The thermal emission of Saturn’s icy moons

Effects of topography and regolith properties

Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France
e-mail: ferrari@ipgp.fr

Received: 30 April 2021
Accepted: 27 August 2021

Abstract

Context. The effects of space weathering and other alteration processes on the upper surface of Saturn’s icy moons are yet to be explored.

Aims. We present a thermophysical model parametrised by way of regolith properties such as porosity, grain size, and composition, as well as the local topography. The modelled surface temperature and apparent emissivity are intended to be compared to measurements taken by Cassini’s Composite Infrared Spectrometer (CIRS), using its focal plane FP1. We study how they are impacted by the topographic model and the regolith properties.

Methods. As an example, we coupled the topography of the Dione moon with our model. Simulations provide the thermal history of the surface elements of the shape model included in the FP1 footprints at the viewing geometries along one CIRS observation. The heat transfer in the regolith may occur through conduction or radiation. Its bolometric albedo, A, and hemispherical emissivity, ε_h, are expressed as a function of grain properties.

Results. The model roughly reproduces the observed variations of surface temperature, T_F, and apparent emissivity, ε_F, in the chosen example, while assuming uniform regolith properties. The dispersion of temperatures within the footprints due to the difference in local time of the surface elements explains most of the directionality of the apparent emissivity, ε_F (Em), at emission angles of Em ≥ 30°. Adding topography at the 8-km scale amplifies this effect by a few percent. Refining the scale to 1 km increases it again by a single percent but at a high computational cost. This particular anisotropy of ε_F (Em) cannot be explained by the directional emissivity, ε_d, of the regolith. The temperature T_F is less affected by this dispersion or by the topographic resolution. Adding regional variations of grain size significantly improves the agreement between the model and observations.

Conclusions. This model demonstrated its good performance and, thus, it is ready for testing current hypotheses on regolith processing by space weathering on Saturn’s icy moons, such as regional changes in grain size.