Ganymede - the largest moon in the solar system - has a salty ocean underneath the icy shell that surrounds it, NASA said on Thursday.

The ocean is believed to be far deeper than any on Earth - some 100 kilometers (60 miles) deep and buried under some 150 kilometers of ice.

The ocean was detected by scientists using the Hubble Space Telescope, which observed Ganymede's magnetic field. The telescope looked at changes that took place to the moon's colorful polar auroras - similar to the Earth's aurora borealis - which give an indication about what lies beneath the surface of moons and planets.

There has long been speculation that Ganymede could have an ocean, although proof had been elusive, despite tantalizing clues sent back by NASA's Galileo spacecraft. Scientists now say they are sure.

"Since the 1970s, there were speculations and models that Ganymede could possess an ocean," said Joachim Saur of the University of Cologne in Germany. "We do not have these ambiguities anymore," he told reporters, with the Hubble findings providing: "the best evidence to date for the existence of an ocean on Ganymede."

The four big moons, with Ganymede the largest, were all discovered the same year - in 1610 - by Galileo

With a diameter of 5,260 kilometers, Ganymede is slightly larger than the planet Mercury.

More 'waterworlds' dicovered

On Wednesday, a separate team of scientists reported that one of Saturn's moons, Enceladus, may have hot springs bubbling beneath its cold surface. There is also believed to be water below the surface of Jupiter's moon Europa, while another of the planet's moons, Callisto, is thought to have subterranean water.

"The solar system is now looking like a pretty soggy place," said NASA headquarters head of planetary science Jill Green, after the latest find.

Ganymede is one of dozens of confirmed moons that orbit Jupiter, and it is among the four large moons of the planet that were discovered in 1610, by the Italian scientist Galileo.

The prospect of a "deep ocean under the icy crust of Ganymede opens up further possibilities for life beyond Earth," said John Grunsfeld, a NASA administrator.

A European Space Agency mission to be launched in 2022 is expected to circle Jupiter and take a closer look at the moons, including Ganymede.

rc/bw (AFP, AP, dpa, Reuters)