Benzene (C6H6) in Titan’s South Pole: from gas phase to ice nucleation

Abstract:

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 high altitude C₆H₆ ice in the South pole ^[1]. Current laboratory data is insufficient to allow models to reproduce the formation of this high-altitude cloud system.

Here, we combine laboratory ^[2,3], modeling ^[4] and observational ^[1] studies from Earth and Planetary Sciences to investigate the chemical and microphysical processes leading to the formation of this cloud system. We present this synergistic work by reporting on the first measurements of the equilibrium vapor pressure of C₆H₆. These measurements were carried out at Titan-like temperatures (135-165 K). Our data indicates that the experimental vapor pressure values fall higher than the most often used extrapolation ^[5] and are closer to earlier ^[6] extrapolations. We have conducted a comparative study using both of these vapor pressure extrapolations, along with temperature profiles and C₆H₆ mixing ratios derived from CIRS data, as input parameters in the coupled microphysics radiative transfer CARMA (Community Aerosol and Radiation Model for Atmospheres) model to constrain nucleation and condensation (cloud altitude, particle sizes and gas relative humidity). Furthermore, we are investigating the role Titan’s organic aerosols could play in the condensation of C₆H₆ ice cloud particles. Analogs of Titan’s aerosols (tholins) have been produced using the Titan Haze Simulation experiment developed on the NASA Ames COSmIC facility ^[3] and are used as solid organic substrates for C₆H₆ ice nucleation experiments. The conditions required for C₆H₆ condensation onto tholins will be discussed.

References:

[1] Vinatier, S. et al., 2018.

[2] Iraci, L. T. et al., 2010.

[3] Sciamma-O’Brien, E. et al., 2017.

[4] Barth, E. L., 2017.

[5] Fray, N. & Schmitt, B., 2009.

[6] Jackowski, A. W., 1974.