科学家认为，木卫三表层下约125英里处有一个盐水大洋，正好位于冰层中间。更深处被认为存在一个液体铁芯，为太阳系中卫星周围唯一磁场提供动力。木卫三每7天环绕木星一周，它的轨道倾斜几乎与地球正面相对。
The Hubble Space Telescope recently caught this shot of Jupiter's moon Ganymede just before it ducked behind the giant planet. The largest moon in our solar system, Ganymede is an icy rock even bigger than Mercury.
It's a gorgeous shot, but the image also reveals important information about Jupiter's atmosphere. As Ganymede passes behind the gas giant, light from the planet bounces off the moon, carrying with it clues about the chemicals that make up the haze above the Jovian clouds.
Also visible in the image is the Arizona-sized impact crater Tros on the moon's surface, with bright streaks of material blasted around it. Scientists think a saltwater ocean lies nearly 125 miles under the moon's surface, sandwiched between layers of ice. Even farther down, a liquid iron core is thought to exist, powering the only magnetic field around a moon in the solar system.
Ganymede circles around Jupiter approximately every seven days in an orbit tilted nearly head-on to Earth.
