木卫一的表面能常保年轻，是因为它有许多活火山。
The unusual surface of Io is kept very young by its system of active volcanoes.
一个故事是木卫四。
木卫一和木卫二跟地球所见的月亮是相同的大小。
The Jovian Io and Europa are about the same size as Earth's moon.
木卫一和木卫二跟地球所见的月亮是相同的大小。
The Jovian Io and Europa are about the same size as Earth's moon.
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