Ganymede, Jupiter's largest moon, is also the solar system's largest satellite, even larger than the planet Mercury. It is also the only celestial body aside from Earth (and the gas giants) to have an intrinsic magnetic field. As if this didn't make the icy body interesting enough, scientists also predict that it has a massive interior ocean with more water than all of Earth's oceans combined. At present, the European Space Agency's (ESA) Jupiter Icy Moons Explorer (JUICE) is in transit to Ganymede to explore it for signs of habitability.