Surface properties of Dione’s trailing hemisphere

Context. The trailing hemispheres of Saturn’s icy moons exhibit red and dark regoliths, most probably resulting from their interaction with the magnetospheric plasma. However, numerous space-weathering processes compete on these surfaces. The study of thermal infrared radiation can help probe the structure of the regolith at depth. The moon Dione was chosen as a case study. Aims. The goal is to constrain the effective grain size of this icy regolith from its thermal emission by relaxing the assumption of blackbody behaviour and doing so consistently with reflectance studies. At the same time, the influence of topography or roughness on this emission can be studied. Methods. A Mie-Hapke hybrid model was used to infer the water ice contaminant mixture and grain size in the uppermost layers consistently with reflectance observations, or the effective grain size and the scattering asymmetry factor, ξ, in deeper ones from Cassini/CIRS spectra. The effect of roughness was tentatively reproduced by including Hapke’s shadowing function assuming zero thermal inertia. Results. Superficial layers of micrometre-sized grains (2–5 μm), composed of water ice contaminated at the molecular scale with less than 0.1% of both amorphous carbon and tholins, are shown to be compatible with band depths and slopes observed in reflectance. These layers are transparent to the radiation emitted by underlying millimetre-sized grains (1–5 mm) as inferred from observed regolith emissivity. The daytime thermal emission is found to be sensitive to illumination and viewing geometry. Mimicking this effect with the Hapke shadowing function yields better predictions of infrared spectra and first estimates of roughness on this hemisphere, which range between 12° and 36° on average. These values are larger than those measured on an eight-kilometre scale from the shape model, and they may be typical of a smaller spatial scale. Retrieved temperatures are higher when including effects of grain size and roughness. The regolith asymmetry factor is therefore not constrained, and assuming ξ = 0 is correct. Nighttime observations can still be analysed assuming blackbody behaviour of the regolith. Assuming amorphous ice in emissivity models provides better predictions.