Experimental Determination of the Solubility of Titan's Aromatic Compounds and Nitriles in Liquid Methane

Abstract:

As confirmed by the Cassini-Huygens mission, Titan, the largest moon of Saturn, is characterized by an active hydrological cycle where light hydrocarbons can precipitate forming pluvial valley, channels and lakes. Titan’s hydrologic cycle is much more complex than the terrestrial one, since there might be multiple materials that compose the fraction of the surface liquids on Titan, among which nitrogen, methane, ethane, and propane. Nitrogen and methane are the dominant components of Titan’s atmosphere. Under Titan’s temperatures regime, methane could cycle between hemispheres on a seasonal timescale while ethane does the same at epochal timescale.

A plethora of organic compounds have been also detected in the atmosphere of Titan, including, among others, benzene, acrylonitrile and cyanoacetylene. These compounds are probably photogenerated in the atmosphere and subsequently precipitate on the surface of the moon. Further chemical evolution of these organic compounds strongly depends on their solubility in Titan’s liquid surfaces. Interestingly, the formation of evaporite deposits seems to take place in dry lake beds after volatile removal. Alternatively, materials with even lower solubility, may remain as a lag deposit after more soluble materials have dissolved and washed away. For this reason, it is of great interest to characterize the solubility of common organic compounds in liquid methane and ethane. Because of the characteristics of their phase transitions, the data on the solubility in liquid methane or ethane are very scarce in the literature.

To fill this gap, we have implemented a laboratory approach to determine the solubility of organic compounds in a simulated liquid Titan environment. Differently from previous work, we make use of a very small quantity of liquid CH₄ by using a PTFE meso-reactor that can be easily cooled down. As a proof-of-concept, preliminary data have been collected by empirically screening the solubility of several organic compounds including benzene, toluene, benzonitrile, acrylonitrile and cyanoacetylene, at three different temperatures ranging from 143 K to 90 K.

The Authors wish to thank the Italian Space Agency for co-funding the Life in Space project (ASI N. 2019-3-U.0)