Revisiting the thermal inertia of Iapetus: Clues to the thickness of the dark material

The energy balance at the surface of an airless planetary body is strongly influenced by the bolometric Bond albedo and the surface thermal inertia. Both of these values may be calculated through the application of a thermal model to measured surface temperatures. The accuracy of either, though, increases if the value of the other is better constrained. In this study, we used the improved global bolometric Bond albedo map of Iapetus derived from Cassini VIMS and ISS and Voyager ISS data in conjunction with Cassini CIRS temperature data to reevaluate surface thermal inertia across Iapetus. Results showed the thermal inertia of the dark terrain varies between 11 and 14.8 J m−2 K−1 s−1/2 while the light material varies between 15 and 25 J m−2 K−1 s−1/2. Using an approximation to the thermal properties of the dark overburden derived from our thermal inertia results, we can implement our thermal model to provide estimates on the dark material thickness, which was found to lie between 7 cm and 16 cm. In order to develop an accurate global thermal model, a weighted function that approximates the surface thermal inertia across Iapetus was developed and verified via our measurements. The global bolometric Bond albedo map, surface thermal inertia map, and the thermal model are then used to synthesize global temperature maps that may be used to study the stability of volatiles.

► The thermal inertia of the light material ranges between 15 and 30 J m−2 K−1 s−1/2.
► We find that the thermal inertia of the dark material ranges between 9.5 and 14.8 mK s.
► Dark overburden thicknesses are estimated to lie between 8 and 16 cm.
► We synthesize a global thermal inertia map.