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

摘要

Vertical distributions and spectral characteristics of Titan's photochemical aerosol and stratospheric ices are determined between 20 and 560cm-1 (500-18μm) 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 HC3N), as inferred from a composite emission feature at ～160cm-1, appear to be located over a narrow altitude range in the stratosphere centered at ～90km. 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 ～60km altitude at 58°S associated with an emission feature at ～80cm-1. We speculate that the identify of this cloud may be due to C2H6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH4) in the stratosphere of Titan. Unlike the highly restricted range of altitudes (50-100km) 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 300km, 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 160km to 1.1-1.4 at 300km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ～140cm-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 NχE derived from 15°S CIRS data at a wavelength of λ=62.5μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583μm. This comparison yields volume extinction coefficient ratios NχE(1.583μm)/NχE(62.5μm) of roughly 70 and 20, respectively, for Titan's aerosol and stratospheric ices. The inferred particle cross-section ratios χE(1.583μm)/χE(62.5μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.