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The Effects of Propane on the Liquids of Titan

Presentation #408.06 in the session “Titan Beach Party”.

Published onOct 26, 2020
The Effects of Propane on the Liquids of Titan

The lakes and seas of Titan are composed primarily of methane (CH4) and ethane (C2H6), with the concentration of dissolved nitrogen (N2) depending on the ratio of methane to ethane, the temperature, and pressure. Propane (C3H8) is formed photochemically in the upper atmosphere of Titan, and condenses at the tropopause. The freezing point of pure propane is 85.5 K, meaning that it would be liquid on the surface of Titan, like methane and ethane. We have begun an exploration of the effect of propane on methane, ethane, nitrogen, and their mixtures in the NAU Astrophysical Materials Laboratory. Cryogenic samples are studied via Raman spectroscopy and photography. As nitrogen was added to a binary hydrocarbon mixture of either propane-methane or propane-ethane, it caused the formation of a second liquid. The droplets form at the meniscus and this nitrogen-rich denser liquid falls once enough material has collected to break surface tension. Ice can form under certain conditions. Differences in behavior of the propane-ethane system and the propane-methane system can be attributed to the difference in nitrogen solubility. For further analysis, the phase diagrams at conditions where the second liquids were observed were calculated using CRYOCHEM. We also modeled a homogeneous N2:CH4:C2H6:C3H8 liquid system to understand the breakdown of ideality. In these simulations, real effects are quantified by calculating the binding free energy between each pair of molecules. We found that increasing alkane length results in a decrease in binding strength between N2 and each of the alkanes, which suggests a molecular explanation for the phase behavior observed in the experiments of these systems at lower temperatures. In summary, pure propane should not freeze on the surface of Titan. However, we see propane ice form under certain conditions that might be possible on Titan. We also see that the liquid-liquid system can form with the addition of propane. We continue to explore the effects of propane on methane, ethane and nitrogen, both individually and additively, and constrain the conditions under which interesting phenomena occur.

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