Plumes of water vapour and other gases escape at high velocity from the surface of Saturn's moon Enceladus.

Credit: NASA/JPL

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SYDNEY: Eruptions of water vapour from Saturn's moon Enceladus may be caused by gravitational tidal forces that distort the moon's shape, according to two new studies. This lends support to previous suggestions that the moon has an interior ocean of liquid water, with the potential for sustaining life.

The papers, published today in the British journal Nature, argue that this so-called cryovolcanic activity is caused by gravitational tidal forces driven by the moon's eccentric orbit. According to the researchers, these forces dramatically affect the moons 'tiger stripes' – previously unexplained fractures in the otherwise smooth icy surface of the moon's south pole.

Huge vapour plumes

Enceladus is found deep within the densest part of Saturn's E ring, and is thought to be the primary source of the ring's rocky matter. In 2005, the U.S. space agency NASA's Cassini probe revealed huge plumes of water vapour gushing from the surface of the moon. This finding propelled Enceladus into the exclusive list of celestial bodies known to be volcanically active: Jupiter's moon Io, Neptune's moon Triton, and of course, Earth.

The first new study attributes this volcanism to friction caused by gravitational tidal forces reshaping the surface of the moon. "The ice heats as the fractures move backwards and forwards against each other, like rubbing your hands together on a cold day," said Francis Nimmo, an astronomer from the University of California in Santa Cruz, who led one of the teams. He thinks that shearing of surface layers heats the surrounding ice, which turns directly to vapour in a vacuum and sprays into space.

Nimmo's model challenges the recent theory that liquid water flows just metres below the surface of Enceladus, escaping as water vapour when cracks form in the thin layer of ice. But, he adds, "in order to get enough motion on the fractures, you still need to have an ocean somewhere beneath the ice, it wouldn't deform enough to give the heating you need if it were directly attached to the rocky core."

Tidal reshaping

The second study, led by astronomer Terry Hurford from NASA's Goddard Space Flight Centre in Maryland, proposes that mechanical stress caused by tidal reshaping of Enceladus forces the tiger stripe rifts to open and close, allowing liquid water contained beneath to escape.

Enceladus does not orbit in a perfectly circular motion, as our Moon does; instead its orbit is slightly elliptical – so the gravitational force on the surface changes throughout its orbit, constantly reshaping it. Hurford's theory suggests that when the moon's orbit is furthest from Saturn, tensile stress forces the fractures to open, releasing liquid water from beneath. As the moon's orbit moves closer to Saturn, compressive forces cause the fractures to close.

Rifts would open periodically, based on the position of the moon in its orbit, so "when a rift opens, it may not open all at once but instead open incrementally, like opening a zipper," said Hurford.

In an accompanying commentary, also published in Nature, Andrew Dombard, of Johns Hopkins University, Maryland, said that while the studies provide new directions for understanding Enceladus, we don't yet have all the answers as to why the moon behaves this way.

"Mimas, another icy satellite of Saturn with a closer orbit and a larger eccentricity, has followed a benign evolution," said Dombard. "Why Enceladus has ended up in such an active state whereas Mimas has not, are questions that will exercise planetary scientists for some time to come."