Subsurface Thermophysical Properties of Europa's Leading and Trailing Hemisphere as Revealed by ALMA

We present best-fit values of porosity – and the corresponding effective ther-12 mal inertiae – determined from three different depths in Europa’s near-subsurface13 (∼ 1 − 20 cm). We find that Europa’s subsurface porosity varies between 75–14 50% (Γef f ≈ 50 − 140 J m−2 K−1 s−1/2) on the leading hemisphere and 50–40%15 (Γef f ≈ 140 − 180 J m−2 K−1 s−1/2) on the trailing hemisphere as a function of16 depth. Residual maps produced by comparison with these models reveal thermally17 anomalous features that cannot be reproduced by global, single porosity models.18 These regions are compared to Europa’s surface terrain and known compositional19 variations. We find that some instances of warm thermal anomalies are co-located20 with known geographical or compositional features on both the leading and trailing21 hemisphere; cool temperature anomalies are well correlated with surfaces previously22 observed to contain pure, crystalline water ice and the expansive rays of Pwyll crater.23 Anomalous regions correspond to locations with subsurface properties different from24 those of our best-fit models, such as potentially elevated thermal inertia, decreased25 emissivity, or more porous regolith. We also find that ALMA observations at ∼ 3 mm26 sound below the thermal skin depth of Europa (∼ 10−15 cm) for a variety of porosity27 models, and thus do not exhibit features indicative of diurnal variability or residuals28 similar to other frequency bands. Future observations of Europa at higher angular29 resolution may reveal additional locations of variable subsurface thermophysical prop-30 erties, while those at other wavelengths will inform our understanding of the regolith31 compaction length and the effects of external processes on the shallow subsurface.