A 14 year old mystery has been solved as astronomers discover that plumes of water gushing from Saturn’s moon Enceladus are the source of water in the planet's upper atmosphere.

Water vapour was detected in a massive torus surrounding Saturn by astronomers using ESA's Herschel Space Observatory. The torus has a thickness equivalent to Saturn's radius and it extends to a distance ten times wider. The torus remained invisible up until now as water vapour is translucent in visible light at such distances, however, infrared observations by Herschel easily revealed Saturn's secret water stash, which then rains down into the ringed planet's upper atmosphere.

Jets of water vapour spewing from the south pole of Saturn's moon Enceladus at rates of around 250 kilograms every second are what feed this torus. The jets were first observed by the Cassini space probe in 2006, and originate from a region of winding fissures dubbed the Tiger Stripes.

Water vapour was first discovered in Saturn’s upper atmosphere in 1997 using ESA’s Infrared Space Observatory, but until now astronomers have been baffled by how it got there. While water can exist in the lower atmosphere, the vapour pressure is too low higher in the atmosphere for it to occur naturally.

“We have a similar situation on the Earth,” explains Paul Hartogh of the Max-Planck-Institut für Sonnensystemforschung and lead author of the Astronomy & Astrophysics paper. “The relative abundance of water in air on the ground can be as high as four percent. With increasing altitude the temperature in the atmosphere is decreasing and in the tropopause region (10-18 kilometres high) it reaches a relative minimum, as low as 190 Kelvin. The vapour pressure of water decreases dramatically at 190 Kelvin so that only about four parts per million of water vapour contributes to air. This is 10,000 times less than on the ground.”

However there is even less water vapour on Saturn due to even lower temperatures in the gas giant’s tropopause. “On Saturn the tropopause temperature is around 85 - 90 Kelvin,” says Hartogh. “At these low temperatures the vapour pressure of water is even more than a billion times lower than in the Earth tropopause. Water vapour cannot exist there and therefore cannot be transported into Saturn's stratosphere from below.”

This means that water vapour cannot exist on Saturn unless it comes from an external source. The amount of water detected in Saturn’s atmosphere by Herschel is equivalent to the amount that is being rained down on the planet from the surrounding torus. Enceladus is now unique as this precipitation makes it the only moon in the Solar System to affect its planet's composition.

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