"If we knew that life had started independently in two places in our solar system, then we could be pretty confident that life also got started on some of the tens of billions of planets and moons around other stars in our galaxy."
Liquid oceans exist kilometres below the surface on Enceladus, so to find out what is happening in the underground seas scientists must rely on the plumes of spray that shoot up into the atmosphere through cracks in the ice.
In October 2015, Nasa sent Cassini into a deep dive through one of those plumes and discovered hydrogen and carbon dioxide.
In a report of their findings published today in the journal Science, scientists said that the "only plausible" source for the hydrogen was chemical reactions between warm water and rocks on the ocean floor.
Crucially, if hydrogen is present it can mix with carbon dioxide to form methane, which is consumed by microbes in the deep, dark seas of our own planet.
"Saturn's moon Enceladus has an ice-covered ocean, and a plume of material erupts from cracks in the ice," said Professor Hunter Waite, of Southwest Research Institute (SwRI) in San Antonio, principal investigator for Cassini's mass spectrometre instrument that detected the hydrogen.
"The plume contains chemical signatures of water-rock interaction between the ocean and a rocky core. We find that the most plausible source of this hydrogen is ongoing hydrothermal reactions of rock containing reduced minerals and organic materials.
"On the modern Earth, geochemically derived fuels such as hydrogen support thriving ecosystems even in the absence of sunlight."
Enceladus is the sixth largest moon of Saturn, and was discovered in 1789 by British astronomer William Herschel. It is about 500km in diameter and approximately 1300 million kilometres from Earth.
Scientists had long suspected water could exist on the moon because of the extreme tidal forces acting on the satellite from Saturn's gravity.
In 2005 Nasa launched Cassini to explore Saturn and its moons, and in 2015 discovered that Enceladus wobbled slightly as it orbited the planet, which could only be accounted for its outer shell was not frozen solid to its interior. A global ocean must be present under the icy surface, the experts concluded.
Since then, researchers have been studying data sent back from the spacecraft to see if the instruments on board had picked up any other clues that life might be present.
The new results are the strongest indication yet that Enceladus has all the conditions needed for life to form. If life is present, it could resemble single-celled tube-like extremophiles, which have lived in hydrothermal vents on Earth for billions of years.
Rothery added: "We now have the last piece of evidence needed to demonstrate that life is possible there.
"This is life that needs neither oxygen nor sunlight, and may be the form in which life on Earth began, before some of it adapted to other conditions."
Enceladus is so far the furthest rock from the sun that could support life in the solar system. Some scientists think Uranus' moon Ariel may have a liquid ocean, but it has not yet been proven.
Although scientists previously thought Enceladus' icy crust was about 20km thick, recent data from Cassini has shown that at the south pole, it could be as little as 4km deep.
Imperial College astrophysicist Dr David Clements said: "We have long suspected that hydrothermal processes are behind the Enceladus plumes and the liquid ocean that fuels them, so this result is fully consistent with that picture.
"This discovery does not mean that life exists on Enceladus, but it is a step on the way to that result.
"It doesn't really tell us anything about how life started on Earth. But it is great to see confirmation that similar hydrothermal processes are at work elsewhere in the solar system."
UCL physics Professor Andrew Coates said: "This is an exciting and remarkable result, which shows that Enceladus may actually be habitable.
"We know that the four requirements for life as we know it are liquid water, the right chemistry, a source of energy and enough time for life to develop. But now, we know that three of the four conditions are there on Enceladus - and this distant moon now joins Mars and Europa as the best potential locations for life beyond Earth in our solar system."
This story was orginally published by the Daily Telegraph.
