NASA's Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter's largest moon. The subterranean ocean is thought to have more water than all the water on Earth's surface. Identifying liquid water is crucial in the search for habitable worlds beyond Earth and for the search for life, as we know it.
NASA's Hubble Space Telescope observed aurorae on the moon that are controlled by Ganymede's magnetic fields. Two auroral ovals can be seen over northern and southern mid-latitudes. Hubble measured slight shifts in the auroral belts due to the influence of Jupiter's own immense magnetic field. This activity allows for a probe of the moon's interior. The presence of a saline ocean under the moon's icy crust reduces the shifting of the ovals as measured by Hubble. As on Earth, Ganymede's aurorae are produced by energetic charged particles causing gases to fluoresce.
Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot electrified gas, in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, it is also embedded in Jupiter's magnetic field. When Jupiter's magnetic field changes, the aurorae on Ganymede also change, "rocking" back and forth.
This discovery adds Ganymede to the list of moons that scientist suspect could have an ocean under the surface, such as Jupiter's icy moon Europa and Saturn's tiny moon Enceladus that spews salt water geysers into space. No doubt astronomers will give this new evidence more attention and uncover new wonders of this alien world.