Low thermal inertias of icy surface

Thermal inertias of icy surfaces of atmosphereless planetary bodies happen to be very low beyond Jupiter orbit.? Their appear to be well insulated by an ice sheet, the further from the Sun the better. To understand why, we have related the thermal inertia to the icy regolith properties such as porosity, grain size, ice form and heat transfer processes. We show that the crystalline or amorphous ice forms, and the quality of contacts between grains mainly govern the magnitude of the thermal inertia. Also beyond the orbit of Jupiter, thermal inertias as low as a few tens J/m^2/K/s^(1/2) are difficult to reproduce with plausible porosity and grains sizes made of crystalline ice unless contacts are loose. This is, on the contrary, straightforward for regoliths of sub-cm-sized grains made of amorphous water ice. The characteristic decrease of thermal inertia with heliocentric distance of icy surfaces is easily explained if amorphous ice is present at cm depths below a thin layer of crystalline ice. We have also examined the thermal dichotomy between the the leading and trailing faces of the Mimas satellite, as observed by the CIRS infrared spectrometer onboard CASSINI and relate it to changing properties probably linked to the diverse space weathering conditions.