Infrared emissivity of icy surfaces: Sensitivity to regolith properties and water-ice contaminants

Context. Most analyses of the infrared emission of Saturn's rings and icy satellites have considered pure water ice as the constituent of regolith and particle surfaces. Visual and near-infrared observations have shown, however, that darkening and reddening contaminants are present at a fraction level of a few percent. In the spectral domain 10-2000 cm(-1), water ice becomes transparent in a few windows, which in particular causes the roll-off of emissivity of icy surfaces that is observed below 50 cm(-1). Their emissivity there may be affected by these contaminants. Aims. We present a quantitative global sensitivity analysis of a hybrid Mie-Hapke model to evaluate the influence of regolith properties and contaminant fraction on the infrared emissivity of icy rings or moons over this spectral range. Methods. A hybrid Mie-Hapke model of the hemispherical emissivity epsilon(h)*(W-n) was made, including various diffraction correction and mixing types with tholins or amorphous carbon grains, or grain size distributions and some anisotropy in emission. A Sobol global sensitivity analysis provided quantitative levels of importance for these factors versus wave number w(n). Results. Given the a priori uncertainties, the most important factor acting on epsilon*h(W-n) remains the size distribution of regolith grains and the average anisotropy factor xi. For W-n> 50 cm(-1), xi, the power-law index p and the minimum a(min) of the size distribution are most influential. In windows of water-ice transparency (10-50, 300-600, and 900-1300 cm(-1)), the emissivity is also sensitive, but to a lesser extent, to the maximum grain size a(max) and the fraction f of contaminants, if mixed at the molecular level. Conclusions. This model provides a self-consistent tool for interpreting multi-modal observations of the thermal emission from icy surfaces. It also offers interesting insights into recent mid-infrared observations of Saturn's rings and Jupiter's moon Ganymede by the JWST-MIRI instrument.