Investigating the Condensation of Benzene (C6H6) in Titan’s South Polar Cloud System with a Combination of Laboratory, Observational, and Modeling Tools

David Dubois,Laura T Iraci,Farid Salama,Sandrine Vinatier,Ella Sciamma-O’Brien,Erika L Barth

doi:10.3847/psj/ac06d5

Abstract

Abstract 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 C6H6 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 C6H6 gas abundances predicted with the new saturation relationship are ∼1000× higher than previous calculations between 150–200 km, which results in larger particle sizes.

Highlights

In Titan’s dense atmosphere, complex hydrocarbon and nitrile species are produced from the dissociation of nitrogen (N2) and methane (CH4) and subsequent chemical reactions in the upper atmosphere, induced by solar UV radiation and electron bombardment from Saturn’s magnetosphere
Using the vapor pressure in Community Aerosol and Radiation Model for Atmospheres (CARMA) simulations allows the computation of the C6H6 gas mixing ratio as a function of altitude below ∼300 km, which cannot be done from Composite Infrared Spectrometer (CIRS) data
Using our new experimental vapor pressure parameterization in CARMA simulations has revealed unexpected effects on the nucleation and growth of C6H6 ice particles in the stratosphere, including a larger C6H6 gas vmr at stratospheric altitudes (Figure 7) below the condensation level, compared to those calculated from the F&S09 extrapolation, which has been used in the past for the interpretation of Titan observational data

Summary

Introduction

In Titan’s dense atmosphere, complex hydrocarbon and nitrile species are produced from the dissociation of nitrogen (N2) and methane (CH4) and subsequent chemical reactions in the upper atmosphere, induced by solar UV radiation and electron bombardment from Saturn’s magnetosphere. These gaseous species can become supersaturated and condense out as ices once they descend to the stratosphere (180–70 K, 1–100 mbar total pressure). A cloud system was observed during the southern fall over the south pole at a much higher altitude (∼300 km) and shown to contain HCN ice particles (Visible Infrared Mapping Spectrometer (VIMS)

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Results

Discussion

Conclusion