Nasa's Galileo spacecraft has found powerful new evidence of a liquid ocean on a moon of Jupiter.

The existence of a subterranean sea on Europa has been suspected but the magnetic data gathered by the spaceprobe during a flyby on 3 January is compelling.

The surface of Europa is covered in an icy crust but swirling waters now seem certain to lie below.

"I think these findings tell us that there is indeed a layer of liquid water beneath Europa's surface," said Dr Margaret Kivelson, principal investigator for the magnetometer carried by Galileo.

The spacecraft flew just 350 kilometres (220 miles) above the frozen moon whilst the magnetometer studied changes in the direction of Europa's magnetic field.

Such changes in the magnetic field would show that the moon must harbour an electrically-conducting material and a salty ocean is by far the most likely candidate for this.

"Jupiter's magnetic field at Europa's position changes direction every 5.5 hours," Dr Kivelson explained. "This changing magnetic field can drive electrical currents in a conductor, such as an ocean. Those currents produce a field similar to Earth's magnetic field, but with its magnetic north pole near Europa's equator and constantly moving. In fact, it is actually reversing direction entirely every 5.5 hours."

The ocean appears to exist in the outer 100 km (60 miles) of the planet, which is the approximate thickness of the ice and water layer.

These latest findings are consistent with previous Galileo images and data which show a tortured surface seemingly formed when Europa's surface ice broke and rearranged itself while floating on a sea below. Further theoretical work is under way to analyse the fluid layer and its properties.