Investigating the Condensation of Benzene (C 6 H 6 ) in Titan’s South Polar Cloud System with a Combination of Laboratory, Observational, and Modeling Tools

• Published in: The Planetary Science Journal
• Main Authors: Dubois, David, Iraci, Laura, T, Barth, Erika, L, Salama, Farid, Vinatier, Sandrine, Sciamma-O’brien, Ella
• Other Authors: NASA Ames Research Center (ARC), Southwest Research Institute, CO, USA, Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics (LESIA), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2021
• Subjects: [PHYS]Physics [physics]; [SDU]Sciences of the Universe [physics]; South Pole; South pole
• Online Access: https://hal.science/hal-03448629; https://hal.science/hal-03448629/document; https://hal.science/hal-03448629/file/Dubois_2021_vap_saturation%20%281%29.pdf; https://doi.org/10.3847/PSJ/ac06d5

International audience We have combined laboratory, modeling, and observational efforts to investigate the chemical and microphysical processes leading to the formation of the cloud system that formed at an unusually high altitude (>250 km) over Titan's south pole after the northern spring equinox. We present here a study focused on the formation of C6H6 ice clouds at 87°S. As the first step of our synergistic approach, we have measured, for the first time, the equilibrium vapor pressure of pure crystalline C6H6 at low temperatures (134-158 K) representative of Titan's atmosphere. Our laboratory data indicate that the experimental vapor pressure values are larger than those predicted by extrapolations found in the literature calculated from higher-temperature laboratory measurements. We have used our experimental results along with temperature profiles and C 6 H 6 mixing ratios derived from observational data acquired by the Cassini Composite Infrared Spectrometer (CIRS) as input parameters in the coupled microphysics radiative transfer Community Aerosol and Radiation Model for Atmospheres (CARMA). CARMA simulations constrained by these input parameters were conducted to derive C6H6 ice particle size distribution, gas volume mixing ratios, gas relative humidity, and cloud altitudes. The impact of the vapor pressure on the CIRS data analysis and in the CARMA simulations was investigated and resulted in both cases in benzene condensation occurring at lower altitude in the stratosphere than previously thought. In addition, the stratospheric C 6 H 6 gas abundances predicted with the new saturation relationship are ∼1000× higher than previous calculations between 150-200 km, which results in larger particle sizes.