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Adding Propane to the Methane–Ethane–Nitrogen System at Titan-like Conditions

Presentation #509.02 in the session Titan Craters, Chemistry and Exploration.

Published onOct 20, 2022
Adding Propane to the Methane–Ethane–Nitrogen System at Titan-like Conditions

Saturn’s largest moon, Titan, is shrouded in a thick nitrogen-rich (N2) atmosphere that supports ongoing methane (CH4) photochemistry, with two of the principal products being ethane (C2H6) and propane (C3H8). Methane, ethane, and dissolved atmospheric nitrogen make up the bulk composition of the polar lakes, and it is suspected that a trace amount of propane resides in them as well. It is likely that even small quantities of C3H8 affects phase behavior, physical properties, and the N2 content of the CH4–C2H6–N2 system, producing phenomena that have not yet been experimentally mapped.

The Astrophysical Materials Lab Group at Northern Arizona University has initiated a study to investigate such behaviors within the CH4–C2H6–C3H8–N2 system. The results showcase the difference between the ternary and quaternary systems and may provide a deeper understanding in the coupled chemical-dynamical behavior of Titan’s lakes.

The work leading up to this study involved recording the changes in freezing point temperatures of the CH4–C2H6 system when N2 is added, as well as the temperature at which a second liquid forms. The present work with C3H8 follows the same process as the CH4–C2H6–N2 experiments in which the alkane mixture is released into the cell as a liquid at 95 K and is then followed by the introduction of gaseous N2 into the system. The N2 is used to maintain a constant vapor pressure of 1.5 bar, simulating both Titan’s surface pressure and its N2-rich atmosphere. The temperature in the cell is incrementally lowered until the first ice is witnessed. N2 dissolution increases as temperature decreases and so more of it must be added as the experiment progresses to maintain constant vapor pressure. So, while the total concentration of the sample changes throughout the experiment, the relative alkane ratio remains the same. Using this process, the freezing point temperatures, the changes in N2 concentration, and the condition for the appearance of the two liquids are recorded and then compared to the ternary results. The comparisons can then demonstrate the impact of C3H8 on the CH4–C2H6–N2 system.

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