彗星在土卫六和地球大气层起源和演化中的重要性线索
Clues on the importance of comets in the origin and evolution of the atmospheres of Titan and Earth
摘要：
地球和土卫六是两个相距甚远的行星体。尽管如此，它们大气层的化学成分显示出共同的迹象，表明它们是由吸积作用以及彗星物质的原位加工产生的。这是值得注意的，因为当地球形成在圆盘的内部时，可能是由贫氧的岩石星子吸积而成，并表现出与顽火辉石类似的化学性质，而泰坦则是在土星的亚星云中由富含氧气和挥发物的天体形成的，称为彗星。从宇宙化学和天体生物学的角度来看，对地球和土卫六上的H、C、N和O同位素的研究可能是破译这两个行星体形成早期阶段过程的关键。本文的主要目标是量化这两个行星体的化学演化的可能方式，特别是它们早期大气中CO和N2的丰度。为了做到这一点，土卫六和地球的原始大气和演变已经从热力学的角度进行了分析。最相关的化学反应，涉及这些物种，并可能在其早期阶段的重要进行了讨论。然后，我们解释了本研究的结果，根据卡西尼-惠更斯任务对这些物种及其同位素所获得的结果。鉴于H、C、N和O优先从形成我们星球的内盘物质中耗尽，它们的同位素分馏的相似性，以及随后地球和泰坦大气的密切演化，都指向地球大气的彗星起源。因此，我们的方案也支持的关键作用，后期单板（彗星和富含水的碳质小行星）丰富的挥发性成分的地球在晚期重轰炸的类地行星。
英文摘要：
Earth and Titan are two planetary bodies formed far from each other. Nevertheless the chemical composition of their atmospheres exhibits common indications of being produced by the accretion, plus ulterior in-situ processing of cometary materials. This is remarkable because while the Earth formed in the inner part of the disk, presumably from the accretion of rocky planetesimals depleted in oxygen and exhibiting a chemical similitude with enstatite chondrites, Titan formed within Saturn's sub-nebula from oxygen- and volatile-rich bodies, called cometesimals. From a cosmochemical and astrobiological perspective the study of the H, C, N, and O isotopes on Earth and Titan could be the key to decipher the processes occurred in the early stages of formation of both planetary bodies. The main goal of this paper is to quantify the presumable ways of chemical evolution of both planetary bodies, in particular the abundance of CO and N2 in their early atmospheres. In order to do that the primeval atmospheres and evolution of Titan and Earth have been analyzed from a thermodynamic point of view. The most relevant chemical reactions involving these species and presumably important at their early stages are discussed. Then we have interpreted the results of this study in light of the results obtained by the Cassini-Huygens mission on these species and their isotopes. Given that H, C, N and O were preferentially depleted from inner disk materials that formed our planet, the observed similitude of their isotopic fractionation, and subsequent close evolution of Earth's and Titan's atmospheres points towards a cometary origin of Earth atmosphere. Consequently, our scenario also supports the key role of late veneers (comets and water-rich carbonaceous asteroids) enriching the volatile content of the Earth at the time of the Late Heavy Bombardment of terrestrial planets.
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