摘要：The Cassini mission performed 127 targeted flybys of Titan during its 13yr 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, HC 3 N, CH 3 CN, C 2 H 3 CN, C 2 H 5 CN, and C 3 H 8 were obtained using ALMA in 2017 May at moderate (≈0 2, or ≈1300km) spatial resolution, revealing the effects of seasonally variable chemistry and dynamics on the distribution of each species. The ALMA submillimeter observations of HCN and HC 3 N are consistent with Cassini infrared data on these species, obtained in the same month. Chemical/dynamical lifetimes of a few years are inferred for C 2 H 3 CN and C 2 H 5 CN, in reasonably close agreement with the latest chemical models incorporating the sticking of C 2 H 5 CN 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 600km) for HCN, HNC, HC 3 N, and C 2 H 5 CN. 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.
关键词：planets and satellites: atmospheres;planets and satellites: individual (Titan);submillimeter: planetary systems;techniques: imaging spectroscopy;techniques: interferometric
