Cassini CIRS observations in May 2013 revealed the presence of a benzene (C6H6) ice cloud in Titan’s south pole stratosphere. Previous analyses of the CIRS data put the cloud top near 280 km, and placed an upper limit of ~1.5 μm for the equivalent radius of pure C6H6 ice particles . Using microphysical modeling we have investigated the size and number of C6H6 cloud particles as a function of altitude in the southern polar atmosphere. Simulations were initialized with CIRS data, and using new lab measurements of C6H6 vapor pressures we measured at Titan-relevant temperatures resulted in more accurate solutions to the equations governing cloud formation and growth.
The Community Aerosol and Radiation Model for Atmospheres (CARMA) simulates the microphysical evolution of aerosol particles in a column of atmosphere. Cloud particle formation and growth is controlled by the vapor pressure of C6H6. Using the NASA Ames Atmospheric Chemistry Laboratory (ACL), we performed new experiments to monitor the condensation of C6H6 in the IR and determined, for the first time, the equilibrium vapor pressure of pure C6H6 at Titan-relevant temperatures (134–158 K) . Our experimental measurements of the vapor pressure of C6H6 differ in slope and magnitude from the extrapolation of , previously used by the Titan community, indicating colder temperatures and higher pressures.
We initialized CARMA with a temperature/pressure profile and a new C6H6 mixing ratio derived from CIRS data at 87 S to model the southern polar atmosphere. Cloud particles are created through heterogeneous nucleation using haze particles as cloud condensation nuclei. We see cloud particles begin to form at altitudes comparable to that from the reanalysis of CIRS data using our new vapor pressure, with a vertical profile extending down through Titan’s tropopause. The population of cloud particles grows from an effective radius ~0.5 μm near the cloud top to ~1.5 μm in the troposphere. Using our new C6H6 vapor pressure in both the simulations and a reanalysis of CIRS data, results in a slightly lower cloud top altitude (~250 km) and larger particle sizes than analyses with .
Funding for this project is provided through NASA CDAP.
 Vinatier et al. 2018, Icarus, 310, 89-104.  Dubois et al. 2021, Planet. Sci. J. 2, 121.  Fray & Schmitt 2009, Planet. Space Sci., 57, 2053-2080.