Optical constants of Titan's stratospheric aerosols in the 70-1500cm -1 spectral range constrained by Cassini/CIRS observations

Sandrine Vinatier*, Pascal Rannou, Carrie M. Anderson, Bruno Bézard, Remco De Kok, Robert E. Samuelson

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

52 Citations (Scopus)

Abstract

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 190km 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 0.05μm, and a fractal dimension of 2, we are able to constrain the refractive index of Titan's aerosol between 70 and 1500cm -1 (143 and 6.7μm). 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 CH 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 193-274km altitude range, which implies that Titan's aerosols exhibit the same chemical composition in all investigated latitude and altitude regions.

Original languageEnglish
Pages (from-to)5-12
Number of pages8
JournalIcarus
Volume219
Issue number1
DOIs
Publication statusPublished - May 2012
Externally publishedYes

Keywords

  • Atmospheres, composition
  • Spectroscopy
  • Titan, atmosphere

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