Jupiter's giant moon Ganymede is the only known moon to have its own magnetic field — and it may be heating up in a process "not yet observed anywhere else," new research suggests.

One of the four Galilean satellites swirling around Jupiter, Ganymede is the largest moon in the solar system. At nearly 3,300 miles (5,300 kilometers) in diameter, it's more than 1,000 miles (1,600 km) wider than Earth's moon and slightly bigger than Mercury, our tiniest planetary tot. (Jupiter has more than 100 confirmed moons, with the largest four known as the Galilean moons.)