Multidisciplinary investigation of benzene (C₆H₆) condensation in Titan’s South Pole

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) at the South pole. Simultaneously, a strong cooling with temperatures dropping below 120 K favored the condensation of hydrocarbon molecules at unusually high altitudes (>250 km). Cassini detected for the first time an IR spectral signature consistent with the presence of C₆H₆ ice in the South Pole at these high altitudes. 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, modeling and observational effort to investigate the chemical and microphysical processes leading to the formation of C₆H₆ ice clouds. We report on the first measurements of the equilibrium vapor pressure of C₆H₆ <180 K. The impact of the vapor pressure on the CIRS data analysis and simulations was investigated and resulted in benzene condensation occurring at lower altitude in the stratosphere than previously thought. In addition, an increase in the altitude-dependent C₆H₆ gas volume mixing ratio is predicted which results in larger ice particle sizes.