Experimental Study of Evaporites on Titan

Titan’s lakes and seas are composed of predominantly methane and ethane [1], however thermodynamic modeling suggests that various other molecules exist in the lakes at lower abundances. Some of these molecules include: acetylene, ethylene, benzene, and acetonitrile e.g., [2-5]. As the lakes evaporate, they may leave behind “bathtub rings” that are bright in the Cassini VIMS 5 µm band [6]. Experimental work [5, 7-10] has improved our understanding of Titan’s evaporite composition and has opened up a new class of “hydrocarbon minerals” [11] to be studied.

Using a custom-built Titan chamber [12], we condense various combinations of liquid (methane and/or ethane) and solid (e.g., acetylene) hydrocarbons onto a dish (90 K) within the chamber where the compounds are constantly exposed to a 1.5 bar N₂ atmosphere. We use FTIR spectroscopy (Nicolet 6700, 1-2.5 µm), mass measurements, and cameras to characterize spectral absorptions, band shifts, solubility values, and the morphology of the sample. Here, we present a culmination of results from evaporite experiments associated with the dissertation research for this project.

Ethylene experiments [9] show that the evaporite forms more quickly in pure methane, the evaporite was identified by spectral band shifts, and our evaporation rates match those in the literature. Acetylene:benzene co-crystal experiments [10] show that the co-crystal forms within minutes at 135 K, is stable down to 90 K, and was identified by several new spectral bands and sample morphology changes. Acetonitrile experiments show that when combined with methane and allowed to evaporate, the residual spectrum is quite different from pure acetonitrile.

These results can be applied to co-condensation processes in Titan’s atmosphere, and the ongoing effort to better characterize the composition of Titan’s evaporites. Knowledge of these spectral and optical changes will be useful for future missions such as Dragonfly, which can closely examine molecular minerals like co-crystals on Titan’s surface.

This work was funded by the NESSF Grant #80NSSC17K0603.

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