ALMA Spectral Imaging of Titan Contemporaneous with Cassini's Grand Finale

The Cassini mission performed 127 targeted flybys of Titan during its 13 yr mission to Saturn, culminating in the Grand Finale between 2017 April and September. Here we demonstrate the use of the Atacama Large Millimeter/submillimeter Array (ALMA) to continue Cassini’s legacy for chemical and climatological studies of Titan’s atmosphere. Whole-hemisphere, interferometric spectral maps of HCN, HNC, HC₃N, CH₃CN, C₂H₃CN, C₂H₅CN, and C₃H₈ were obtained using ALMA in 2017 May at moderate (≈0.″2, or ≈1300 km) spatial resolution, revealing the effects of seasonally variable chemistry and dynamics on the distribution of each species. The ALMA submillimeter observations of HCN and HC₃N are consistent with Cassini infrared data on these species, obtained in the same month. Chemical/dynamical lifetimes of a few years are inferred for C₂H₃CN and C₂H₅CN, in reasonably close agreement with the latest chemical models incorporating the sticking of C₂H₅CN to stratospheric aerosol particles. ALMA radial limb flux profiles provide column density information as a function of altitude, revealing maximum abundances in the thermosphere (above 600 km) for HCN, HNC, HC₃N, and C₂H₅CN. This constitutes the first detailed measurement of the spatial distribution of HNC, which is found to be confined predominantly to altitudes above 730 ± 60 km. The HNC emission map shows an east-west hemispheric asymmetry of 13% ± 3%. These results are consistent with very rapid production (and loss) of HNC in Titan’s uppermost atmosphere, making this molecule an effective probe of short-timescale (diurnal) ionospheric processes.