ALM揭示的木星卫星欧罗巴前导半球与后随半球的地下热物理性质
Subsurface Thermophysical Properties of Europa's Leading and Trailing Hemispheres as Revealed by ALM
AI总结：
本研究利用ALMA观测数据，通过最佳拟合模型揭示了欧罗巴前导与后随半球地下孔隙率和热惯量的差异，并识别出与地表特征相关的热异常区域，为理解其浅层地下性质提供了新见解。
摘要：
我们给出了从欧罗巴近地表（约1-20厘米）三个不同深度确定的孔隙率——以及相应的有效热惯量——的最佳拟合值。欧罗巴地下上层约20厘米的孔隙率在前导半球变化于75%-50%之间（$\Gamma_{eff}\approx50-140$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$），而在后随半球变化于50%-40%之间（$\Gamma_{eff}\approx140-180$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$）。通过与这些模型比较产生的残差图揭示了无法用全球均匀孔隙率模型重现的热异常特征。我们将这些区域与欧罗巴的表面地形和已知成分变化进行了比较。我们发现，一些温暖热异常的实例在前导半球和后随半球上均与已知的地理或成分特征共位；冷温度异常与先前观测到的含有纯净、结晶水冰的表面以及Pwyll陨石坑的广阔射线良好相关。异常区域对应于地下性质不同于我们最佳拟合模型的位置，例如可能升高的热惯量、降低的发射率或更多孔的月壤。我们还发现，ALMA在约3毫米波段的观测探测深度低于欧罗巴的热肤深度（约10-15厘米），因此不表现出指示昼夜变化的特征或与其他频段相似的残差。未来以更高角分辨率对欧罗巴的观测可能会揭示更多地下热物理性质变化的区域，而其他波长的观测将有助于我们理解月壤压实长度以及外部过程对浅层地下的影响。
英文摘要：
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 75-50% ($Γ_{eff}\approx50-140$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$) on the leading hemisphere and 50-40% ($Γ_{eff}\approx140-180$ 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.
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