Regional mapping of aerosol population and surface albedo of Titan by
the massive inversion of the Cassini/VIMS dataset
Abstract
Mapping Titan’s surface albedo is a necessary step to give reliable
constraints on its composition. However, even after the end of the
Cassini mission, surface albedo maps of Titan, especially over large
regions, are still very rare, the surface windows being strongly
affected by atmospheric contributions (absorption, scattering). A full
radiative transfer model is an essential tool to remove these effects,
but too time-consuming to treat systematically the
~50000 hyperspectral images VIMS acquired since the
beginning of the mission. We developed a massive inversion of VIMS data
based on lookup tables computed from a state-of-the-art radiative
transfer model in pseudo-spherical geometry, updated with new aerosol
properties coming from our analysis of observations acquired recently by
VIMS (solar occultations and emission phase curves). Once the physical
properties of gases, aerosols and surface are fixed, the lookup tables
are built for the remaining free parameters: the incidence, emergence
and azimuth angles, given by navigation; and two products (the aerosol
opacity and the surface albedo at all wavelengths). The lookup table
grid was carefully selected after thorough testing. The data inversion
on these pre-computed spectra (opportunely interpolated) is more than
1000 times faster than recalling the full radiative transfer at each
minimization step. We present here the results from selected flybys. We
invert mosaics composed by couples of flybys observing the same area at
two different times. The composite albedo maps do not show significant
discontinuities in any of the surface windows, suggesting a robust
correction of the effects of the geometry (and thus the aerosols) on the
observations. Maps of aerosol and albedo uncertainties are also
provided, along with absolute errors. We are thus able to provide
reliable surface albedo maps at pixel scale for entire regions of Titan
and for the whole VIMS spectral range.