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

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 700 km with a vertical resolution of ∼10 km. 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 ∼310 km 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 2.3 μm 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 2.3 μm 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 2.3 μm, to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane also contributes significantly to the strong absorption at 3.2-3.5 μm that was previously attributed only to C-H stretching bands from aerosols. Ethane bands may affect the surface windows too, especially at 2.7 μm. 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 3.4 μm, we detect a strong and narrow absorption at 3.28 μm 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 4.5 μm. Finally, we obtain the CO mixing ratio between 70 and 170 km, through the inversion of its 4.7 μm band. The average result of 46 ± 16 ppm is in good agreement with previous studies.