We present best-fit values of porosity—and the corresponding effective thermal inertiae—determined from three different depths in Europa’s near-subsurface (∼1–20 cm). The porosity of the upper ∼20 cm of Europa’s subsurface varies between 75% and 50% (Γ_eff ≈ 50–140 J m⁻² K⁻¹ s^−1/2) on the leading hemisphere and 50%–40% (Γ_eff ≈ 140–180 J m⁻² K⁻¹ s^−1/2) on the trailing hemisphere. Residual maps produced by comparison with these models reveal thermally anomalous features that cannot be reproduced by globally homogeneous porosity models. These regions are compared to Europa’s surface terrain and known compositional variations. We find that some instances of warm thermal anomalies are co-located with known geographical or compositional features on both the leading and trailing hemisphere; cool temperature anomalies are well correlated with surfaces previously observed to contain pure, crystalline water ice and the expansive rays of Pwyll crater. Anomalous regions correspond to locations with subsurface properties different from those of our best-fit models, such as potentially elevated thermal inertia, decreased emissivity, or more porous regolith. We also find that ALMA observations at ∼3 mm sound below the thermal skin depth of Europa (∼10–15 cm) for a range of porosity values, and thus do not exhibit features indicative of diurnal variability or residuals similar to other frequency bands. Future observations of Europa at higher angular resolution may reveal additional locations of variable subsurface thermophysical properties, while those at other wavelengths will inform our understanding of the regolith compaction length and the effects of external processes on the shallow subsurface.

中文翻译：

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ALMA 揭示的木卫二前半球和后半球的地下热物理特性

我们提出了从木卫二近地下（∼1-20 cm）的三个不同深度确定的孔隙率的最佳拟合值以及相应的有效热惯性。木卫二地下上部约 20 cm 的孔隙度在前半球的 75% 到 50% 之间变化（ Γ _eff ≈ 50–140 J m ⁻² K ^-1 s ^-1/2），在前半球上变化为 50%–40% ( Γ ) _eff ≈ 140–180 J m ⁻² K ⁻¹ s ^−1/2 ) 在后半球上。与这些模型进行比较生成的残差图揭示了热异常特征，而这些特征无法通过全局均匀孔隙度模型重现。将这些区域与木卫二的表面地形和已知的成分变化进行比较。我们发现，一些暖热异常实例与前半球和后半球的已知地理或成分特征位于同一地点；低温异常与先前观察到的含有纯净结晶水冰的表面以及普威尔陨石坑的膨胀射线密切相关。异常区域对应于地下特性与我们的最佳拟合模型不同的位置，例如潜在的热惯性升高、发射率降低或更多孔隙的风化层。我们还发现，对于一系列孔隙度值，ALMA 在木卫二热表层深度（∼10-15 cm）以下约 3 毫米处进行观测，因此没有表现出与其他频段类似的日变化或残差特征。未来以更高的角分辨率对木卫二进行的观测可能会揭示可变的地下热物理特性的其他位置，而其他波长的观测将有助于我们了解风化层压实长度以及外部过程对浅层地下的影响。