At least four distinct plumes of water ice spew out from the south polar region of Saturn's moon Enceladus. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 25, 2009. NASA/JPL/Space Science Institu

On Saturn, rain doesn't come from a cloud: it comes from the moon Enceladus, astronomers have recently discovered, solving a 14-year-old mystery.

For more than a decade scientists have pondered about Saturn's water source in the upper atmosphere, which they have found comes from the icy moon Enceladus.

Scientists from the ESA's Herschel space observatory found that the water expelled from the moon created a giant torus of water vapor around Saturn. Enceladus is the only moon in the Solar System known to affect the chemical composition of its parent planet.

Enceladus emits around 550 pounds (250 kilograms) of water vapor every second, through a set of jets from the south polar region, known as the Tiger Stripes because of their unique surface markings.

"The reason why so much water is produced is not clear," Paul Hartogh, of the Max Planck Institute for Solar System Research, told 3News. "Of course you have this warm area in the south - and that is probably created by some tidal forces - but the exact mechanism is not known. The amount of energy dissipating due to tidal forces is actually not enough to explain why so much water is produced there."

Enceladus is the sixth largest moon of Saturn. It was first discovered in 1789 by William Herschel, but little was known about it until recent years.

Scientist observed that the water forms a doughnut-shaped torus of vapor surrounding the ringed planet. The total width of the torus is more than 10 times the radius of Saturn, yet it is only about one Saturn radius thick.

Enceladus orbits Saturn at a distance of about four Saturn radii, refilling the torus with its jets of water. Even though its size is enormous, it has avoided detection until now because water vapor is transparent to visible light but not at the infrared wavelengths Herschel was designed to see.

"There is no analogy to this behaviour on Earth," said Hartogh, who led the collaboration on the analysis of these results. "No significant quantities of water enter our atmosphere from space. This is unique to Saturn."

Teams using ESA's Infrared Space Observatory first reported that source of water in 1997. Computer models of these latest observations show that about 3 to 5 percent of the water emitted by Enceladus ends up falling into Saturn.

Most of the water from Enceladus is lost into space and freezes on the rings or perhaps falls onto Saturn's other moons. The small portion that does fall into the planet ? enough to explain the water detected in its upper atmosphere ? is also responsible for producing additional oxygen-bearing compounds, such as carbon dioxide.

Despite its small size, Enceladus has multiple terrains to include old, heavily cratered surfaces and some young, tectonically distorted ones, some of which have regions with surface ages as young as 100 million years old.

In 2005, NASA's Cassini spacecraft did several close flybys of Enceladus, and saw a water-rich plume emitting from the moon's south polar region. There was also the presence of escaping internal heat and showed that Enceladus is geologically active today.

With all these emerging observations, it still remains that eventually, water in Saturn's upper atmosphere is transported to lower levels, where it will condense but the amounts are so tiny that the resulting clouds are not observable.

"Herschel has proved its worth again. These are observations that only Herschel can make," said Göran Pilbratt, ESA's Herschel Project Scientist. "ESA's Infrared Space Observatory found the water vapour in Saturn's atmosphere. Then NASA/ESA's Cassini/Huygens mission found the jets of Enceladus. Now Herschel has shown how to fit all these observations together."