NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States.
Department of Chemistry, Southern Methodist University, Dallas, Texas 75275, United States.
Acc Chem Res. 2021 Aug 3;54(15):3050-3059. doi: 10.1021/acs.accounts.1c00250. Epub 2021 Jul 23.
In this Account, we highlight recent work in the developing field of mineralogy of Saturn's moon Titan, focusing on binary co-crystals of small organic molecules. Titan has a massive inventory of organic molecules on its surface that are formed via photochemistry in the atmosphere and likely processing on the surface as well. Physical processes both in the atmosphere and on the surface can lead to molecules interacting at cryogenic temperatures. Recent laboratory work has demonstrated that co-crystals between two or more molecules can form under these conditions. In the organic-rich environment of Titan, such co-crystals are naturally occurring minerals and a critical area of research to understand the physical, chemical, and possibly even biological and prebiotic processes occurring in this alien world.With a future NASA mission, , slated to land on Titan in the next decade, much work is needed to understand organic mineralogy in order to properly interpret the data from this and past Titan missions, such as . By cataloging Titan minerals and their properties, we can begin to connect these behaviors to large-scale surface features observed on Titan (labyrinth terrain, lake evaporites, karst, dunes, etc.), and possible processes leading to their formation (erosion, deposition, etc.). To date, seven co-crystals (aside from clathrates and hydrates) have been experimentally reported to form under Titan-relevant conditions, with an eighth predicted by theoretical modeling. This Account will summarize the formation and properties of these cryominerals and discuss the implications for surface processes on Titan. Enhanced thermal expansion and decreased crystal size, for example, may lead to fracturing and/or more rapid erosion of co-crystal-based deposits; density changes upon co-crystal formation may also play a role in organic diagenesis and metamorphism on Titan. Some cryominerals with stability only under certain conditions may preserve the evidence of Titan's history, such as cryovolcanic activity, ethane fluvial/pluvial exposure, and outgassing of CO from the interior of the moon.In this Account, we will also highlight areas of future work, such as the characterization of pure molecular solids and the search for ternary (and more complex) co-crystals. We note that on Titan, organic chemistry dominates, which gives a unique opportunity for chemists to play an even more significant role in planetary science discoveries and likewise in discoveries motivated by planetary science to inform fundamental organic and physical chemistry research.
在这篇综述中，我们重点介绍了土卫六矿物学这一新兴领域的最新研究进展，特别是关于小有机分子二元共晶的研究。土卫六表面存在大量的有机分子，它们是通过大气中的光化学过程形成的，并且可能在表面也经历了进一步的化学转化。无论是在大气中还是在表面，物理过程都可能导致分子在低温下相互作用。最近的实验室研究表明，在这些条件下，两种或更多分子之间可以形成共晶。在土卫六富含有机物的环境中，这些共晶是自然存在的矿物质，也是研究该天体物理、化学，甚至生物和前生物过程的关键领域。未来美国宇航局的一项任务，计划在未来十年内在土卫六着陆，为了正确解释该任务以及过去的土卫六任务（如）的数据，需要开展大量工作来了解有机矿物学。通过对土卫六矿物及其性质进行编目，我们可以开始将这些行为与在土卫六上观测到的大规模表面特征（迷宫地形、湖泊蒸发盐、喀斯特地貌、沙丘等）以及可能导致其形成的过程（侵蚀、沉积等）联系起来。迄今为止，已有七种共晶（除了包合物和水合物之外）在与土卫六相关的条件下被实验报道形成，还有一种共晶通过理论建模预测。本综述将总结这些低温矿物的形成和性质，并讨论它们对土卫六表面过程的影响。例如，增强的热膨胀和晶体尺寸的减小可能导致共晶沉积物的断裂和/或更快速的侵蚀；共晶形成时的密度变化也可能在土卫六的有机成岩作用和变质作用中发挥作用。一些仅在特定条件下稳定的低温矿物可能保存了土卫六历史的证据，例如低温火山活动、乙烷河流/雨水暴露以及从月球内部释放的 CO。在这篇综述中，我们还将重点介绍未来的工作领域，例如纯分子固体的特性研究以及三元（和更复杂）共晶的寻找。我们注意到，在土卫六上，有机化学占主导地位，这为化学家在行星科学发现中发挥更重要的作用提供了独特的机会，同样，行星科学的发现也为基础有机和物理化学研究提供了信息。
