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Supersonic jets of water vapour are blasting from Saturn's moon Enceladus, reveal observations from NASA's Cassini spacecraft. Some say the newly discovered features bolster the case for a reservoir of liquid water - which could potentially host life - just beneath the moon's surface.

The high-powered jets lie within wider plumes of water vapour and ice, found in 2005, which extend hundreds of kilometres from the moon's south pole.

The supersonic jets were detected during a flyby on 24 October 2007, when the probe's Ultraviolet Imaging Spectrograph (UVIS) observed a star as it passed behind the plumes. By measuring how much starlight was absorbed during the passage, scientists discerned four distinct jets in the plume.

The jets were still tightly focused at an altitude of 15 km above the surface, suggesting they were moving faster than 2100 km per hour. Such high speeds imply that the jets are fed by pressurised water vapour that shoots through narrow openings - which act like rocket nozzles - in the moon's icy surface.

The simplest way to generate such pressures is by evaporating a reservoir of liquid water that lies close to the moon's surface, says lead author and UVIS team member Candice Hansen of NASA's Jet Propulsion Laboratory in Pasadena, California. The heat needed to evaporate the water might be generated in part by Saturn's gravity, which squeezes and stretches the moon.

But some researchers say the plumes might originate from subsurface ice instead. In one scenario, the strains from Saturn's tugs could cause a shell of ice to grind and vaporise, particularly if the shell lay over a 'lubricating' ocean of liquid water buried relatively deep below the surface.

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