The Search for New Molecular Species in the Atmosphere of Titan

The atmosphere of Saturn’s moon, Titan, contains a vast array of minor molecular constituents originating from the dissociation of its primary compounds, N2and CH4. The extent of Titan’s photochemical network was revealed by ionospheric measurements from the Cassini orbiter, leaving substantial gaps in the corresponding neutral inventory typically probed through infrared spectroscopy. Utilizing the unprecedented sensitivity and high spectral resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) at longer wavelengths, we searched for additional trace gases in Titan’s atmosphere during 2016, 2017,and 2019to further characterize Titan’s atmospheric and surface composition. These observations have resulted in the spectroscopic detections of the small cyclic molecule, cyclopropenylidene (c-C3H2), and the largest nitrile to date, methyl cyanoacetylene (CH3C3N), for the first time in Titan’s atmosphere –or anywhere else in the Solar System. We employed radiative transfer models to determine the volume mixing ratio of both species in Titan’s middle and upper atmosphere (altitudes > 350 km)and constrained the total column densities for the incorporation into future photochemical models of Titan’s atmosphere. Discrepancies between these results and current models necessitates the need for dedicated laboratory and modeling studies regarding the production pathways for trace complex organics in Titan’s atmosphere. Additionally, the measurement of previously undetected molecular species provides further incentive for the study of planetary atmospheres in the (sub)millimeter regime with facilities such as ALMA.