Current Status of the New Field of Titan Mineralogy, with a Focus on Co-crystals
Abstract
Titan, Saturn's largest satellite, contains an impressive inventory of organic molecules and is considered a prebiotic chemical laboratory on a planetary scale. Photochemistry in the atmosphere induced by solar radiation and charged particles from Saturn's magnetosphere causes a chemical cascade, dissociating and ionizing N$ _{2}$ and CH$ _{4}$ and generating a plethora of small and large organic molecules. Most of these molecules are transported down to the surface, where they may react and interact as they become integrated into the unique geology of Titan. Many of the organic molecules on Titan's surface appear to be able to associate in a manner akin to minerals on Earth, forming molecular solids, co-crystals, and hydrates. These 'molecular minerals' exhibit unique physical and mechanical properties compared to their pure components, and may affect formation mechanisms and timescales of landscape evolution on Titan. Here, we focus on a subset of these molecular minerals: co-crystals. We have discovered four co-crystals to date, which have been characterized using Raman spectroscopy and cryogenic powder X-ray diffraction. The benzene-ethane co-crystal, the equivalent to a 'hydrated mineral' on Titan as ethane is liquid at Titan surface temperatures, forms in short timescales and would likely be the first material to precipitate from an evaporating hydrocarbon lake. The acetylene-butane co-crystal might be the most ubiquitous in these polar regions of Titan, as both of these compounds should be abundant on Titan and both are highly soluble in liquid hydrocarbons, making them the major components of evaporite materials in and around the Titan lakes. The acetylene-ammonia co-crystal could form via co-condensation in the atmosphere, or if acetylene dissolved in liquid methane or ethane flowed over an ammonia-rich surface deposit during a fluvial/pluvial event. Finally, the acetonitrile-acetylene co-crystal may also be plentiful on Titan, in equatorial regions where butane has been washed away by fluvial/pluvial processes. Intermolecular interactions of molecular minerals such as these might change dissolution and reprecipitation kinetics and equilibria, and therefore could affect the chemical erosion, transport, and deposition of molecules in terrain on Titan's surface. Many of these materials have order/disorder transitions that can lead to significant changes in volume, which might lead to unique geological features. Differences in physical or mechanical properties may also lead to chemical gradients, which life could potentially exploit. The catalytic hydrogenation of acetylene, for example, has been cited as a possible energy-yielding reaction for metabolizing microbes. Co-crystal formation might be a possible mechanism for storing acetylene, similar to the way carbon dioxide is stored in carbonate deposits on Earth, preserving rich deposits of this resource in the Titan subsurface where it might be more readily accessible to a putative microbial community. Future work will involve further characterization of these co-crystalline materials, as well as the search for new molecular minerals.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.485C