NASA announced today that researchers using the Hubble space telescope have detected the presence of a saltwater ocean on Ganymede, Jupiter's largest moon.

Scientists speculated for years about the existence of a liquid ocean under the icy surface of the moon, and geologists even mapped Ganymede's rocky exterior to get a better idea of its features. But until now, they weren't completely sure what was under the crust.

The key to NASA's recent conclusion was Ganymede's vivid aurora, which was first observed in the 1990s. Ganymede's aurora occurs in two belts near its magnetic poles, just like the auroras on Earth. And just like our planet, scientists believe that Ganymede has an iron core, which produces a magnetic field. When a planet or moon's magnetic field interacts with gasses in the atmosphere, auroras thrive.