A diagram illustrates the possible interior of Saturn's moon Enceladus based on a gravity investigation by Nasa's Cassini spacecraft and NASA's Deep Space Network, courtesy of NASA. The gravity measurements suggest an ice outer shell and a low density, rocky core with a regional water ocean sandwiched in between at high southern latitudes. REUTERS/NASA

Nasa's Cassini spacecraft has discovered a large underground ocean of liquid water beneath the surface of Enceladus, Saturn's small moon that is barely 500 km in diameter.

Gravity measurements of Enceladus from 2010 to 2012 by Cassini revealed a dense material beneath the moon's icy surface in the South Polar Region.

Scientists say the material is most likely liquid water, according to the study published in the 4 April issue of the journal Science.

"There must be a denser material at depth that compensates the missing mass: very likely liquid water, which is seven percent denser than ice," lead author of the study, Luciano Iess of Sapienza University of Rome, said in a statement.

Enceladus has jets of water vapour and ice gushing from its south pole.

The subsurface sea is about 10km deep and extending up to south latitudes. It exists beneath 30-40km thick ice shell, according to Nasa.

"This then provides one possible story to explain why water is gushing out of these fractures we see at the South Pole," David Stevenson of the California Institute of Technology, one of the study's co-authors, said.

Scientists see the hidden ocean inside Saturn's moon as a potential area to look for extraterrestrial microbes.

"The subsurface ocean evidence supports the inclusion of Enceladus among the most likely places in our solar system to host microbial life."

"Material from Enceladus' south polar jets contains salty water and organic molecules, the basic chemical ingredients for life," said Linda Spilker, Cassini's project scientist.

"Their discovery expanded our view of the 'habitable zone' within our solar system and in planetary systems of other stars. This new validation that an ocean of water underlies the jets furthers understanding about this intriguing environment."