Material Properties of Organic Liquids, Ices, and Hazes on Titan

Abstract

Titan has a diverse range of materials in its atmosphere and on its surface: the simple organics that reside in various phases (gas, liquid, and ice) and the solid complex refractory organics that form Titan’s haze layers. These materials all actively participate in various physical processes on Titan, and many material properties are found to be important in shaping these processes. Future in situ explorations on Titan would likely encounter a range of materials, and a comprehensive database to archive the material properties of all possible material candidates will be needed. Here, we summarize several important material properties of the organic liquids, ices, and the refractory hazes on Titan that are available in the literature and/or that we have computed. These properties include thermodynamic properties (phase-change points, sublimation and vaporization saturation vapor pressure, and latent heat), and physical properties (organic liquid densities and organic ice and haze densities). We have developed a new database to provide a repository for these data and make them available to the science community. These data can be used as inputs for various theoretical models to interpret current and future remote sensing and in situ atmospheric and surface measurements on Titan. The material properties of the simple organics may also be applicable to giant planets and icy bodies in the outer solar system, interstellar medium, protoplanetary disks, and exoplanets.

The Fate of Simple Organics on Titan's Surface: A Theoretical Perspective

“…Our simple estimation aligns with the findings of photochemical models incorporating condensation schemes, for HCN in Lavvas et al (2011), for the nitrile species in Krasnopolsky (2014), Willacy et al (2016), and all the major hydrocarbon and nitrile species in Vuitton et al (2019). Future work that incorporates more updated SVPs from Yu et al (2023a) into a sophisticated photochemical model could facilitate the estimation of the deposition rate of each condensate on Titan's surface.…”

“…In our theoretical examination of the fate of simple organics on Titan's surface, guided by the data from Yu et al (2023a), we draw the following conclusions:…”

“…Using phase change points and saturation vapor pressures (SVPs) of detected gas-phase simple organics species in an updated Titan material property database (Yu et al, 2023a), we determine the physical phase of 17 hydrocarbon and nitrile species detected in Titan's atmosphere. These species include methane (CH 4 ), ethane (C 2 H 6 ), ethylene (C 2 H 4 ), acetylene (C 2 H 2 ), propane (C 3 H 8 ), propene (C 3 H 6 ), allene (C 3 H 4 -a), propyne (C 3 H 4 -p), diacetylene (C 4 H 2 ), benzene (C 6 H 6 ), hydrogen cyanide (HCN), cyanoacetylene (HC 3 N), acetonitrile (CH 3 CN), propionitrile (C 2 H 5 CN), acrylonitrile (C 2 H 3 CN), cyanogen (C 2 N 2 ), and dicyanoacetylene (C 4 N 2 ).…”

“…Existing laboratory data on crystalline ices indicate porosity values from a few percent to ∼35% (Satorre et Yu et al, 2023a). Amorphous ices can exhibit higher levels of porosity, up to 40%-50% (Yu et al, 2023a). However, the surface temperature of Titan is too warm for amorphous ice formation.…”

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Titan's thick methane (CH 4 ) and nitrogen (N 2 ) atmosphere has enabled rich photochemistry to occur in its upper atmosphere. Photochemistry leads to the creation of a myriad of organic molecules, with at least 17 gas-phase simple organic species already identified in Titan's atmosphere (Yu et al, 2023a). The increasing complexity of the photochemical reactions eventually leads to the formation of complex refractory organic particles, which constitute Titan's characteristic haze layers (Tomasko et al, 2005).

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