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Its photographs of Callisto, darkest and outermost of the Galilean moons, showed that this gloomy sphere may be at least half water. Ganymede also seems watery, but appears to have an ice crust and possibly a mud core. Remarkably, both moons are dotted with dark spots that some scientists are tentatively calling ice volcanoes—craters that spew ice instead of fire and lava. Still closer to Jupiter, Europa apparently hides a rocky core beneath its bright icy surface.
But the most curious of all these major moons is the innermost, lo (pronounced eye-oh). Roughly the size of the earth's own moon, it has reddish polar caps, a yellowish sodium cloud cover and a strange surface chemistry that may be a consequence of intense radiation bombardment. On its closest approach, Voyager will come within 18,800 km (11,650 miles) of this mysterious moon. Then, as Voyager sweeps away, its instruments will get glimpses of the other Jovian moons, perhaps even a tiny 14th moon, which was spotted several years ago. ∙