Optical constants of Titan's stratospheric aerosols in the 70-1500 cm-1 spectral range constrained by Cassini/CIRS observations
Résumé
We utilized aerosol extinction coefficient inferred from Cassini/CIRS spectra in the far and mid infrared region to derive the extinction cross section near an altitude of 190 km at 15°S (from far-IR) and 20°S (from mid-IR). By comparing the extinction cross section that are derived from observations with theoretical calculations for a fractal aggregate of 3000 monomers, each having a radius of 0.05 micron, and a fractal dimension of 2, we are able to constrain the refractive index of Titan's aerosol between 70 and 1500 cm-1 (143 and 6.7 μm). As the real and imaginary parts of the refractive index are related by the Kramers-Kronig equation, we apply an iterative process to determine the optical constants in the thermal infrared. The resulting spectral dependence of the imaginary index displays several spectral signatures, some of which are also seen for some Titan's aerosol analogues (tholins) produced in laboratory experiments. We find that Titan's aerosols are less absorbent than tholins in the thermal infrared. The most prominent emission bands observed in the mid-infrared are due to C-H bending vibrations in methyl and methylene groups. It appears that Titan's aerosols predominantly display vibrations implying carbon and hydrogen atoms and perhaps marginally nitrogen. In the mid infrared, all the aerosol spectral signatures are observed at three additional latitudes (56°S, 5°N and 30°N) and in the 193-274 km altitude range, which implies that Titan's aerosols exhibit the same chemical composition in all investigated latitude and altitude regions.