Updated Radiative Transfer Model for Titan: Validation on VIMS/Cassini
Observations of the Huygens Landing Site and Application to the Analysis
of the Dragonfly Landing Area
Abstract
Titan is a prime target for astrobiological research. Organic materials
from atmospheric chemistry precipitate on the surface and are subject to
geological processes (e.g. eolian and fluvial erosion) that lead to the
formation of dune fields, river networks and seas similar to their
terrestrial counterparts. The analysis of the surface reflectance in the
near-infrared (NIR) allows to constrain the surface composition, which
is crucial to understand these atmosphere/surface interactions. However,
Titan’s atmosphere prevents the surface from being probed in the NIR,
except in 7 transmission windows where the methane absorption is
sufficiently low (centered at 0.93, 1.08, 1.27, 1.59, 2.01, 2.7- 2.8 and
5 μm). We use an updated version of the Radiative Transfer (RT) model of
Hirtzig et al. (2013), with updated gases and aerosols opacities, in
order to better simulate atmospheric absorption and scattering and
retrieve surface albedos in the 7 NIR transmission windows with an
enhanced accuracy. Our RT model is based on the SHDOMPP and CDISORT
(Evans, 2007 and Buras, 2011) solvers to solve the RT equations in
plane-parallel and pseudo-spherical approximations respectively. We
recently improved atmospheric inputs of the model with up-to-date
gaseous CH4, CH3D,
13CH4,
C2H2, HCN and CO abundances profiles and
absorption coefficients (Vinatier et al. 2007, Niemann et al. 2010;
Maltagliati et al. 2015; Serigano et al. 2016; Rey et al. 2018; Thelen
et al. 2019; Gautier et al. 2021), and improved aerosol optical
properties. In particular, optical properties of Titan’s aerosols are
now computed from a fractal aggregate model (Rannou et al. 2003)
constrained by measurements of the Huygens probe (Tomasko et al. 2008
and Doose et al. 2016). The new version of our RT model is benchmarked
with the help of the most recent RT model for Titan (Coutelier et al.
2021) and validated using observations of the Descent Imager/Spectral
Radiometer (DISR) onboard Huygens. Coupled with an efficient inversion
scheme, our model can be apply to the Cassini’s Visual and Infrared
Mapping Spectrometer (VIMS) dataset to retrieve atmospheric opacity and
surface albedos at regional and global scales. This will help to analyze
future James Webb Space Telescope (JWST) observations of Titan (Nixon et
al. 2016) and prepare the Dragonfly mission (Lorenz et al. 2018).