Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.0708s&link_type=abstract
American Astronomical Society, DPS meeting #41, #7.08
Astronomy and Astrophysics
Astronomy
Scientific paper
Analyses of Cassini CIRS far-infrared limb spectra of Titan at low latitudes have yielded the spectrum of Titan's aerosol between 60 and 560 cm-1, as well as its vertical abundance distribution from an altitude of 300 km to the surface. Scale height is variable, but everywhere substantially exceeds that of a uniformly mixed atmosphere, indicating a source well above 300 km. The Titan aerosol spectrum is similar to the spectrum of tholin material generated in the laboratory (Khare et al., 1984, Icarus 60, 127 - 137) between 400 and 560 cm-1, but rapidly diverges from the laboratory spectrum below 400 cm-1, suggesting rather different chemical formation processes. A broad Titan aerosol emission feature centered at 145 cm-1 implies a low-energy bond or group of bonds typical of out-of-plane bending or torsion modes of large molecules, such as polycyclic aromatic hydrocarbons (PAHs) or nitrogenated aromatics.
Limb scans at latitudes 15N and 15S reveal a separate broad emission feature between 70 and 270 cm-1, restricted to the altitude range 60 - 100 km. These altitudes correspond very closely to those where various nitriles are expected to condense, and the spectral shape of the observed feature resembles that expected for a combination of HCN and HC3N ices, the two most abundant nitriles found in Titan's stratosphere. This is the first time stratospheric ices have been identified at low latitudes. A preliminary study suggests that stratospheric nitrile ices are about five times more abundant at 62N than at 15S. An analysis of the 506 cm-1 HC3N ice band at 70N, where some vertical resolution of high spectral resolution data is possible, reveals a particle radius of 2 - 3 microns and suggests a cloud thickness of no more than a few kilometers. Physical implications will be discussed.
Anderson Martha C.
Samuelson Robert E.
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