) as the best candidates for the N- and O-bearing compounds, respectively. Inferred organic concentrations in individual ice particles vary but may reach tens of mmol levels. The low-mass nitrogen- and oxygen-bearing compounds are dissolved in the ocean, evaporating efficiently at its surface and entering the ice grains via vapour adsorption. The potentially partially water soluble, low-mass aromatic compounds may alternatively enter ice grains via aerosolization. These amines, carbonyls, and aromatic compounds could be ideal precursors for mineral-catalysed Friedel-Crafts hydrothermal synthesis of biologically relevant organic compounds in the warm depths of Enceladus' ocean.