Delivery of Ices to Titan’s Surface within Methane Raindrops

Photochemical processes in Titan’s upper atmosphere produce a number of hydrocarbon and nitrile gases which reach their condensation temperatures in Titan’s stratosphere. These ices form around the organic haze particles which give Titan its characteristic orange color. The microphysics of these ice particles was modeled using the Titan mode of PlanetCARMA (based on the Community Aerosol & Radiation Model for Atmospheres). CARMA models the vertical transport, coagulation, nucleation, condensation, and evaporation of particles in a column of atmosphere. Ice composition includes hydrogen cyanide (HCN), benzene (C₆H₆), diacetylene (C₄H₂), propane (C₃H₈), acetylene (C₂H₂), and ethane (C₂H₆). CARMA tracks the mass of each ice on the atmospheric particles and calculates a flux of material across the tropopause.

Once in the troposphere, these particles can become seed nuclei for the methane clouds seen from groundbased and Cassini observations. Methane is the only condensable gas abundant enough in Titan’s atmosphere to grow cloud particles to raindrop sizes, which then fall to the surface transporting any haze and ice mass within. The Titan Regional Atmospheric Modeling System (TRAMS) is a fully dynamic, compressible, regional-scale numerical model of Titan’s atmosphere. Coupled to CARMA, TRAMS is used to explore the microphysics and dynamics of Titan’s methane storms. We will report on results from TRAMS simulations of methane clouds and storms to quantify the mass and composition of ices deposited at Titan’s surface and implications for local changes in Titan’s surface albedo.