Ganymede

Ganymede is the largest moon, and one of the four Galilean moons of the planet Jupiter. Ganymede is larger than the planets 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 moon in the solar system to possess a magnetic field, has a tenuous oxygen atmosphere, and is believed to have 6 times the water on Earth in the form of several subsurface saltwater oceans sandwiched between layers of ice.

Type

Planetoid / Moon

Location under

Jupiter