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The Fate of Simple Organics on Titan’s Surface

Presentation #216.01 in the session Titan III: Surface and Interior (Oral Presentation)

Published onOct 23, 2023
The Fate of Simple Organics on Titan’s Surface

Titan’s atmospheric photochemistry continuously converts methane and nitrogen into simpler organic gas-phase molecules. As these molecules descend toward Titan’s surface, many of them can condense into liquids or ices. In this work, we investigate the fate of these organic molecules when they land on the “dry” and “wet” (hydrocarbon lakes/seas-dominated) areas of Titan’s surface. Utilizing updated thermophysical properties that govern the phase change of these molecules, we found that most organic molecules, including all nitriles, triple-bonded hydrocarbons, and benzene, would land on Titan’s surface as solids. Conversely, methane, ethane, propane, and propene would be in liquid form, while ethylene remains gaseous on Titan’s surface. For the molecules that arrive as solids, we investigated their interactions with lake liquids on Titan’s ‘wet’ surface and determined their ultimate fates. We found that simple organic ices might float on Titan’s lake: 1) if they either have sufficient pore space (25-60% of porosity, depending on the ice composition) to balance buoyancy against gravity or 2) if they exhibit a non-zero surface tension-induced capillary force. The latter, however, is only possible for hydrogen cyanide or acetylene ices on ethane-rich lake liquids. Otherwise, the ices would sink into the lake liquids, forming lakebed sediments. Pore space-induced flotation seems a more plausible explanation for the transient ‘magic islands’ phenomena on Titan’s lakes. This is because such flotation would lead to the eventual sinking of the ices once all pore spaces are filled with lake liquids, whereas flotation induced by capillary force would more likely persist. Furthermore, if the ice behaves similarly to pumice rafts on Earth, they might be able to float on Titan’s lake liquids for prolonged periods before sinking. This behavior could explain why transient features can last weeks to months on Titan’s lakes.

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