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Combining Laboratory Experiments, Models, and Observations to Investigate Ice Cloud Formation in Titan’s Stratosphere

Presentation #208.05 in the session Titan I: Atmosphere (Oral Presentation)

Published onOct 23, 2023
Combining Laboratory Experiments, Models, and Observations to Investigate Ice Cloud Formation in Titan’s Stratosphere

Following the northern spring equinox, the global circulation of Titan’s atmosphere reversed, enhancing the mixing ratios of many species at the south pole. Simultaneous strong cooling (T < 120 K) permitted several of these molecules to condense at unusually high altitudes (> 250 km). Microphysical modeling of ice cloud formation can be combined with analysis of Cassini Composite Infrared Spectrometer (CIRS) data to assess the contribution of specific molecular species in the south polar cloud.

Here we report the results of a new laboratory study which provides fundamental input data describing the critical saturation (Scrit) conditions required to nucleate solid phase benzene (C6H6) via vapor deposition at relevant temperatures (135- 170 K). Formation of a condensed phase requires supersaturation conditions, and studies of condensates relevant to other solar system bodies have shown significant supersaturation requirements at low temperatures[1,2]. The temperature dependence of benzene nucleation conditions measured here will be combined with previous measurements of the equilibrium vapor pressure of benzene[3,4] in the Titan CARMA (Community Aerosol & Radiation Model for Atmospheres)[5] microphysical model to simulate nucleation, condensation, evaporation, sedimentation, and coagulation of cloud particles. We will evaluate the influence of laboratory-derived temperature-dependent Scrit values on the size and number of benzene ice particles compared to simulations using estimated Scrit values derived from previous lab measurements with other hydrocarbons. These model results will more accurately quantify the contribution of condensed-phase benzene to the south polar cloud system that appeared during the autumn at 300 km. Re-analysis of Cassini CIRS (Cassini Composite Infrared Spectrometer) data in the 9-17 um spectral region from 65ºS to 90ºS in forthcoming studies will complete this holistic investigation of the contribution of benzene to the observed south polar clouds.

Acknowledgments: This work is supported by the NASA SMD CDAP R&A Program (22-CDAP22_2-0028).

References: [1] Santiago-Materese et al., Icarus, 303, 280 (2018) [2] Iraci et al., Icarus, 210, 985 (2010) [3] Dubois et al., PSJ, 2, 121 (2022) [4] Hudson et al., PSJ, 3, 120 (2022) [5] Barth, Atmosphere, 10, 1064 (2020)

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