The Galileo space probe, which has been orbiting Jupiter for nearly three years, has sent back the strongest evidence yet that salt-water oceans exist on Europa and Callisto. Finding deep oceans is vital in the search for life forms as organisms could not evolve without the benefit of liquid water.
Scientists have already gathered indirect geological evidence that Europa has an icy ocean. Now researchers from Nasa and the University of California, Los Angeles have analysed disturbances of Jupiter's magnetic fields created by Europa and Callisto, indicating that the moons have sub-surface oceans.
The moons do not have magnetic fields so finding that they can disturb Jupiter's magnetic field suggests they must be able to have fields induced in them as a result of deep "conducting layers" under their surface - that is, oceans of salty water.
Nasa has already identified Europa as a possible site of life and is planning to send a probe to investigate.