THE GALILEAN MOONS
CALLISTO lies farthest from the worst of its giant host’s lethal magnetospheric radiation. Additionally, it never fell victim to the savage gravitational tugging, squeezing, and mauling that Ganymede, Europa, and Io have, leading to an interior that never fully melted and is only partially differentiated. The least geologically active of the three icy worlds, battered Callisto evolved more benignly. The low levels of radiation coupled with the moon’s geological stability have seen it considered prime real estate for a future human base. Callisto may have a liquid ocean, though it’s likely buried deep below its surface.
Diameter: 3,000 miles (4,800 km)
1.17 million miles (1.88 million km) from Jupiter GANYMEDE is the largest moon in the solar system; it is wider (but less massive) than Mercury. The moon’s two-toned surface reflects distinct terrain types: one dark and heavily cratered, the other brighter and overlaid by ridges and furrows. Both are ancient, at least 4 billion years old, and researchers divide the moon’s geologic history into three phases: first characterized by impacts; then tectonic upheaval; and finally the current, mostly quiescent phase. The moon’s magnetic field offers persuasive evidence for a liquid nickel-iron core several hundred miles thick and the Galileo mission discovered tantalizing hints of a salty subsurface ocean.
Diameter: 3,270 miles (5,260 km)
665,000 miles (1.1 million km) from Jupiter
EUROPA sits close enough to Jupiter that it is subjected to stronger tidal forces than Ganymede, which have kept it more geologically alive. Europa hosts a mixture of bright plains and mottled terrain, with ridgelike cracks and veinous linear scratches. A relative paucity of craters implies a surface some 20 million to 40 million years old in places, perhaps still in the throes of tectonic activity. Europa also has one of the solar system’s brightest surfaces — it reflects 62 percent of the light that falls on it, akin to glacial ice — and likely possesses beneath its crust a warm, slushy ocean.
Diameter: 1,940 miles (3,100 km) 417,000 miles
(671,000 km) from Jupiter
IO is the innermost Galilean moon. It is a violently volcanic world stretched by tides generated through interactions with Jupiter and its fellow moons. Its surface is pocked with volcanoes and lakes of molten lava, while its tenuous atmosphere is largely sulfur dioxide.
Io has no water; thus, unlike its icy compatriots, it is likely too inhospitable for any type of life as we know it. Io and its passage through Jupiter’s magnetic field have a profound effect on the gas giant, playing a role in generating the lightning and aurorae that light up the planet’s atmosphere.
Diameter: 2,260 miles (3,640 km) 262,000 miles (422,000 km) from Jupiter