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首页> 外文期刊>Planetary and space science >Organic chemistry in Titan's upper atmosphere and its astrobiological consequences: I. Views towards Cassini plasma spectrometer (CAPS) and ion neutral mass spectrometer (INMS) experiments in space
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Organic chemistry in Titan's upper atmosphere and its astrobiological consequences: I. Views towards Cassini plasma spectrometer (CAPS) and ion neutral mass spectrometer (INMS) experiments in space

机译：土卫六高空大气中的有机化学及其天体生物学后果：I.对卡西尼等离子体光谱仪（CAPS）和离子中性质谱仪（INMS）在太空中进行实验的看法

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The discovery of carbocations and carbanions by Ion Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) instruments onboard the Cassini spacecraft in Titan's upper atmosphere is truly amazing for astrochemists and astrobiologists. In this paper we identify the reaction mechanisms for the growth of the complex macromolecules observed by the CAPS Ion Beam Spectrometer (IBS) and Electron Spectrometer (ELS). This identification is based on a recently published paper (Ali et al., 2013. Planet. Space Sci. 87, 96) which emphasizes the role of Olah's nonclassical carbonium ion chemistry in the synthesis of the organic molecules observed in Titan's thermosphere and ionosphere by INMS. The main conclusion of that work was the demonstration of the presence of the cydopropenyl cation - the simplest Huckel's aromatic molecule - and its cyclic methyl derivatives in Titan's atmosphere at high altitudes. In this study, we present the transition from simple aromatic molecules to the complex ortho-bridged bi- and tri-cyclic hydrocarbons, e.g., CH2+ monosubstituted naphthalene and phenanthrene, as well as the ortho- and pen-bridged tri-cyclic aromatic ring, e.g., perinaphthenyl cation. These rings could further grow into tetra-cyclic and the higher order ring polymers in Titan's upper atmosphere. Contrary to the pre-Cassini observations, the nitrogen chemistry of Titan's upper atmosphere is found to be extremely rich. A variety of N-containing hydrocarbons including the N-heterocycles where a CH group in the polycyclic rings mentioned above is replaced by an N atom, e.g., CH2+ substituted derivative of quinoline (benzopyridine), are found to be dominant in Titan's upper atmosphere. The mechanisms for the formation of complex molecular anions are discussed as well. It is proposed that many closed-shell complex carbocations after their formation first in Titan's upper atmosphere, undergo the kinetics of electron recombination to form open-shell neutral radicals. These radical species subsequently might form carbanions via radiative electron attachment at low temperatures with thermal electrons. The classic example is the perinaphthenyl anion in Titan's upper atmosphere. Therefore, future astronomical observations of selected carbocations and corresponding carbanions are required to settle the key issue of molecular anion chemistry on Titan. Other than earth, Titan is the only planetary body in our solar system that is known to have reservoirs of permanent liquids on its surface. The synthesis of complex biomolecules either by organic catalysis of precipitated solutes "on hydrocarbon solvent" on Titan or through the salvation process indeed started in its upper atmosphere. The most notable examples in Titan's prebiotic atmospheric chemistry are conjugated and aromatic polycyclic molecules, N-heterocycles including the presence of imino > C=N-H functional group in the carbonium chemistry. Our major conclusion in this paper is that the synthesis of organic compounds in Titan's upper atmosphere is a direct consequence of the chemistry of carbocations involving the ion-molecule reactions. The observations of complexity in the organic chemistry on Titan from the Cassini-Huygens mission dearly indicate that Titan is so far the only planetary object in our solar system that will most likely provide an answer to the question of the synthesis of complex biomolecules on the primitive earth and the origin of life. (C) 2015 Elsevier Ltd. All rights reserved.
机译：离子中性质谱仪（INMS）和卡西尼等离子体光谱仪（CAPS）仪器在土卫六高层大气中的卡西尼号航天器上发现了碳正离子和碳负离子，这对于天化学家和天体生物学家来说确实是令人惊讶的。在本文中，我们确定了CAPS离子束光​​谱仪（IBS）和电子光谱仪（ELS）观察到的复杂大分子生长的反应机理。该鉴定基于最近发表的论文（Ali等，2013.Planet。Space Sci。87，96），该论文强调了奥拉的非经典碳离子化学在泰坦热层和电离层中观察到的有机分子合成中的作用。 INMS。这项工作的主要结论是在高海拔土卫六大气中证明了环丙烯基阳离子（最简单的Huckel芳族分子）及其环状甲基衍生物的存在。在这项研究中，我们介绍了从简单的芳族分子到复杂的邻桥双环和三环烃（例如CH2 +单取代的萘和菲）以及邻桥和戊桥三环芳环的过渡，例如，萘萘基阳离子。这些环在泰坦的高层大气中可能进一步成长为四环和更高阶的环聚合物。与卡西尼之前的观测相反，发现泰坦高层大气的氮化学非常丰富。发现在土卫六的高层大气中占主导地位的是各种含N的烃类，其中包括上述多环中的CH基被N原子取代的N杂环，例如喹啉（苯并吡啶）的CH2 +取代的衍生物。还讨论了形成复杂分子阴离子的机理。有人提出，许多闭壳复杂的碳正离子首先在土卫六的高层大气中形成后，会经历电子复合的动力学，从而形成开壳的中性自由基。这些自由基物质随后可能会在低温下与热电子通过辐射电子附着形成碳负离子。典型的例子是土卫六高层大气中的萘并萘基阴离子。因此，需要未来对选定碳正离子和相应碳负离子的天文观测来解决Titan上分子阴离子化学的关键问题。除地球以外，土卫六是我们太阳系中唯一已知在其表面上具有永久性液体储集层的行星体。通过在“泰坦”上“在烃类溶剂上”对沉淀的溶质进行有机催化或通过拯救过程的合成复杂的生物分子实际上是在其高层大气中开始的。泰坦的益生元大气化学中最著名的例子是共轭芳族多环分子，N-杂环，包括碳鎓化学中亚氨基> C = N-H官能团的存在。我们在本文中的主要结论是，泰坦高层大气中有机化合物的合成是涉及离子分子反应的碳正离子化学反应的直接结果。卡西尼-惠更斯（Cassini-Huygens）任务对土卫六有机化学复杂性的观察表明，土卫六是迄今为止太阳系中唯一的行星物体，很可能会为原始生物合成复杂生物分子的问题提供答案。地球和生命的起源。 （C）2015 Elsevier Ltd.保留所有权利。

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