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Phase Diagram Mapping of the N₂+CH₄+CO System to Better Understand Pluto’s Glaciers

Presentation #308.07 in the session Pluto System (Oral Presentation)

Published onOct 23, 2023
Phase Diagram Mapping of the N₂+CH₄+CO System to Better Understand Pluto’s Glaciers

N2, CH4 and CO are highly abundant volatiles at places such as Pluto, Triton, Eris and Makemake. Pluto’s Sputnik Planitia, for example, is a giant glacial reservoir of this ternary mixture. It is mobile species, like these three, that shape the geology of a body, making it important to study how they interact with one another. However, there are gaps in the literature of fully mapped phase diagrams for these materials, which limits our knowledge of these interactions. This affects models of outer solar system bodies because they must assume a simpler compositional makeup for a given region of interest. For instance, Sputnik Planitia is often assumed to be pure N2, even though the addition of CH4 and CO affects glacial behaviors, such as what phases are present under different surface conditions. This work aims to address these unknowns by using laboratory methods and thermodynamic modeling to create an accurate equation of state (EOS) for the ternary system. This EOS will describe what phases are present for any temperature, pressure, and composition point. In the Astrophysical Materials Laboratory at NAU, two laboratory methods are being implemented to map the three binary phase diagrams within the ternary system. The first method maps the three-phase solid-liquid-vapor curves of N2+CO, N2+CH4, and CO+CH4. This method is complete, and results are published (doi: 10.1063/5.0097465). The second method, which is underway, involves cooling a single composition, locating its phase boundaries using Raman spectroscopy, then later adjusting for supercooling. The phase boundary points are added to plots of temperature vs. composition for each binary system. This method is nearly complete for N2+CO, which will update the current curves in the literature, which are rough estimates, and do not account for supercooling. Work has also begun with the N2+CH4 system, which will be crucial to validate, since many models rely on the phase diagram derived from X-ray diffraction (Prokhvatilov & Yantsevich, 1983). Finally, the CO+CH4 temperature-composition phase diagram will be experimentally mapped for the first time. The laboratory results will fine-tune a thermodynamic model, CRYOCHEM, to create the EOS. CRYOCHEM is based on the Thermodynamic Perturbation Theory and couples the Perturbed-Chain Statistical Associating Fluid Theory for the fluid component with the Lennard-Jones Weeks-Chandler-Andersen approach for the solid part. The derived EOS will provide compositions and densities of the equilibrium phases for use by the community in geophysical, global circulation, and sublimation-condensation evolution models.

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