Saturn’s grip explains odd moon


Enceladus, a white moon of Saturn with ice-spewing volcanoes, owes its strangeness to tides of gravitational forces exerted by its parent planet, a study has found.

Discovered in 1789 by William Herschel, Enceladus measures only 504 km across yet is one of the great oddities of the solar system. Its surface is a gorgeous white shell of ice, rather than asteroid-pocked rock and dust, and the surface is pristine except for a network of fractures near its south pole.

These cracks — dubbed “tiger stripes” — emit fountains of water vapor that instantly turn into icy grains on contact with the vacuum of space. Some astrophysicists conclude that it harbors an ocean of salt water, which in turn makes it a good candidate as a source for life.

But how can a subsurface sea exist if the ambient temperature is close to absolute zero and the sun is a distant dot?

The answer, say theorists, lies with a phenomenon called tidal forces. They argue that the gravitational pull exerted by Saturn squeezes Enceladus’ innards, causing friction whose heat allows the water to remain in a liquid state.

When Enceladus is closest to Saturn, the plume is at its dimmest, a sign that the fractures are being closed up by a mighty gravitational pull from the giant planet, according to the new study. When Enceladus is at its farthest point from Saturn, the plume is several times brighter, suggesting that the fractures open out and more water is disgorged.

The evidence comes from 252 infrared images taken by the great U.S. explorer probe Cassini during its lonely swings around the planet. They provide “strong evidence that tidal forces do play an important role in controlling Enceladus’ plume activity,” says the paper, led by Matthew Hedman of Cornell University in New York.

Many of the icy grains from Enceladus fall back on its surface, which explains its dazzling white surface. The ice may also be the origin of one of the rings of Saturn that give the gas giant its special beauty, according to some theories.