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Path to the N₂+CO+CH₄ Equation of State: Results from Laboratory Experiments and Theoretical Modeling

Presentation #307.05 in the session “Centaur/KBO/Pluto Gathertown”. Cross-listed as presentation #114.03.

Published onOct 03, 2021
Path to the N₂+CO+CH₄ Equation of State: Results from Laboratory Experiments and Theoretical Modeling

Nitrogen, methane, and carbon monoxide are the most abundant substances on Pluto, Triton, Eris and Makemake. Sputnik Planitia, for example, is a giant reservoir of this mixture. Understanding this ternary phase diagram is a crucial step to understanding these outer solar system planetary bodies. This study uses experiments run in the Astrophysical Materials Laboratory at Northern Arizona University to map out this ternary system, by first starting with multiphase experiments for the three constituting binary systems (N2 + CO, CO + CH4, N2 + CH4). Notably, there are no experimental data that involve solid phases of CO + CH4 in the literature. We investigate these by using Raman spectroscopy to monitor where phase changes occur in the binary systems. In Raman, a photon is absorbed and then another photon is emitted. The Raman shift is then the difference in wavelength between those two photons, usually corresponding to a vibrational energy level. This spectroscopy method allows us to effectively monitor phase changes. The laboratory studies are being compared to a pair of distinct thermodynamic models. Computational molecular dynamics simulations are used to study the thermodynamic properties of each binary system considered. We use these simulations to predict densities, from which we quantify real solution effects, and analyze the specific molecular interactions causing the observations with pair distribution functions. The second model uses CRYOCHEM equation of state, which is based on the Thermodynamic Perturbation Theory (TPT) by coupling the Perturbed-Chain Statistical Associating Fluid Theory (PCSAFT) for the fluid part with the Lennard-Jones Weeks-Chandler-Andersen approach for the solid part. The solid-phase binary interaction parameters in this model are being fine-tuned by the laboratory data, and the model will ultimately provide the compositions and densities of the equilibrium phases for use in geophysical models.

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