Rapid Aggregation and Dissolution of Organic Aerosols in Liquid Methane on Titan.

Complex organic aerosols in the upper atmosphere of Saturn's moon Titan reach the troposphere and surface, where a methane (CH4)‐based hydrological cycle occur. Previous studies have assumed no interactions between organic aerosols and liquid CH4, although the dissolution of low‐molecular‐weight photochemical products in liquid CH4 has been considered. Here we report experimental results of soaking a laboratory analog (so‐called tholin) of Titan's organic aerosols in liquid CH4 at 93–98 K for several hours and then evaporating the liquid, simulating wet–dry cycling on Titan. After wet–dry cycling, residual tholin particles form aggregates through cementation. Solid evaporitic deposits formed by evaporation of interacted liquid contain nitrogen‐bearing aromatics, suggesting selective dissolution of aromatics. Our results suggest that organic aerosols or high‐molecular‐weight compounds adsorbed on them partly dissolve in liquid CH4 on Titan, even during short‐term wetting events, promoting the growth of aerosols to dune particles via aggregation and providing aromatics to evaporites. Plain Language Summary: Saturn's largest moon, Titan has a dense atmosphere composed of nitrogen and methane. Active organic chemistry induced by sunlight occurs in the atmosphere to form complex organic aerosols. These organic aerosols eventually settle on the surface, where rivers, lakes, and seas of liquid methane are present. Upon seasonal changes in precipitation and evaporation, wet–dry cycling of liquid methane is believed to occur on Titan, similar to Earth's water cycles. Can organic aerosols settled on the surface be dissolved into liquid methane? If so, what components of aerosol can be dissolved, and how could this affect Titan's geology? Here we perform laboratory experiments to address these questions. Unlike predictions by previous studies, we find that organic aerosols or aromatics adsorbed on them would partly dissolve into liquid methane efficiently. These processes could play key roles in the formation of unique landforms, such as dunes and evaporites, on Titan. Key Points: We experimentally simulate interactions between laboratory analogs (tholin) of Titan's aerosols and liquid methane at low temperaturesComplex aromatics are dissolved from tholin, forming evaporites after drying; while, residual tholin form aggregatesWet–dry cycling of aerosols on Titan may provide aromatics to evaporites and could explain size growth of aerosols to dune particles [ABSTRACT FROM AUTHOR]