Khurana K K, Kivelson M G, Stevenson D J, Schubert G, Russell C T, Walker R J, Polanskey C
Institute of Geophysics and Planetary Physics, University of California, Los Angeles 90095, USA.
Nature. 1998 Oct 22;395(6704):777-80. doi: 10.1038/27394.
The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites-Io, Europa, Ganymede and Callisto-on each orbit. Initial results from the spacecraft's magnetometer have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence in the case of Europa.
自1995年12月7日以来，伽利略号航天器一直在环绕木星运行，并且在每次轨道运行时都会与四颗伽利略卫星之一——木卫一、木卫二、木卫三及木卫四相遇。航天器磁力计的初步结果表明，与木卫三以及可能的木卫一不同，木卫二和木卫四都没有明显的内部磁场。在此，我们报告了在木卫二和木卫四附近外部磁场（与木星内磁层相关）的扰动情况。我们将这些扰动解释为由卫星响应周期性变化的等离子体环境而产生的感应磁场引起。电磁感应要求卫星内部有涡流流动，而我们的计算表明，最有可能的解释是两颗卫星表面之下都存在具有显著电导率的层。我们认为，这些导电层很可能是由咸液态水海洋的存在所导致，在木卫二的情况下，已经有间接的地质证据证明这一点。
