Jupiter's moon Ganymede is the largest moon in the solar system, bigger than the planet Mercury. It's also the only moon known to have its own magnetic field.

Jupiter's moon Ganymede is the largest moon in the solar system, bigger than the planet Mercury, NASA says.

Ganymede is bigger than both Mercury and the dwarf planet Pluto. NASA says there is strong evidence that it has an underground saltwater ocean.

It is also the only moon known to have its own magnetic field, something typically found on planets like Earth. The field causes auroras, ribbons of glowing gas that circle the moon's poles.