On Earth, grinding tectonic plates unleash earthquakes and volcanic eruptions. But on Saturn's moon Enceladus, such friction produces enough heat to launch ice into space, creating spectacular geysers. That's the conclusion of a new study that could provide valuable clues about where water--and life--might be lurking in our solar system and beyond.
Among Saturn's 59 known moons, the 500-kilometer-wide, ice-covered Enceladus is the only one that launches large geysers of icy particles high above its surface. So much of this material blows into space that it created Saturn's faint E ring. But researchers have puzzled over what's behind these icy jets. To find out, scientists analyzed images and data from NASA's Cassini spacecraft. They concluded that the geysers originate from a region called the tiger stripes, a set of dark streaks along the moon's surface that look like tectonic fault lines on Earth. The moon's "hottest measured temperatures are at the tiger stripes," says planetary scientist and co-author Robert Pappalardo of NASA's Jet Propulsion Laboratory in Pasadena, California.
In two papers in tomorrow's Nature, Pappalardo and colleagues report that Enceladus's exaggerated elliptical orbit causes the moon's icy crust to flex at these fault lines. As huge pieces of crust rub together, they generate enough heat to sublimate the ice lurking within the faults, in the same way that comets sprout tails when they veer too close to the sun's heat. The data also suggest that Enceladus harbors underground seas that allow the icy shell to flex enough to produce the heat that powers the geysers. Although the process seems to be responsible for building Saturn's E ring, "it is doubtful that Saturn's main rings are generated in a similar manner," Pappalardo says. That's because those rings are made of larger particles and are located much closer to the planet.
The researchers' conclusion about the cause of the ice jets seems reasonable because similar processes can be seen working on Jupiter's moon Io, says astrophysicist John Bally of the University of Colorado in Boulder. There, the push and pull of the gas-giant's gravity triggers volcanic eruptions. Heating Enceladus' ice along the faults would produce enough water vapor to blow the particles into space and "render the plumes visible," Bally says.