Microphysical modeling of mixed composition ices in Titan’s stratosphere

Titan’s atmosphere includes many trace hydrocarbon and nitrile species that reach their condensation temperatures in the stratosphere. These, for the most part, will condense out as ices given sufficient condensation nuclei, which are provided by the organic haze particles. Barth (2017) explored the physics behind the size and abundance of pure ice particles that could be present in Titan’s atmosphere and found they would condense out in layers between about 80 and 60 km given thermal conditions at the Huygens landing site (Fig. 1).

We now expand that study to multiple latitudes and include mixtures of trace species in the ice particles. Anderson et al. (2018) have shown that in Titan’s stratosphere, where many of the trace gases are saturated at the same altitude, they are likely co-condensing onto the haze particles. This changes the optical properties of the particles, but microphysically the formation process is similar to modeling the pure ices, as long as the vapor pressures are adjusted for the mixture.

Modeling is done using the Community and Aerosol Radiation Model for Atmospheres (CARMA; Barth 2020). CARMA models the physics of vertical transport and coagulation in a column of atmosphere and the interaction of particles and gases through nucleation, condensation, and evaporation. The growth routines have been modified to include particles composed of multiple volatiles, with each volatile component growing or evaporating in response to the environment (Barth & Toon, 2006). Particles are represented by a number of discrete mass bins, such that the size distribution of ice particles can be explored at all altitudes in the column. The model keeps track of the changes with time of the number of particles (including core mass for clouds) and mass density of volatiles.

This work is supported by NASA CDAP 80NSSC20K0485.

References: Anderson, C. M., R. E. Samuelson and D. Nna-Mvondo 2018. Organic ices in Titan’s stratosphere. Space Sci. Rev. 214, 125; Barth, E.L. 2020. PlanetCARMA: A New Framework for Studying Planetary Atmospheres. Atmosphere, 11(10), 1064; Barth, E. L. 2017. Modeling survey of ices in Titan’s stratosphere. Planet. Space Sci. 137, 20–31; Barth, E. L., and O. B. Toon 2006. Methane, ethane, and mixed clouds in Titan’s atmosphere: Properties derived from microphysical modeling. Icarus 182, 230–250.