Saturn's Tidal Tugs Energize Enceladus' Icy Plumes

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Enceladus shows off its plume to NASA's Cassini spacecraft cameras.
NASA/JPL-Caltech/Space Science Institute

One of the biggest surprises of the ongoing NASA Cassini mission at Saturn was the discovery of giant plumes of water vapor and ice particles blasting into the space from the surface of Enceladus, one of the planet’s 62 known moons.

NASA and ESA astronomers released movies of Saturn's northern and southern lights, glimpsed edge-on for the first time by the Hubble Space Telescope.
NASA

Scientists wondered why the relatively diminutive moon, which measures about 310 miles (500 kilometers) in diameter, wasn’t frozen solid. They also began creating computer models to try to unravel the physics behind the stunning geological phenomenon.

Now, after analyzing 252 images of Enceladus’ plumes, scientists have part of the answer: Gravitational variations during the moon’s slightly eccentric, 1.37-day orbit around Saturn create tidal forces that directly impact how much material is shot into space from four fissures around the moon’s south pole.

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“It’s not a subtle variation. You can look at some of the images and you can actually see it with your eyes. It’s very dramatic,” planetary scientist Matthew Hedman, with Cornell University, told Discovery News.

After accounting for lighting variations and viewing angles in the images, which were taken between 2005 and 2012, Hedman and colleagues found that when Enceladus was farthest from Saturn its plumes were three times brighter than when it was closest.

On average, Enceladus circles about 148,000 miles (238,000 kilometers) from the center of Saturn. Its closest and farthest approach differ by less than 1 percent, or about 1,243 miles (2,000 kilometers.)

The analysis indicates that the fissures, which scientists nicknamed “tiger stripes,” open wider when the tidal forces are strongest, allowing more material to escape. On average, the cracks, which are warmer than the surrounding area, are about 80 miles (130 kilometers) long, 1.2- to 2 miles (2- to 3 kilometers) wide, and about 1,650 (500 meters) deep.

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“Different theoretical models made different predictions for how the cracks might open and close or heat up and cool down as the moon goes around the planet,” Hedman said. “With this information we’re in a position now to confront those models with real data.”

The information also is expected to help scientists better understand how similar forces might be at work elsewhere in the solar system.

For example, Jupiter’s icy moon Europa, which is believed to house an underground ocean, is laced with double ridges with grooves running down the middle, similar to what is found on Enceladus.

“They’re not active as far as we know at this point, but when you get a result like this one, you’re seeing that there actually might be quite simple forces at work along those fractures when they are active,” planetary scientist John Spencer with the Southwest Research Institute in Boulder, Colo., told Discovery News.

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Similar features have been spotted on Jupiter’s large moon Ganymede, other moons of Saturn and on Miranda, a moon of Uranus.

“There are lots of places where we think similar processes may have occurred in the past, but we can’t watch them happen like we can on Enceladus,” Spencer said.

Scientists have more work ahead to understand how the flexing of Enceladus is heating its interior and how extensive its suspected subterranean sea might be.

“The more extensive the ocean, the easier it is for the surface to flex, so there’s a lot of work that can be done based on this information,” Spencer said.

The research appears in this week’s Nature.