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).

We explore the possibility that Callisto’s ocean sits beneath its high-pressure ice, rather than above it. Oceans perched between ice phases are considered to be stable configurations for Ganymede, Callisto, and Titan. High-pressure ices under the liquid water ocean will transport heat and solutes into the ocean as long as the convective adiabat for the ices remains close to the melting temperature (Choblet et al. 2017, Kalousova and Sotin 2018). However, this configuration may become unstable when the perched ocean is close to freezing and its salinity increases, if the ocean becomes denser than the underlying ice. Among the oceans in the solar system, Callisto’s must be among the coldest and most saline because the internal heat appears to be low in the absence of tidal dissipation. Surface geology indicates its lithosphere is fully stagnant (Moore et al. 2004). Solid-state convection may continue beneath less than 100 km or dirty non-convecting ice (McKinnon 2006). And just below this layer may reside a liquid water ocean that is the lag deposit of Callisto’s thicker primordial ocean, the concentrated result of 4 Gyr of freezing. Using representative interior structures based on the current constraints from the Galileo mission (Anderson et al. 2001) coupled with recently obtained thermodynamic data (Vance et al. 2018), we demonstrate the possibility for using magnetic induction to identify where the ocean currently resides in Callisto. Anderson, J. D. et al. (2001). Shape, mean radius, gravity field, and interior structure of Callisto. Icarus, 153(1):157–161. Choblet, G. et al. (2017). Heat transport in the high-pressure ice mantle of large icy moons. Icarus, 285:252–262. Kalousovà, K. and Sotin, C. (2018). Melting in high-pressure ice layers of large ocean worlds - implications for volatiles transport. Geophysical Research Letters. McKinnon, W. (2006). On convection in ice I shells of outer solar system bodies, with detailed application to Callisto. Icarus, 183(2):435–450. Moore, et al. (2004). Callisto. Jupiter. The Planet, Satellites and Magnetosphere, 1:397–426. Moore, J. and Pappalardo, R. (2011). Titan: An exogenic world? Icarus, 212:790–806. Vance, S. D. et al. (2018). Geophysical investigations of habitability in ice-covered ocean worlds. Journal of Geophysical Research: Planets, 123, 180–205.

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.