  The Cassini mission performed 127 targeted flybys of Titan during its 13-year
mission to Saturn, culminating in the Grand Finale between April-September
2017. 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
May 2017 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 sub-mm 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
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 $\pm$ 60 km. The HNC emission map shows an east-west hemispheric
asymmetry of (13$\pm$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.
