A cross-laboratory comparison study of Titan haze analogs and a material property database of organic liquids, ices, and hazes on Titan

In Titan’s N₂-CH₄-rich atmosphere, photochemistry leads to the formation of many simple gas-phase organic molecules. During their journey through Titan’s atmosphere, these simple organics can condense to form organic liquids or ices. These organic molecules can also coagulate and react to form complex refractory organic particles that constitute Titan’s thick haze layers. Overall, Titan has a diverse range of materials in its atmosphere and on its surface: the simple organics that reside in various phases and the solid complex refractory organics. All these materials actively participate in various physical processes in Titan’s atmosphere and on Titan’s surface. Future in-situ exploration on Titan would likely encounter a range of materials, and an updatable material property database that includes the diverse properties of possible material candidates will be needed.

For complex organics, laboratory-made aerosol analogs, “tholins”, are often used as the model material. Many different laboratories have synthesized tholins since the 1970s and have characterized them to different extents. However, we are still uncertain whether or which tholins are representative of the actual haze particles on Titan. We believe the best way to proceed is to conduct uniform measurements of tholins produced in different laboratories and provide a range of property values for the community. In our first cross-laboratory comparison study, we measured an important surface property for seven tholin samples from three independent laboratory facilities, the surface energy. We found that tholins have varying surface energies based on their experimental production conditions, which resulted in a range of surface energy values (47-81.7 mJ/m²) [1].

For simple organics, their material properties are scattered in the literature over a century, making it difficult to utilize these data. In our forthcoming paper and web-portal, we compiled and computed several material properties of organic liquids, ices, and haze analogs on Titan, including thermodynamic properties (phase change point, sublimation and vaporization saturation vapor pressure, and latent heat), physical property (density), and surface properties (liquid surface tension and solid surface energy) [2].

[1] Li, J., Yu, X., Sciamma-O’Brien, E., et al. (2022). A Cross-laboratory Comparison Study of Titan Haze Analogs: Surface Energy. PSJ, 3(1), 2.

[2] Yu, X., Yu, Y., Li, J., et al. in prep.