"Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.
"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at Headquarters in Washington.
"These results demonstrate the interconnected nature of NASA's science missions that are getting us closer to answering whether we are indeed alone or not," Zurbuchen said.
The study from researchers with the Cassini mission, published in the journal Science, indicates hydrogen gas - which could potentially provide a chemical energy source for life - is pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor.
The presence of ample hydrogen in the moon's ocean means that microbes - if any exist there - could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water.
This chemical reaction, known as "methanogenesis" because it produces methane as a byproduct, is at the root of the tree of life on Earth, and could even have been critical to the origin of life on our planet.
Life as we know it requires three primary ingredients: liquid water; a source of energy for metabolism; and the right chemical ingredients, primarily carbon, hydrogen, nitrogen, oxygen, phosphorous and sulphur.
With this finding, Cassini has shown that Enceladus - a small, icy moon a billion miles farther from the Sun than Earth - has nearly all of these ingredients for habitability.
Cassini has not yet shown phosphorous and sulphur are present in the ocean, but scientists suspect them to be, since the rocky core of Enceladus is thought to be chemically similar to meteorites that contain the two elements.
The Cassini spacecraft detected the hydrogen in the plume of gas and icy material spraying from Enceladus during its last, and deepest, dive through the plume on October 28, 2015.
Cassini also sampled the plume's composition during flybys earlier in the mission.
From these observations scientists have determined that nearly 98 per cent of the gas in the plume is water, about one per cent is hydrogen and the rest is a mixture of other molecules including carbon dioxide, methane and ammonia.