Radiative Transfer on Titan: Towards a Massive Inversion of Atmospheric and Surface Properties From VIMS/Cassini Observations of Titan

Titan, the largest moon of Saturn, is the only one to possess a dense, extended and hazy atmosphere, primarily composed of N2 and a few percents of CH4. Nitrogen and methane are photo-chemically dissociated to produce a plethora of complex nitrogenous and organic compounds, leading to the formation of an extensive haze of organic aerosols. CH4 absorptions and scattering from haze particles contribute to the almost complete hiding of Titan's surface at UV-visible-NIR wavelengths, letting Titan's surface until recently largely unknown. Since 2004, the Visual and Infrared Mapping Spectrometer (VIMS) aboard the Cassini spacecraft has provided a wealth of hyperspectral observations of Titan (more than 30,000 data cubes). VIMS can image Titan's surface in seven narrow near-IR spectral windows, where atmospheric methane absorptions are the weakest. In order to retrieve the absolute surface albedo, high-fidelity radiative transfer models are used, taking as inputs physical properties of gases and aerosols as a function of the altitude. These calculations are extremely time consuming and thus used to analyze only a few number of isolated Titan's spectra, although with a very high level of accuracy. Our goal is to massively invert the VIMS dataset. A smart inversion scheme is thus required, providing a good compromise between accuracy and computation time. We will proceed in four steps. First, we will choose the best-suited radiative transfer model for the geometry of the observation. Indeed, plane-parallel radiative transfer models are very accurate for low to moderate incidence and emergence angles but provide wrong results for high air mass (usually for incidence or emergence angles higher than 70°). On the other hand, spherical 3D Monte Carlo models are slower than plane-parallel model but give accurate results for extreme geometries. A sensitivity analysis is underway to define the geometry conditions in which 3D Monte Carlo computations are needed. The second step consists in inverting the absolute surface albedo of several homogeneous Titan's regions imaged for very different geometries of observation in order to check the consistency of the inversion scheme. Then we will build look-up tables (LUT) for a range of discrete values of incidence, emergence and azimuth angles, opacity of the aerosols and surface albedo, using the best-suited radiative transfer model as a function of the geometry. The computation time could be strongly reduced by the use of surface-atmosphere coupling analytical set of equations. The final step will consist in inverting the atmospheric and surface properties for several VIMS cubes, then for the whole VIMS dataset and thus draw maps of Titan's surface absolute infrared albedos.