Investigating the condensation of benzene (C₆H₆) in Titan’s South polar cloud system

Titan has a 29-year seasonal cycle as it orbits Saturn. Following the northern spring equinox in August 2009, Titan’s global atmospheric circulation reversed within the next two years. This event increased the mixing ratios of benzene (C₆H₆) and other species at the South pole. Simultaneously, a strong cooling with temperatures dropping below 120 K favored the condensation of organic hydrocarbon molecules at unusually high altitudes (>250 km). The Cassini Composite Infrared Spectrometer (CIRS) detected for the first time an IR spectral signature consistent with the presence of C₆H₆ ice in the South polar region at these high altitudes^[1]. Current laboratory data, however, is insufficient to allow models to reproduce the formation of this high-altitude cloud system. Here, we combine a synergistic laboratory^[2], modeling^[3] and observational^[1] effort to investigate the chemical and microphysical processes leading to the formation of C₆H₆ ice clouds at 87° S. We report on the first measurements of the equilibrium vapor pressure of C₆H₆ at low temperatures, representative of Titan’s atmosphere, between 135-165 K. Our laboratory data indicates that the experimental vapor pressure values fall higher than the most often used extrapolation by Fray & Schmitt (2009)^[4], and are closer to the Jackowski (1974) extrapolation ^[5]. We have used both Jackowski and Fray & Schmitt extrapolations of vapor pressure, along with temperature profiles and C₆H₆ mixing ratios obtained from CIRS data, as input parameters in the coupled microphysics radiative transfer CARMA (Community Aerosol and Radiation Model for Atmospheres) model. C₆H₆ ice particle distribution and gas volume mixing ratios were derived to constrain nucleation and condensation (cloud altitudes, particle sizes and gas relative humidity). The impact of the vapor pressure was investigated. Using the Jackowski extrapolation does not bring the C₆H₆ cloud top to higher altitudes than with the Fray & Schmitt extrapolation, but leads to an increase in the altitude-dependent C₆H₆ gas volume mixing ratio and C₆H₆ ice particle size distribution up to 250 km.