A RT-based Technique for the Analysis and the Removal of Titan's Atmosphere by Cassini/VIMS-IR data
Abstract:Since 2004, the Visual and Infrared Mapping Spectrometer (VIMS), together with the CIRS and UVIS spectrometers, aboard the Cassini spacecraft has provided insight on Saturn and Titan atmospheres through remote sensing observations. The presence of clouds and aerosols in Titan's dense atmosphere makes the analysis of the surface radiation a difficult task. For this purpose, an atmospheric radiative transfer (RT) model is required. The implementation of a RT code, which includes multiple scattering, in an inversion algorithm based on the Bayesian approach, can provide strong constraints about both the surface albedo and the atmospheric composition.
The application of this retrieval procedure we have developed to VIMS-IR spectra acquired in nadir or slant geometries allows us to retrieve the equivalent opacity of Titan's atmosphere in terms of variable aerosols and gaseous content. Thus, the separation of the atmospheric and surface contributions in the observed spectrum is possible. The atmospheric removal procedure was tested on the spectral range 1-2.2μm of publicly available VIMS data covering the Ontario Lacus and Ligeia Mare regions.
The retrieval of the accurate composition of Titan's atmosphere is a much more complex task. So far, the information about the vertical structure of the atmosphere by limb spectra was mostly derived under conditions where the scattering could be neglected [1,2]. Indeed, since the very high aerosol load in the middle-low atmosphere produces strong scattering effects on the measured spectra, the analysis requires a RT modeling taking into account multiple scattering in a spherical-shell geometry. Therefore the use of an innovative method we are developing based on the Monte-Carlo approach, can provide important information about the vertical distribution of the aerosols and the gases composing Titan's atmosphere.