ESA’s Herschel space observatory has solved the 14-year mystery of where the water in Saturn's upper atmosphere comes from - it's being expelled from the planet's moon Enceladus.
The moon's expelling around 250 kg of water vapour every second, via a set of jets in the south polar region known as the Tiger Stripes because of their surface markings.
The water creates a ring of vapour surrounding Saturn at a distance of about four Saturn radii, which was first reported in 1997 by teams using ESA’s Infrared Space Observatory.
Computer models indicate that about three to five percent of the water expelled by Enceladus ends up falling into Saturn. It is also responsible for the production of additional oxygen-bearing compounds, such as carbon dioxide.
It means Enceladus is the only moon in the solar system known to influence the chemical composition of its parent planet.
"There is no analogy to this behaviour on Earth," says Paul Hartogh of the Max Planck Institut für Sonnensystemforschung.
"No significant quantities of water enter our atmosphere from space. This is unique to Saturn."
Although most of the water from Enceladus is lost into space, freezes on the rings or perhaps falls onto Saturn’s other moons, the small fraction that does fall into the planet is enough to explain the water observed in its upper atmosphere.
Ultimately, the water in Saturn's upper atmosphere is transported to lower levels, where it will condense - but in such tiny quantities that it's not possible to see the resulting clouds.