Anderson J D, Lau E L, Sjogren W L, Schubert G, Moore W B
Jet Propulsion Laboratory, California Institute of Technology, Pasadena 91109-8099, USA.
Nature. 1997 May 15;387(6630):264-6. doi: 10.1038/387264a0.
Before the arrival of the Galileo spacecraft at Jupiter, models for the interior structure of the four galilean satellites--Io, Europa, Ganymede and Callisto-ranged from uniform mixtures of rock and ice (that is, undifferentiated objects) or rocky cores surrounded by a mantle of water ice. Now it appears that Io has a large metallic core and that Ganymede is strongly differentiated, most probably into a three-layer structure consisting of a metallic core, a silicate mantle and a deep outer layer of ice. Direct information on the interior structure of Callisto determined from previous spacecraft fly-bys was essentially limited to an estimate of the mean density being intermediate between pure ice and pure rock. Here we report measurements of Callisto's gravitational field which reveal that, in contrast to Io and Ganymede, this galilean satellite is most probably a homogeneous object consisting of a solar mixture of 40% compressed ice and 60% rock (including iron and iron sulphide). Callisto's undifferentiated state is consistent with the apparent lack of an intrinsic magnetic field, and indicates that the outermost galilean satellite has not experienced a heating phase sufficiently high to separate its rock and metal components from the lighter ices.
在伽利略号宇宙飞船抵达木星之前，关于木卫一、木卫二、木卫三及木卫四这四颗伽利略卫星内部结构的模型，范围涵盖从岩石与冰的均匀混合体（即未分化天体）到被水冰地幔环绕的岩石内核。如今看来，木卫一有一个巨大的金属内核，木卫三则有显著分化，很可能形成了一个由金属内核、硅酸盐地幔以及深层外层冰组成的三层结构。此前通过宇宙飞船近距离飞越所确定的关于木卫四内部结构的直接信息，基本上仅限于估计其平均密度介于纯冰和纯岩石之间。在此我们报告对木卫四引力场的测量结果，该结果显示，与木卫一和木卫三不同，这颗伽利略卫星很可能是一个由40%压缩冰和60%岩石（包括铁和硫化铁）的太阳混合体构成的均匀天体。木卫四未分化的状态与明显缺乏固有磁场相符，这表明最外层的伽利略卫星未曾经历过足以使其岩石和金属成分与较轻的冰分离的高温加热阶段。
