Titan's atmosphere as observed by Cassini/VIMS solar occultations : CH4, CO and evidence for C2H6 absorption

Luca Maltagliati*, Bruno Bézard, Sandrine Vinatier, Matthew M. Hedman, Emmanuel Lellouch, Philip D. Nicholson, Christophe Sotin, Remco J. de Kok, Bruno Sicardy

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

26 Citations (Scopus)

Abstract

We present an analysis of the VIMS solar occultations dataset, which allows us to extract vertically resolved information on the characteristics of Titan's atmosphere between ~100 and 700km with a vertical resolution of ~10km. After a series of data treatment procedures to correct problems in pointing stability and parasitic light, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. The transmittances show clearly the evolution of the haze, with the detection of the detached layer at ~310km in September 2011 at mid-northern latitudes. Through the inversion of the transmission spectra with a line-by-line radiative transfer code we retrieve the vertical distribution of CH4 and CO mixing ratio. For methane inversion we use its 1.4, 1.7 and 2.3μm bands. The first two bands are always in good agreement and yield an average stratospheric abundance of 1.28±0.08%, after correcting for forward-scattering effects, with no significant differences between the occultations. This is significantly less than the value of 1.48% obtained by the GCMS/Huygens instrument. We find that the 2.3μm band cannot be used for the extraction of methane abundance because it is blended with other absorptions, not included in our atmospheric model. The analysis of the residual spectra after the inversion shows that such additional absorptions are present through a great part of the VIMS wavelength range. We attribute many of these bands, including the one at 2.3μm, to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane also contributes significantly to the strong absorption at 3.2-3.5μm that was previously attributed only to C-H stretching bands from aerosols. Ethane bands may affect the surface windows too, especially at 2.7μm. Other residual bands are generated by stretching modes of C-H, C-C and C-N bonds. In addition to the C-H stretch from aliphatic hydrocarbons at 3.4μm, we detect a strong and narrow absorption at 3.28μm which we tentatively attribute to the presence of PAHs in the stratosphere. C-C and C-N stretching bands are possibly present between 4.3 and 4.5μm. Finally, we obtain the CO mixing ratio between 70 and 170km, through the inversion of its 4.7μm band. The average result of 46±16ppm is in good agreement with previous studies.

Original languageEnglish
Pages (from-to)1-24
Number of pages24
JournalIcarus
Volume248
DOIs
Publication statusPublished - 1 Mar 2015
Externally publishedYes

Keywords

  • Atmospheres, composition
  • Infrared observations
  • Occultations
  • Spectroscopy
  • Titan, atmosphere

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