For the past 14 years, Saturn's water source has been a mystery, but astronomers now know that the icy moon Enceladus has been providing water to the planet's upper atmosphere.
The European Space Agency's Herschel space observatory has found that that the water given off from the moon Enceladus created a giant torus of water vapor around the planet. This latest discovery means that Enceladus is the only moon in the Solar System that is known to affect the chemical composition of Saturn, its parent planet.
Enceladus, the sixth largest moon of Saturn, was first discovered in the late 17th century, but little was known about it until recent years.
Enceladus ejects around 550 pounds (250 kilograms) of water vapor every second. This is done through a set of jets from the south polar region, known as the Tiger Stripes, named for their unique surface markings.
Observations further show that the water forms a doughnut-shaped torus, or surface, of vapor surrounding the ringed planet. The total width of the torus is more than 10 times the radius of Saturn. However, it is only about one Saturn radius thick.
Enceladus orbits the planet at a distance of about four Saturn radii, replenishing 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 Paul Hartogh of the Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany, 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."
Saturn's atmosphere is known to have traces of gaseous water in its deeper layers, but the particular puzzle was how water ended up in its upper atmosphere.
That source of water was first reported in 1997 by teams using ESA's Infrared Space Observatory. Computer models of these latest Herschel observations show that about 3 to 5 percent of the water ousted 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 is enough to explain the water detected in its upper atmosphere. It is also responsible for producing additional oxygen-bearing compounds, such as carbon dioxide.
Enceladus, discovered in 1789 by William Herschel, is the sixth-largest of the moons of Saturn. Despite its small size, the moon has various terrains ranging from old, heavily cratered surfaces to young, tectonically distorted terrain. Some of them have regions with surface ages as young as 100 million years old. Enceladus is one of only three outer solar system bodies to include Jupiter's moon Io and Neptune's moon Triton, where active eruptions have been seen.
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."