Aerosols in titan s atmosphere are important in determining its thermal structure and can act as condensation nuclei for clouds. The global circulation reversed within the two years following the…

This is a research grant awarded by NASA to the Southwest Research Institute (SwRI), a nonprofit applied research organization. The grant, valued at $120,580.83, will fund a study on the role of aerosols and cloud formation in Titan's atmosphere. The objectives are to: 1) measure the vapor pressure of benzene at Titan-relevant temperatures, 2) produce analogs of Titan's aerosols and study benzene condensation, 3) use experimental data to model cloud formation, and 4) compare results to observations from the Cassini mission's Composite Infrared Spectrometer (CIRS). The research seeks to enhance the scientific return of the Cassini mission by better understanding the physical and chemical processes in Titan's atmosphere, particularly the post-equinox changes observed in the south polar region. This is a civilian research grant, with no set-aside designation.

Aerosols in titan s atmosphere are important in determining its thermal structure and can act as condensation nuclei for clouds. The global circulation reversed within the two years following the northern spring equinox in august 2009, increasing the mixing ratios of benzene (C6H6) and other species at the south pole. A simultaneous strong cooling (T250KM). C6H6 has been detected by cirs in the south polar cloud system, but the existing laboratory data is insufficient to allow models to reproduce the formation of the observed cloud. Objectives: we will combine laboratory, modeling and observational studies to investigate the condensation of benzene on titan s aerosols as a potentially important component of the cloud system that appeared during the autumn at 300 KM above titan s south pole. The project goals are: 1) (lab) to measure the vapor pressure of benzene at titan-relevant temperatures, 2) to produce analogs of titan s aerosols and investigate the conditions required for condensation of benzene on them, 3) (model) to use the experimental data to constrain nucleation and condensation in microphysical models, in order to determine expected cloud altitudes and particle sizes and 4) (observation) to compare our experimental data and modeling outputs to observations from cirs in the 9-17 m spectral region, to understand the molecular composition of this cloud system. Significance to cdap: the proposed study is relevant to cdap as IT will enhance the scientific return of the cassini mission by (1) providing laboratory data that will address the physical and chemical processes that occur in titan s atmosphere, particularly at the south polar region after the northern spring equinox, and (2) using these experimental data as input in microphysical models and for direct comparison to cirs data. IT will be a pluridisciplinary effort, drawing laboratory expertise from both planetary and earth sciences, as well as modeling and cirs observational expertise, broadening the scientific participation in the analysis of cassini data. Methods: we will use two laboratory facilities at NASA ames. Analogs of titan s aerosols will be produced at 150 k in the titan haze simulation (THS) apparatus. IR monitoring of benzene condensation will be conducted in the atmospheric chemistry laboratory (ACL). In the THS, tholins will be produced in a jet-cooled N2-CH4-based plasma expansion and deposited onto silicon. In the meantime, sublimation pressure of pure C6H6 will be determined at titan-relevant temperatures using the ACL facility, providing much-needed data for equilibrium models and reference data for the condensation on tholins study. After producing the THS tholins, they will be collected and transferred under an inert atmosphere into the ACL chamber. Spectra (2.5-17 m) of the tholins will be acquired from 295-120 k. A second set of temperature-dependent spectra will be acquired while C6H6 gas is introduced, monitoring for evidence of surface chemical reactions, adsorption, and/or condensation. In the last year of the proposed work, a pilot study will be conducted with HCN and mixtures of C6H6 and HCN. The resulting laboratory data will be compared to cirs spectra and used as input in the titan carma microphysical model which simulates nucleation, condensation, evaporation, sedimentation, and coagulation of cloud particles. The predicted cloud vertical distribution will be compared to the vertical profile derived from cirs observations, guiding the interpretation of the cloud system composition and vertical structure. There is a clear need for a synergy between critical laboratory experiments, and the modeling and observation communities in order to better understand the initial formation of the south polar cloud system. The combined laboratory, modeling and observational efforts that are proposed here are specifically designed to address this issue.