Mystery Solved: Water in Saturn’s Atmosphere from Enceladus’ Geysers

  Enceladus_jets

 Until now, Saturn's atmosphere was known to contain traces of gaseous water in its deeper layers. An enigma has been the presence of water in its upper atmosphere. ESA's Herschel space observatory has shown that water expelled from the moon Enceladus forms a giant donut-shaped torus of water vapour around Saturn.


The discovery solves a 14-year mystery by identifying the source of the water in Saturn’s upper atmosphere. Herschel’s latest results confirm that Enceladus is the only moon in the Solar System known to influence the chemical composition of its parent planet.

Enceladus ejects around 250 kg of water vapour every second, through a series of jets from the south polar region known as the Tiger Stripes because of their distinctive surface markings.

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 the planet at a distance of about four Saturn radii, replenishing the torus with its jets of water.  Despite its enormous size, the torus has escaped detection until the Herschel discovery because water vapour is transparent to visible light but not at the infrared wavelengths Herschel was designed to see.

Herschel, the largest infrared space telescope, is stationed at the second Lagrange point of the Sun-Earth system. Its 3.5-metre diameter collects long-wavelength infrared radiation from some of the coolest and most distant objects in the Universe. Herschel covers a wide range of wavelengths, from far-infrared to sub-millimetre. The longest of these wavelengths have not been covered before. The satellite was launched on 14 May 2009 with ESA's Planck microwave observatory, on board an Ariane 5 from Europe's Spaceport in Kourou, French Guiana.

First reported in 1997 by teams using ESA’s Infrared Space Observatory, the source of Saturn's water was unknown until now. Computer models of these latest Herschel observations show that about 3-5% of the water expelled by Enceladus ends up falling into Saturn.

“There is no analogy to this behaviour on Earth,” says Paul Hartogh, Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany, who led the collaboration on the analysis of these results.
 
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 sufficient to explain the water observed in its upper atmosphere. It is also responsible for the production of additional oxygen-bearing compounds, such as carbon dioxide.

Ultimately, 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.

The Daily Galaxy via European Space Agency

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