Titan's Aerosol and Stratospheric Ice Opacities between 18 and 500 μm: Vertical and Spectral Characteristics from Cassini CIRS

Cassini Composite Infrared Spectrometer (CIRS) spectra of Titan are utilized between 20 and 560 cm-1 (500 to 18 μm) to determine aerosol and stratospheric ice opacities, as well as vertical abundance distributions below 300 km. We derive broad emission features that peak at ~140 cm-1 for the aerosol and ~160 cm-1 for the ice at latitudes 15°N, 15°S, and 58°S. The ice feature spans 70 to 270 cm-1 and is restricted to altitudes between 50 and 100 km, where condensations of various nitriles are expected. This ice feature most likely stems from a composite of HCN and HC3N ices, which are the two most abundant nitriles in Titan’s atmosphere. We speculate that the aerosol emission feature arises from lattice vibrations of large 2D planar molecules, such as polycyclic aromatic hydrocarbons (PAHs) and/or nitrogenated aromatics. Between 400 and 560 cm-1, Titan’s aerosol spectrum resembles the spectrum of laboratory generated aerosol or tholin material (Khare et al., 1984, Icarus 60, 127-137), but rapidly diverges from the laboratory spectrum below 400 cm-1. The chemical composition of Titan’s aerosol does not appear to change with altitude or latitude, suggesting that the rates of formation and chemical evolution of the aerosol are lower than the rate of dynamical turnover for the atmosphere. This is not the case for the stratospheric ice composite, which appears to chemically change with altitude and latitude, due to differences in ice abundances and condensation temperatures.