Capability Development for a Titan Microphysics Model: Particle Shape and Cloud Composition, 15-R9711

Principal Investigator
Erika L. Barth

Inclusive Dates:  04/01/07 – 08/01/07

Background - Titan, Saturn's largest moon, is the most likely object in the Solar System to exhibit Earth-like weather. Titan's air is composed mostly of nitrogen, similar to the Earth's atmosphere, but the trace gases are mainly hydrocarbons – including the predominant volatile species, methane. Temperatures allow methane condensation, both as ice and liquid, and so methane may participate in a cycle similar to the Earth's hydrological cycle. Recent discoveries from NASA's Cassini/Huygens mission call for more sophisticated atmospheric models. The Huygens probe experiment has clearly demonstrated a need for changes in the shape of the haze particles such that they are fractal aggregates above 80 km and are best represented as spherical particles at lower altitudes. Additionally, the Huygens probe measurements of Titan's temperature and methane profiles show a surface too dry for stable methane droplets. Since ethane and methane are miscible, the lower vapor pressure of ethane could allow droplets composed of a mixture of methane, ethane and nitrogen to be stable at Titan's surface.

Approach - The goal of this project was to enhance the capabilities of an existing Titan cloud microphysics model by adding the ability to model Titan's cloud particles as mixtures of methane, ethane and nitrogen (Phase I), and adding the ability to model Titan's haze particles as fractal aggregates (Phase II). Titan-CARMA (Community Aerosol and Radiation Model for Atmospheres) is a column microphysics model that includes the physics for nucleation, condensation, evaporation, coagulation and sedimentation of methane, ethane and tholin (haze) particles in Titan's atmosphere. In Phase I, the cloud growth schemes were altered to allow for mixing of methane, ethane and nitrogen in the simulated cloud droplets. The code changes involved re-writing the vapor pressure routine to include a contribution from the mole fraction of each volatile and adding a new calculation for the freezing point temperature. Phase II involved adding the option to model the haze particles as fractal aggregates, including changes in the production, sedimentation and coagulation functions.

Accomplishments - The focus on the freezing point of cloud droplets led to another avenue of research that treats melting and freezing as a multi-step process whereby the droplet responds to the latent heat of the phase change (previous modeling had used a fixed rate for phase changes). For methane, evaporative cooling keeps the particle from completely melting before reaching the surface, indicating that methane hail can reach Titan's surface. This is a particularly interesting result in light of the Huygens landing site, where the humidity near the surface was too low to sustain raindrops, but moisture was detected on the ground. As ethane is much less volatile, it melts almost immediately when the ice particle falls to a temperature warmer than the freezing point. The haze particle work is ongoing, but currently a significant decrease in the number of haze particles in the stratosphere and troposphere is seen compared to the steady state results from the previous spherical haze model. A decrease in the number of tropospheric haze particles (from previous modeling) is consistent with Huygens probe data.

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