Titan’s Upper Atmospheric HCN and Temperature Profiles Measured by ALMA

Hydrogen cyanide (HCN) is a stable and abundant molecular species produced through Titan’s robust, nitrogen and methane-based atmospheric photochemistry. HCN has been routinely observed on Titan at various wavelengths, including rotational transitions in the (sub)millimeter that enable sensitive measurements of Titan’s atmospheric state over a large vertical extent (~100-1000 km). Here, we detail the investigation of Titan’s upper atmospheric (>600 km) HCN abundance and temperature profiles through the analysis of observations of HCN J=4-3 (354.5 GHz) and J=3-2 (265.9 GHz) transitions obtained between 2012 and 2017 with the Atacama Large Millimeter/submillimeter Array (ALMA). We compare HCN abundances determined with ALMA to previous observations in the infrared and (sub)millimeter, and photochemical model results. Upper atmospheric thermal profiles retrieved from these ALMA data show variability in the temperature and altitude of Titan’s mesopause during this epoch, which we compare to the highly variable temperature measurements from the Cassini Ion and Neutral Mass Spectrometer and Ultraviolet Imaging Spectrometer instruments. These results provide additional insight for dynamical and photochemical models of Titan’s atmosphere, particularly during Titan’s northern spring, summer, and into the post-Cassini era.