Tiger Stripes fuel Saturn's torus

The doughnut-shaped torus of water surrounding Saturn originated from Enceladus, making it the only moon in the Solar System known to influence the chemical composition of its parent planet.

Water plumes shoot from Enceladus Credit: NASA/JPL/Space Science Institutes

The transparent water vapour torus has previously escaped detection, but was revealed by the Herschel space observatory, thanks to its use of submillimeter spectroscopy with infrared wavelengths. The discovery solves a 14-year riddle as to the source of water vapour in Saturn's upper atmosphere.

Cryovolcanic activity near the south pole of Enceladus produces plumes of water-dominated gases and ice particles which escape and populate the torus. The jets – known as the the Tiger Stripes – expel around 250kg of water vapour every second. Most is lost into space, some freezes onto the rings or falls on other moons, but about 3-5% ends up falling on Saturn.

“There is no analogy to this behaviour of Earth,” said Paul Hogarth from the Max Planck Institute who led the analysis. “No significant quantities of water enter our atmosphere from space. This is unique to Saturn.”

The small quantities of water vapour that do fall on the planet is sufficient to explain the presence of water in the upper atmosphere. It also explains additional oxygen-bearing compounds like carbon dioxide. Herschel probed the physical conditions and structure of the torus matter and revealed it torus was the ultimate source of water in the upper atmosphere of Saturn, but not its moon, Titan.

The research – published in Astronomy and Astrophysics – revealed four rotational lines of water in the Enceladus water-vapour torus. The total width of the torus is more than 10 times the radius of Saturn, and is one Saturn radius thick.  Enceladus orbits the planet at a distance of four Saturn radii, and replenishes the torus with the jets from the Tiger Stripes.

“Herschel has proved its worth again,” said Göran Pilbratt, ESA Herschel project scientist. “These are observations only Herschel can make.”