Temporal Evolution of Titan’s Stratospheric Temperatures and Trace Gases from a Two-dimensional Retrieval of Cassini Composite Infrared Spectrometer Data

Abstract

We use a two-dimensional (2D) radiative transfer model of Titan, which allows the atmospheric structure to vary in both altitude and latitude, to retrieve the spatial distribution of temperature, haze extinction, and C2H2, C2H6, C3H8, CH3C2H, C4H2, and HCN gases, from Cassini Composite Infrared Spectrometer (CIRS) limb-mapping observations over the duration of the Cassini mission. We compare our results with previous analyses of CIRS limb observations using radiative models that only allow the atmosphere to vary in altitude. The temperature, haze, and gas composition retrieved with the 2D model mostly show the same broad spatial and temporal trends as previously published results from 1D models. However, there are some significant differences in the retrieved structure at the fall and winter poles poleward of 60°. Most noticeably, the HCN abundance in the depleted region near 65°N at 350 km in northern winter is stronger in the 2D retrievals than in previous 1D retrievals, and the 2D retrievals show very different structure from earlier 1D retrievals in the north polar C2H2 structure during early northern spring, with a strong depletion around 70°N at 0.02 mbar.