The Chemical History and Evolution of Titan’s Atmosphere as Revealed by ALMA
Saturn’s largest moon, Titan, possesses a substantial atmosphere containing significant minorities of nitrile and hydrocarbon species, predominantly due to the photodissociation of the major gases, N2 and CH4. Titan’s methane cycle, liquid lakes, and complex organic chemistry make it an intriguing target through its similarities to Earth and the allure of its astrobiological potential. Though the existence of heavy nitrile species – such as CH3C3N, HC5N, and C3H7CN – has been inferred through Cassini Ion and Neutral Mass Spectrometer (INMS) data, confirmation of these species has yet to be made spectroscopically. Other hydrocarbon species, such as C3H4 and C3H8 have been detected using Voyager’s Infrared Spectrometer (IRIS; Maguire et al., 1981) and later mapped by the Composite Infrared Spectrometer (CIRS; Nixon et al., 2013) onboard Cassini, but abundance constraints for these species in the mesosphere is poor. To fully understand the production of these species and their spatial distribution in Titan’s atmosphere, vertical abundance profiles must be produced to use with current photochemical models. Utilizing early science calibration images of Titan obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA), Cordiner et al. (2014; 2015) determined the vertical distribution of various nitriles and hydrocarbons in Titan’s atmosphere, including at least one previously undetected molecule – C2H5CN. For my dissertation project, I will calibrate and model sub-millimeter emissions from molecules in Titan’s atmosphere, and quantify variations in the spatial distribution of various species throughout its seasonal cycle by utilizing high resolution ALMA data. The main goals of this project are as follows:
1. To search for previously undetected molecules in Titan’s atmosphere through analysis of the existing public ALMA data, and/or through ALMA proposals of my own;
2. Constrain abundance profiles of detected molecular species, and provide upper abundance limits for those we cannot detect;
3. Map the spatial distribution of detected species in order to improve our understanding of Titan’s atmospheric transport and circulation;
4. Determine how these spatial distributions change over Titan’s seasonal cycle by utilizing multiple years of public ALMA data.
The majority of this work will employ the Non-linear Optimal Estimator for MultivariatE Spectral analySIS (NEMESIS) software package, developed by Oxford University (Irwin et al., 2008), to retrieve abundance and temperature information through radiative transfer models. These results will allow us to investigate the chemical evolution and history of Titan’s rich, pre-biotic atmosphere by providing valuable abundance measurements and constraints to molecular photochemical and dynamical models. We will compare our results with measurements made by the Cassini spacecraft, thereby enhancing the scientific return from both orbiter and ALMA datasets. The increased inventory of complex, organic molecules observable with ALMA’s sub-mm frequency range and high spatial resolution may also yield detections of species fundamental to the formation of living organisms, such as amino acids. Thus, by informing photochemical and dynamical models and increasing our known inventory of complex molecular species, we will also assess Titan’s potential habitability.