Ganymede is the largest moon, and one of the four Galilean moons of planet Jupiter. Ganymede is larger than Mercury and Pluto, and 2/3 the size of Mars. It would be considered a planet if it were orbiting the Sun instead of Jupiter.
Ganymede is the only mooon in the solar system to possess a magnetic field, has a tenuous oxygen atmosphere, and is believed to have six times the water on Earth in the form of several subsurface saltwater oceans sandwiched between layers of ice.
Ganymede, along with Callisto, Europa, and Io, were discovered by the Italian astronomer Galileo Galilei in 1610.
Pan, zoom, or go full-screen with this megapixel image of Ganymede.
A few notable surface features on Ganymede:
Ganymede, which had been a million kilometers away across the system a few seconds before, zoomed toward them, flicked past, and was lost to sight behind them.
- Dan Simmons, Illium