Ganymede is the largest moon in the solar system and new observations suggest the massive satellite may host a vast saltwater ocean.

The Hubble Space Telescope found evidence of oceans on the Jovian satellite that could hold more water than is present on Earth. Since the 1970's, some astronomers have suspected water oceans exist on the distant moon.  

Ganymede, roughly the same size as the planet Mercury, is far too cold for liquid water to exist on its surface. The ocean detected by astronomers would be located at least 100 miles under the frozen crust.

Aurorae above Ganymede were studied by astronomers using the Hubble Space Telescope. A saltwater ocean under the surface of the moon would create specific effects on the magnetic field surrounding the satellite. Studying the glow of hot, electrified gases over the poles of the satellite showed patterns suggesting a massive ocean exists within the body of the moon.

"I was always brainstorming how we could use a telescope in other ways. Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon's interior," Joachim Saur, at the University of Cologne, said.

Ganymede is the only moon in the solar system which possesses a magnetic field. As it orbits around Jupiter, the powerful magnetic field of the solar system's largest planet alters the field of the satellite. Observations of these changes, seen as a rocking motion, provided evidence of the sub-surface ocean.

A large body of salt water would result in the development of a secondary magnetic field that would counteract the field from Jupiter, in a process called magnetic friction. The Hubble team found the rocking motion of Ganymede's magnetic field covered just two degrees instead of a predicted level of six degrees, suggesting the effect of a vast saline ocean.

Life is found on Earth in any location where water is available, and many researchers believe that alien life may also require the liquid.  

"Identifying liquid water on other worlds - big or small - is crucial in the search for habitable planets beyond Earth," the Hubble team reported.

The findings from Hubble are the best evidence ever discovered for the presence of oceans of water on Jupiter's largest satellite.

Detection of the ocean deep beneath the surface of Ganymede was detailed in the Journal of Geophysical Research.