Bubbly Ocean on Enceladus Explains Plume Mystery

Mardi, 05 Octobre 2010 06:00

Bubbly Ocean on Enceladus Explains Plume Mystery

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Bubbly seawater below a crust of ice could explain the famous plumes on Saturn’s moon Enceladus. A new model based on data from the

Cassini orbiter suggests the water, gas, dust and heat observed in the moon’s plumes all come from seawater circulating from the ocean to the surface of the ice and back.

“You have one process that delivers all those things, whereas before you had to try to find out how to do each of those things separately,” said NASA planetary scientist Dennis Matson of the Jet Propulsion Laboratory in an October 4 press briefing at the American Astronomical Society’s Division for Planetary Sciences meeting in Pasadena, California.

Enceladus’s fantastic wet plumes make it one of the solar system’s best candidates for life. But astronomers have gone back and forth on whether the moon’s southern spritzes mean it has a liquid ocean. Most of the debate has centered around salt. While Cassini found sodium in Saturn’s outermost ring, which is believed to be formed from material spat out by Enceladus, observations from Earth-based telescopes found no sodium in the plumes.

The missing salt could be explained by a “Perrier” ocean of slightly bubbly seawater, Matson said. The bubbles come from gas dissolved in the water, and an ocean of just one or two percent gas would do it, according to their model. Bubbly seawater is less dense than ice, so it would rise easily to the surface through cracks in an ice crust. At the surface, popping bubbles would throw a fine salty spray that would show up in the solid ring, but not in vapors in the plume.

“The sodium was hiding in the little grains,” Matson said. “In the case of Enceladus, sodium isn’t in the vapor, it’s in the solid particles. This was something entirely new that had not been seen elsewhere.”

The model also explains why spots on the moon are so unexpectedly warm. “Hot” spots at a feature called the tiger stripes reach -135 degrees Fahrenheit. This is chilly by Earth standards, but a sauna compared to their -370 Fahrenheit surroundings.

In the bubbly ocean model, heat from the moon’s interior is transferred in ocean water to the surface of the ice. At the surface, the water cools, dissolves the bubble gases and returns to the ocean via cracks in the ice.

“The realization that there’s a circulation system inside of Enceladus is a new way of thinking, and it’s not one that’s been employed to explain any other satellite behavior,” Matson said.