麻豆传媒

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

The supersonic jets were detected during a flyby on 24 October 2007, when the probe鈥檚 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 鈥� which act like rocket nozzles 鈥� in the moon鈥檚 icy surface.

The simplest way to generate such pressures is by evaporating a reservoir of liquid water that lies close to the moon鈥檚 surface, says lead author and UVIS team member Candice Hansen of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. The heat needed to evaporate the water might be generated in part by Saturn鈥檚 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鈥檚 tugs could cause a shell of ice to grind and vaporise, particularly if the shell lay over a 鈥榣ubricating鈥� ocean of liquid water buried relatively deep below the surface.

鈥楯ury鈥檚 out鈥�

Directly turning ice into water vapour, however, would require even more heat to create the high-speed jets than vaporising liquid water, Hansen told 麻豆传媒. Since researchers still can鈥檛 account for all the energy needed to create the plumes, she argues that the discovery of the supersonic jets strengthens the case for liquid water.

Cassini team member Bonnie Burrati, also of JPL, agrees. If the source of the jets was ice rather than liquid water, 鈥測ou might get a little puff, but nothing as jetty as what they鈥檙e measuring鈥�, she told 麻豆传媒.

But Andrew Ingersoll of Caltech, a member of Cassini鈥檚 imaging team, says it鈥檚 too soon to draw that conclusion: 鈥淭he jury鈥檚 still out.鈥� Some shapes for the nozzle-like openings in the surface could spew water vapour out at supersonic speeds regardless of whether the water was originally liquid or ice, he says.

The new measurements could help resolve what is powering the plumes. So far, the main source of energy for heating Enceladus鈥檚 interior is thought to come from the strains induced by Saturn鈥檚 pull on the moon.

But it鈥檚 unclear how much these 鈥榯idal forces鈥� deform the moon, and they may not be not strong enough to account for the energy needed to release the plumes, says Terry Hurford of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland. 鈥淲e don鈥檛 think there鈥檚 enough heat,鈥� Hurford says.

Wobbling moon

The new measurements could introduce a fix, by altering models of how Saturn鈥檚 tides open and close the moon鈥檚 鈥榯iger stripes鈥�, 130-kilometre-long cracks that are the source of the plumes.

These stripes are thought to open and close every time Enceladus completes an orbit around Saturn, which occurs every 1.37 days.

Strangely, UVIS observations show more gas was being released from the south pole in 2007, when the moon was further away from the planet and the stripes were thought to be closed, than in 2005, when the moon鈥檚 fissures were thought to be open.

That suggests the moon might wobble on its rotational axis. That could subtly change the orientation of the cracks enough to influence the timing of their opening and closing, Hurford says. This wobble could also further stress material in the moon, increasing the heat available to feed the plumes.

Cassini鈥檚 UVIS device will next measure the jets during Cassini鈥檚 last Enceladus flyby, set for 18 May 2010. At that time, the Sun will be perfectly aligned to pass behind the plumes, allowing scientists to study how its light is absorbed by the watery features.

Journal reference: (vol 456, p 477)