Peeking beneath Titan’s haze: molecular simulations unraveling the mysteries of Titan’s surface materials and liquids
Creators: name: Thakur, Atul Chudaman
Publisher: name: No Publisher Supplied
Year: 2024

description: Titan, Saturn's largest moon, is a world of extraordinary astrobiological potential, characterized by its dense atmosphere, stable surface liquids, and seasonal weather cycles. With surface conditions that are cryogenic yet chemically rich, Titan hosts a complex interplay of organic molecules that could parallel early Earth's prebiotic chemistry. This unique environment, coupled with evidence of dynamic geological processes such as cryovolcanism and fluvial erosion, positions Titan as one of the most intriguing targets for exploring the possibility of life beyond Earth. However, a microscopic understanding of Titan's surface materials is missing, which is crucial for interpreting the complex interactions and processes occurring on its surface. This thesis aims to establish a foundational understanding of Titan's surface materials, particularly its cryominerals and hydrocarbon liquids, through state-of-the-art molecular simulations. We begin by focusing on the acetylene:ammonia (1:1) co-crystal, a representative Titan cryomineral. Our simulations reveal that at 90~K, the co-crystal exists in a dynamically disordered plastic phase, with ammonia molecules exhibiting rapid orientational disorder. We further demonstrate that under Titan’s cryogenic conditions, nuclear quantum effects can significantly accelerate this disorder by weakening intermolecular interactions. The implications of this phase transition on the mechanical properties of the co-crystal are explored, showing that dynamic disorder leads to anisotropic softening of the material's elastic constants. Additionally, we observe similar dynamic disorder in the acetonitrile (1:2) co-crystal, indicating that such disordered cryominerals are likely common on Titan’s surface. Along with the Titan's solids, this thesis also introduces dielectric corrected (DC) models for simulating liquid hydrocarbons, specifically methane and ethane, which are the primary components of Titan's lakes. These models incorporate fixed dipole moments to accurately represent the dielectric properties of the liquids, enabling the study of solvation phenomena that are crucial for understanding non-aqueous chemistry on Titan. The DC models are shown to provide a realistic description of solvation free energies for polar and charged species, thereby offering new insights into potential chemical processes in Titan’s unique environment. Throughout the thesis, local neural network interatomic potentials are employed to study Titan materials. In later sections, we investigate the limitations of these local models, particularly the consequences of neglecting long-range interactions. We examine the role of long-range interactions in orientational relaxations, solvation dynamics, and dielectric responses of polar liquids. Our findings indicate that while local models perform exceptionally well in predicting the structure and dynamics of bulk liquids, explicit long-range electrostatic interactions become critical at interfaces. Finally, we discuss the broader implications and future extensions of our work for understanding the surface and subsurface processes on Titan, as well as the potential for prebiotic chemistry. The insights gained from this thesis lay the groundwork for future investigations into the chemical, mechanical and thermodynamic aspects of Titan's materials, offering valuable predictions for upcoming missions such as NASA's Dragonfly rotorcraft, set to explore Titan in the 2030s.