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	Ganymede At a Glance
	Discovery 1610, Galileo Galilei Diameter 3,270 miles 5,262 km Distance from Jupiter 665,000 miles 1.1 million km Orbital Period 7.2 days Compare Moons

Ganymede is the solar system's largest moon -- larger than the planet Mercury. It consists of about half ice and half rock and metal. Grooves and ridges that crisscross its surface indicate that it has undergone great changes over the eons.

During several passes, the Galileo spacecraft saw mountains of ice, plus sheets of ice that erupted from volcanoes. It also saw deep canyons and broad, smooth plains created by the motions of Ganymede's crust.

Callisto At a Glance
Discovery 1610, Galileo Galilei Diameter 2,985 miles 4,806 km Distance from Jupiter 1.2 million miles 1.9 million km Orbital Period 16.7 days Compare Moons

Callisto
Like Europa, Callisto's icy surface may conceal an ocean. The case for an ocean is more tentative, but it is bolstered by a huge basin on one side of the moon. It was created by a powerful impact billions of years ago.

But there is no jumble of rocks and mountains on the opposite side of Callisto, as there is with big impact basins on our own moon. A deep ocean could have cushioned the impact, preventing shock waves from piling up rocks half a world away.

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