A Cross-Laboratory Comparison Study of Titan Haze Analogs

The methane and nitrogen in Titan’s upper atmosphere enable rich photochemistry, creating numerous organic compounds in Titan’s atmosphere. These simple organic molecules in Titan’s atmosphere can further react, polymerize, and coagulate to form the complex refractory organic particles that constitute Titan’s thick haze layers. These complex organics actively participate in various processes such as cloud formation, lake interaction, and sediment transport on Titan. Their unique material properties are found to play an important role in shaping these processes and therefore represent critical input parameters for modeling and preparing for the future in-situ explorations of Titan (i.e., the Dragonfly mission).

Since no sample returned from Titan is available, the data on their properties rely upon the laboratory-made organic aerosol analogs (“tholins”). Many laboratory facilities have been synthesizing tholins since the 1970s. However, modern research studies often adopt the properties of tholins from one particular laboratory. It is an open question whether/which tholins are representative of the actual complex organics on Titan. To approach this question, we think the best way to proceed would be to compare material properties of tholins produced in different laboratory facilities to existing/future data on the complex organics on Titan. To minimize the effect of measuring techniques/conditions on the measured sample properties, we used a standardized approach and measured tholin samples produced by three independent facilities (Johns Hopkins University/PHAZER, NASA Ames Research Center/COSmIC, and University of Northern Iowa/PAC). All samples are made with gas mixtures ranging from 1-5% CH4 and 95-99% N2. After all the samples are made and deposited on the same substrate provided to each facility, they are shipped in custom-built vacuum vessels and then measured in an inert atmosphere to avoid sample contamination/oxidation. We have characterized a range of important material properties of these samples, including surface energy, elastic modulus, hardness, and optical constants from the UV to the mid-infrared (190 nm to 30 µm). While these tholin samples exhibit a certain degree of variation in their material properties, they also share common characteristics, bolstering our confidence in using these properties to understand complex organics on Titan and inform future in-situ exploration missions. This comparison study also paves the way for future collaborative laboratory research to enhance our understanding of hazes on other outer solar system planets and exoplanets.