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Phase Behaviors of Pluto’s Volatiles

Presentation #318.01 in the session Outer Solar System Ices (iPosters).

Published onOct 20, 2022
Phase Behaviors of Pluto’s Volatiles

Nitrogen (N2), methane (CH4), and carbon monoxide (CO) are the most abundant volatiles on Pluto, Triton, Eris, and Makemake. Pluto’s Sputnik Planitia, for example, is a giant reservoir of these volatiles. It is important to understand the phase behaviors of mixtures, as they directly affect how a surface evolves and/or interacts with the atmosphere. This study aims to quantify and map the phase diagrams of this ternary chemical system as a function of temperature, pressure, and composition using two experimental methods run in the Astrophysical Materials Laboratory at NAU, starting with multiphase experiments for the three constituting binary systems (N2 + CO, CO + CH4, N2 + CH4). Notably, there are gaps in planetary materials and ice databases, particularly in phases of ices and compositions of mixtures. The first method cools a single composition slowly, using Raman spectroscopy to detect where phase changes occur within the system. This method (the “isoplethic method”) allows us to obtain temperatures of the phase boundaries for each composition. We then plot the phase-change locations on the corresponding binary phase diagram. Collecting data at multiple compositions is time consuming and expensive, so we couple this method with a second method that alternately applies gas injections with cooling along the three-phase vapor-liquid-solid equilibrium curve on the temperature-pressure phase diagram. This method is advantageous because it allows for more efficient collection of data points at multiple compositions along the three-phase equilibrium curve relative to the isoplethic method. We have performed the second method for all three binary systems, and the results are recently published (doi: 10.1063/5.0097465). These laboratory methods are being compared and their results are modeled using a thermodynamic equation of state, CRYOCHEM, 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. We fine-tune the solid-phase binary interaction parameters in this model with the laboratory data, and the model will ultimately provide the compositions and densities of the equilibrium phases for use by the community in various application models (e.g., geophysical models, global circulation models, and sublimation-condensation evolution models).

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