The structure of Titan's atmosphere from Cassini radio occultations: One- and two-way occultations

Abstract

We present the results of the remaining ten soundings of Titan's atmosphere by radio occultations using the Cassini spacecraft that have not been previously reported. Three were in 2008 and 2009, and used the Ultra Stable Oscillator onboard the spacecraft. The rest were in 2014 and 2016 after the USO had failed and were executed in two-way mode. Comparison of the later soundings with those earlier in the Cassini mission provide an indication of the seasonal change at mid latitudes. The southern hemisphere has shown a dramatic cooling during the onset of autumn; change in the northern hemisphere is noticeable, but more subdued. Changes at low latitudes are even smaller. The additional retrievals from 2008 to 2009 add a better constraint on the latitude structure of the previously reported destabilization of the temperature profiles at northern latitudes during winter and early spring. They show that the most pronounced effect is at polar latitudes (>70 N), suggesting that they are associated with the descending circulation over the winter pole that has been proposed by several authors. The full set of occultations indicates minimal variation with latitude and time in the troposphere, likely attributable to the large atmospheric radiative damping time and efficient meridional heat transports, but somewhat more variation near the surface, which has a smaller thermal inertia.