AI 中文总结
本研究利用金石雷达和绿岸望远镜在2011-2024年对木卫二开展雷达观测，获得迄今经度覆盖最全面的测量数据，测得其雷达反照率与极化比等参数，验证了相干背散射反对效应并给出雷达波穿透深度上限。
AI 中文摘要
木星的三颗伽利略卫星——木卫二（Europa）、木卫三（Ganymede）和木卫四（Callisto）——因其冰壳和疑似地下海洋而具有特别的科学研究价值。然而，自1987-1991年的观测以来，人们一直未对这些冰卫星的雷达特性进行测量。由于无线电波可以穿透纯冰到达相当深的位置，雷达观测为表征这些卫星冰壳的地下特性提供了强有力的手段，可为行星演化研究提供关键见解。为填补这些卫星雷达研究中长期存在的空白，我们在2011-2024年间使用金石（Goldstone）3.5厘米太阳系雷达和绿岸望远镜（GBT）对木卫二进行了观测。本文中，我们展示了迄今为止覆盖经度范围最全面的木卫二雷达测量数据集，并描述了其盘面积分雷达特性。基于单静态金石观测数据，我们测得两种圆极化的雷达反照率值分别为$\boldsymbol{\rm \hatσ_{OC}}$ = 0.92 ± 0.11和$\boldsymbol{\rm \hatσ_{SC}}$ = 1.35 ± 0.13（未加权均值与均方根离散度），圆极化比$μ_c$ = 1.44 ± 0.12（加权均值与均方根离散度）。通过绿岸望远镜以双静态方式获得的数值与上述结果相近。$μ_c$的数值表明木卫二的雷达散射特性存在前导侧与后随侧的二分性。我们的研究结果支持相干背散射反对效应（CBOE）的存在，该效应是目前解释冰质伽利略卫星异常雷达散射特性最被广泛接受的物理机制。由于我们采用双静态配置观测木卫二，因此可以确定木卫二CBOE峰宽度的下限为36角秒，这也给出了X波段雷达波在木卫二上穿透深度的上限为32米（约1000个波长）。
英文摘要
Three of Jupiter's Galilean satellites - Europa, Ganymede, and Callisto - are of particular scientific interest due to their icy shells and suspected subsurface oceans. However, the radar properties of the icy satellites have not been measured since observations in 1987-1991. Because radio waves can penetrate pure ice to considerable depths, radar observations provide a powerful means of characterizing the subsurface properties of the icy shells of these satellites, offering key insights into planetary evolution. We have observed Europa using the Goldstone 3.5-cm Solar System Radar and the Green Bank Telescope (GBT) in 2011-2024 in order to address a longstanding gap in the radar studies of these moons. In this paper, we present the most longitudinally comprehensive set of radar measurements of Europa to date and describe its disk-integrated radar properties. On the basis of monostatic Goldstone data, we find radar albedo values in two circular polarizations of $\hatσ_{\rm OC}$ = 0.92 $\pm$ 0.11 and $\hatσ_{\rm SC}$ = 1.35 $\pm$ 0.13 (unweighted mean and root-mean-square dispersion), with a circular polarization ratio of $μ_c$ = 1.44 $\pm$ 0.12 (weighted mean and root-mean-square dispersion). Values obtained bistatically at the GBT are similar. The $μ_c$ values suggest a leading-vs-trailing side dichotomy in radar scattering properties on Europa. Our results support the existence of the coherent backscatter opposition effect (CBOE), currently the most widely accepted physical mechanism that explains the unusual radar scattering properties of the icy Galilean satellites. Because we observed Europa with a bistatic configuration, we can place a lower bound on the width of Europa's CBOE peak equal to 36 arcsec, which provides an upper bound of 32 m ($\sim$1000 wavelengths) on the penetrating depth of X-band radar waves at Europa.
Comments31 pages, 17 figures, 8 tables, supplementary figures for the figure set and CSV files for the spectra are included as ancillary files. Accepted for publication in PSJ
