Search

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]

Doubly-substituted methane isotopologues (13CH3D and 12CH2D2) can provide unique information to resolve the origin, transport, and conversion of environmental methane. We used a high-resolution mass spectrometer (Thermo Fisher Scientific 253 Ultra) to precisely determine Δ13CH3D and Δ12CH2D2 (precision and accuracy of ∼0.35‰ and ∼1.35‰ (1σ), respectively) in this study. The impacts of temperature, humidity, and slit quality are found to be critical for the reproducibility of the results, while the fragmentation rate and bellows effects on the clumped isotope results were negligible. A heating equilibrium experiment performed at 500 °C, 400 °C and 300 °C (n = 19) calibrates the results to an absolute reference. A pretreatment system that can extract methane having concentrations from ∼15% to ∼99.8% is proposed, while the fractionation produced throughout the pretreatment is negligible within precision (Δ13CH3D = +0.16 ± 0.32‰ and Δ12CH2D2 = +0.14 ± 1.12‰ (n = 16)). Analyses of natural gases from shale basins and variable methane-rich gases emitted from artificial lakes validate the capability of the current pretreatment and spectrometer systems to determine methane clumped isotopes in a wide range of natural environmental samples. [ABSTRACT FROM AUTHOR]