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

## TL;DR

This study uses ALMA to perform spectral imaging of Titan's atmosphere, complementing Cassini data, revealing detailed spatial and vertical distributions of various molecules and their seasonal and dynamical variations.

## Contribution

First detailed ALMA measurements of Titan's atmospheric molecular distributions, providing new insights into chemical lifetimes and spatial asymmetries, complementing Cassini observations.

## Key findings

- ALMA maps of HCN, HNC, HC3N, C2H3CN, C2H5CN, C3H8 show seasonal and dynamical effects.
- HNC is confined above 730 km with east-west hemispheric asymmetry.
- Chemical lifetimes of C2H3CN and C2H5CN are a few years, matching chemical models.

## Abstract

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.