During the Galileo spacecraft’s flyby of Europa, magnetic field measurements detected an inductive signal due to the response of Europa’s interior conductors to temporal fluctuations in the Jovian magnetic field. In contrast, no signatures of intrinsic magnetic field originating from the dynamo motion in the metallic core were acquired. These measurements suggest that a global sub-surface ocean containing electrolytes exists beneath the solid ice shell and that the metallic core lacks convection. Europa’s interior is expected to be divided into the metallic core, rocky mantle and hydrosphere based on the moment of inertia factor estimated from gravity field measurements. Specifically, the thickness of the outermost water layer is 120–170 km, and the radius of the metallic core is 0.12–0.43 times the surface radius. No systematic investigation of Europa’s internal evolution has been conducted to estimate the current state of the subsurface ocean and to explain the absence of a core dynamo field within such uncertainty for internal structure and material properties (especially ice properties). Herein, I performed a numerical simulation of the long-term thermal evolution of Europa’s interior and investigated the temporal changes in the ocean thickness as well as the temperature and heat flow of the metallic core. If the ice reference viscosity is greater than 5 × 10 Pas, the sub-surface ocean can persist even in the absence of tidal heating. In the case of a tidal heating of 10 mW/m and 20 mW/m, the ice shell thickness is 90 km if the ice reference viscosity is 1 × 10 and 1 × 10 Pas, respectively. Regardless of the ice reference viscosity, if the tidal heating is 50 mW/m, the shell thickness will be 40 km. The thermal history of the metallic core is determined by the hydrosphere thickness and the metallic core density, and is unaffected by variations in the ice shell (ocean) thickness. Preferred conditions for the absence of the core dynamo include CI chondritic abundance for the long-lived radioactive isotopes, lower initial core–mantle boundary (CMB) temperature and thicker hydrosphere. The core may be molten without convection if the composition is near the eutectic in a Fe–FeS alloy, or not molten (without convection) if the composition is near the Fe or FeS endmember. Specifically, if the rocky mantle has a CI chondritic radioisotope abundance, any core composition and hydrosphere thickness allow the absence of the core dynamo if the initial temperature at the CMB is lower than 1,250 K. If the rocky mantle has the ordinary chondritic radioisotope abundance, or a higher initial temperature (1,500 K) at the CMB, the core density lower than 6,000 kg/m is preferred for the absence of the core dynamo. In the case of the core composition near the eutectic one, a hydrosphere thicker than 150 km is required for the lacking core dynamo. The lower pressure of Europa’s rocky mantle due to its thinner hydrosphere compared with that of Ganymede may facilitate heat transfer in the mantle, lowering its temperature and making dynamo motion more challenging.

中文翻译：

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木卫二存在地下海洋和不存在金属核心驱动磁场的结构条件

在伽利略号航天器飞越木卫二期间，磁场测量检测到了由于木卫二内部导体对木星磁场时间波动的响应而产生的感应信号。相反，没有获得源自金属核心中的发电机运动的固有磁场的特征。这些测量表明，固体冰壳下方存在含有电解质的全球地下海洋，并且金属核心缺乏对流。根据重力场测量估计的惯性矩因子，木卫二的内部预计将分为金属核心、岩石地幔和水圈。具体来说，最外层水层的厚度为120-170公里，金属核的半径为表面半径的0.12-0.43倍。尚未对木卫二的内部演化进行系统的调查，以估计地下海洋的当前状态，并解释在内部结构和材料特性（尤其是冰特性）如此不确定的情况下缺乏核心发电机场的原因。在这里，我对木卫二内部的长期热演化进行了数值模拟，并研究了海洋厚度的时间变化以及金属核心的温度和热流。如果冰参考粘度大于 5 × 10 Pas，即使没有潮汐加热，地下海洋也可以持续存在。在潮汐加热为 10 mW/m 和 20 mW/m 的情况下，如果冰参考粘度分别为 1 × 10 和 1 × 10 Pas，冰壳厚度为 90 km。无论冰参考粘度如何，如果潮汐加热为 50 mW/m，则壳厚度将为 40 km。金属核的热历史由​​水圈厚度和金属核密度决定，并且不受冰壳（海洋）厚度变化的影响。不存在核心发电机的首选条件包括长寿命放射性同位素的 CI 球粒陨石丰度、较低的初始核心-地幔边界 (CMB) 温度和较厚的水圈。如果 Fe-FeS 合金中的成分接近共晶，则核心可能会在没有对流的情况下熔化；如果成分接近 Fe 或 FeS 端元，则核心可能不会熔化（没有对流）。具体来说，如果岩石地幔具有 CI 球粒陨石放射性同位素丰度，则如果 CMB 的初始温度低于 1,250 K，则任何核心成分和水圈厚度都允许不存在核心发电机。如果岩石地幔具有普通球粒陨石放射性同位素丰度，或 CMB 处较高的初始温度（1,500 K），如果没有核心发电机，则优选核心密度低于 6,000 kg/m。在核心成分接近共晶成分的情况下，缺少核心发电机需要厚度超过 150 km 的水圈。与木卫三相比，木卫二的岩石地幔由于其水圈更薄而压力较低，这可能有利于地幔中的热传递，降低其温度并使发电机运动更具挑战性。