NASA has long suspected that Jupiter's biggest moon, Ganymede, has a subsurface ocean, but that's hard to prove when it's in the outer solar system. Now, a team of scientists has found a way to provide evidence that an ocean does exist underneath its ice crust using the Hubble telescope. How? Well, Ganymede is the only moon in our solar system that has its own magnetic field. That, in turn, is connected to Jupiter's field since the moon's so close to the planet. This connection causes aurorae (like the northern lights here on Earth) that circle the moon's northern and southern poles, which turned out to be just what the scientists needed.