Thermophysical Properties of Europa's Surface Constrained by Galileo Photopolarimeter-Radiometer Temperature Measurements

Abstract

Thermal measurements constrain the physical properties of icy satellite surfaces, including grain size, porosity, and regolith structure. On Europa, analyses of the Galileo Photopolarimeter-Radiometer (PPR) dataset revealed thermal inertia heterogeneities, but limited resolution hindered detailed characterization. We reanalyze the PPR dataset to derive maps of Europa's albedo and thermal inertia, and infer the microphysical properties of its icy regolith. Using the KRC thermal model, we fit brightness temperatures and interpret the results with conductivity models of porous ice to constrain grain size, porosity, and sintering processes. We find a mean Bond albedo of 0.64 pm 0.06 and a mean thermal inertia of 56 pm 17 tiu. Thermal inertia varies significantly, with a low-inertia equatorial band (39 pm 7 tiu) and higher values at mid-latitudes and on the trailing hemisphere, likely reflecting compositional differences. These values imply a porous regolith with grain sizes from micrometers to centimeters and an average porosity of 0.61 pm 0.1. Thermal inertia shows little correlation with geological units except for the Pwyll ejecta, which exhibit higher values. Instead, its agreement with sputtering rates suggests sputtering-driven sintering as a key process. Electron-driven sintering appears inefficient, while temperature-gradient metamorphism may enhance grain growth at depth. Modeled surface temperatures range from 67 to 148 K. These results provide a framework for interpreting future observations from Europa Clipper and JUICE.