"We have not found evidence of the presence of microbial life in the ocean of Enceladus, but the discovery of hydrogen gas and the evidence for ongoing hydrothermal activity offer a tantalizing suggestion that habitable conditions could exist beneath the moon's icy crust".
Meanwhile, scientists using the Hubble Space Telescope observed a warm jet of water shooting out of the warmest part of Europa, one of Saturn's moons that is composed of an icy layer over a salty liquid ocean of water.
The findings were reported Thursday in the journal Science by a team from Southwest Research Institute in San Antonio. The final step is the molecular hydrogen being produced, which has the chemical energy to support microbial systems in the ocean.
Cassini has no instruments that can detect life, so it will be up to future robotic visitors to seek out possible life on Enceladus, the scientists said.
"It really represents a capstone finding for the mission", said Cassini's project scientist, Linda Spilker, noting that the spacecraft has been circling Saturn for more than a decade.
This established that, while Enceladus is freezing on its surface, underneath is a liquid ocean.
"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment", NASA's Thomas Zurbuchen said.More news: Elon Musk says Tesla pickup and semi-trucks are coming
There are three essential ingredients for life: water, a source of energy for metabolism, and a mixture of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. Plumes of water vapor spew from cracks at the moon's south pole.
It means Enceladus may have the same single-celled organisms which began life on Earth, or more complex life still. This is because the rocky core of the icy Saturn moon is believed to have similar chemical properties to meteorites, which contain both sulfur and phosphorus.
This illustration shows Cassini diving through the Enceladus plume in 2015.
During earlier flybys, the spacecraft also sampled the gas plumes' composition. Cassini detected some of the chemical elements needed for life in plumes of gas and particles erupting from the moon's surface.
Meanwhile, Dr. David Clements, astrophysicist at Imperial College London, said: 'This discovery does not mean that life exists on Enceladus, but it is a step on the way to that result'.
Enceladus, a frozen moon nearly 10 times as far from the sun as Earth, at 900 million miles, was one of the least likely candidates.
Older results have suggested that the hot water is intermingling with the rock underneath the sea.