Titan's aerosol and stratospheric ice opacities between 18 and 500 µm: Vertical and spectral characteristics from Cassini CIRS

Abstract

Vertical distributions and spectral characteristics of Titan's photochemical aerosol and stratospheric ices are determined between 20 and 560 cm ^-1 (500-18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined. In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ∼3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC ₃N), as inferred from a composite emission feature at ∼160 cm ^-1, appear to be located over a narrow altitude range in the stratosphere centered at ∼90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S. There is some evidence of a second ice cloud layer at ∼60 km altitude at 58°S associated with an emission feature at ∼80 cm ^-1. We speculate that the identify of this cloud may be due to C ₂H ₆ ice, which in the vapor phase is the most abundant hydrocarbon (next to CH ₄) in the stratosphere of Titan. Unlike the highly restricted range of altitudes (50-100 km) associated with organic condensate clouds, Titan's photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3-2.4 at about 160 km to 1.1-1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ∼140 cm ^-1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics. Finally, volume extinction coefficients N χ_E derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios N χ_E(1.583 μm)/N χ_E(62.5 μm) of roughly 70 and 20, respectively, for Titan's aerosol and stratospheric ices. The inferred particle cross-section ratios χ_E(1.583 μm)/ χ_E(62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.