The universe is pretty lousy with water, it's actually one of the most common molecules in the universe. But most of it is in the form of either ice or vapor. When humans look at sites for space exploration, a real bonus is a source of liquid water. Although it's still pretty rare it's actually more common than you might think. Saturn's moon Enceladus and Jupiter's moon Europa are already known to have liquid oceans under their frozen surfaces. Now according to a recent article on sciencenews.org, we can add the Jupiter moon Ganymede to that list.
Sometimes scientists can literally observe water and ice directly erupting from a geyser on a distant body in space. However, it wasn't so easy for the researchers from the University of Cologne, Germany. To find out if Ganymede had a liquid ocean, they had to use the Hubble Space Telescope to observe it's Aurora. Like the Aurora Borealis on earth, this is mainly an interaction between charged particles in Ganymede's atmosphere and magnetic fields. Jupiter's massive magnetic field should have caused Ganymede's Aurora to shift back and forth by about 6 degrees.
What they found however was only a shift of about 2 degrees. Their leading hypothesis for this discrepancy is a conductive liquid under the surface that is causing a secondary magnetic field, counteracting Jupiter's. And based off other bodies in that system, the leading candidate for a conductive liquid is a saltwater ocean underneath the surface.
The bad news is that if we ever land any kind of mission on Ganymede, that water won't be easy to get to. The abstract of their original paper suggests that the the ocean is roughly between 150 and 250 km below the surface. With the most extreme estimate that it could be up to 330 km down before the surface is reached.
Jupiter's moon Callisto and the dwarf planet Ceres series may also contain some liquid water. It's unlikely, but non-technically impossible that life could survive in such extreme conditions. Either way it will be some time before we find out.
