Ganymede is the only known moon with an active dynamo. No mission has discovered intrinsic magnetism at the other Galilean satellites: Io, Europa, and Callisto. A dynamo requires a large magnetic Reynolds number, which in turn demands, for these moons, a large metallic core that is cooling fast enough for convection. Here we quantify these requirements to construct a regime diagram for the Galilean satellites. We compute the internal heat fluxes that would sustain a dynamo over the wide ranges of plausible radii for their metallic cores. Below a critical radius, no plausible heat flux will sustain a dynamo. Europa likely sits on the opposite side of this limit than Ganymede and Io. We predict that future missions may confirm a small (or absent) core, meaning that Europa could not sustain a dynamo even if its interior were cooling as quickly as Ganymede's core.

中文翻译：

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伽利略卫星 Dynamo 行动的关键核心尺寸


木卫三是已知的唯一一颗拥有活跃发电机的卫星。没有任务发现其他伽利略卫星的内禀磁：木卫一、木卫二和木卫四。发电机需要一个大的磁雷诺数，而对于这些卫星来说，这反过来又需要一个大的金属核心，该核心的冷却速度足够快，以便对流。在这里，我们量化了这些要求，以构建伽利略卫星的政权图。我们计算了在金属芯的宽合理半径范围内维持发电机的内部热通量。在临界半径以下，任何合理的热通量都无法维持发电机。木卫二可能位于这个限制的另一侧，而不是木卫三和木卫一。我们预测未来的任务可能会确认一个小（或不存在）的核心，这意味着即使木卫二内部的冷却速度与木卫三的核心一样快，它也无法维持发电机。