Saturn's moon Enceladus holds 'three of four requirements' for life to exist
Life could exist on Saturn's moon Enceladus around hydrothermal vents similar to those found at the bottom of Earth's oceans, scientists believe.
The "exciting" discovery was made after the space probe Cassini flew through spray bursting from the moon's cracked icy surface.
Chemical analysis of the plume suggested conditions favourable for methanogenesis - the generation of methane by microbes that use hydrogen and carbon dioxide to obtain energy.
On Earth, methane-making bugs flourish in the vicinity of hydrothermal vents, fissures in the ocean floor that gush water heated by volcanic activity.
Like Jupiter's moon Europa, Enceladus is believed to be surrounded by a global watery ocean covered by thick ice.
In 2015, the American space agency's Cassini probe made a deep dive into a geyser-like plume of water and other material erupting from cracks in the south polar region of Enceladus.
The spacecraft's instruments registered molecular hydrogen and carbon dioxide, two ingredients critical for methanogenesis.
Hydrogen levels were high enough to imply a continual source, and were consistent with hydrothermal activity.
Writing in the journal Science, the US team led by Dr Hunter Waite, from the Southwest Research Institute in San Antonio, Texas, concluded: "Our analysis supports the feasibility of methanogenesis as an energy-releasing process that can occur over a wide range of geochemical conditions plausible for Enceladus' ocean."
However, the scientists pointed out that just because Enceladus has conditions suitable for methanogenesis, that does not prove anything is living there.
Leading British expert Professor Andrew Coates, from University College London, 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."
Enceladus, which is 502 kilometres (312 miles) across, is one of numerous moons orbiting Saturn, the largest of which, Titan, is bigger than the planet Mercury.
It has a rocky interior and icy surface with what is believed to be a salty ocean sandwiched between the two. Tidal heating caused by the moon's interaction with Saturn's powerful gravity prevents the ocean from freezing.
Soon after the Cassini orbiter began circling Saturn in 2005 it discovered water plumes venting into space from cracks at the moon's south pole.
Analysis has shown the plumes mainly to consist of tiny particles of water ice, with traces of methane, ammonia, carbon dioxide, salts, and simple organic molecules.
Silica nanoparticles were also detected, indicating a hot rocky interior reacting chemically with alkaline water.
When Cassini made its final dive through the plumes on October 28 2015, scientists focused on the search for hydrogen.
The results confirmed the presence of hydrogen as well as carbon dioxide, providing the raw material for methanogenesis and possibly life.
Nasa announced the discovery at a press conference at the agency's headquarters in Washington DC on Thursday.
The administration plans to send a spacecraft called Clipper to Jupiter's water-covered moon Europa in the 2020s to investigate whether conditions there are suitable for life.