‘Overlooked?’ –Saturn’s Moon Dione: “A Fossil of the Wondrous Activity Cassini Discovered At the Geysers of Enceladus”

 
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"A picture is emerging that suggests Dione could be a fossil of the wondrous activity Cassini discovered spraying from Saturn's geyser moon Enceladus or perhaps a weaker copycat Enceladus," said Bonnie Buratti of NASA's Jet Propulsion Laboratory in Pasadena, Calif., who leads the Cassini science team that studies icy satellites. "There may turn out to be many more active worlds with water out there than we previously thought."


“There’s really good evidence” for Dione’s ocean, says Buratti. “I kind of feel this is one of the things we’ve left hanging as we leave the Saturn system, that we just haven’t answered. If Dione has an ocean, it’s another example where there might be a habitable environment because we have liquid water—there’s a heat source and there might be organic molecules in there that contribute to primitive bacterial life.”

 

More than 400,000 kilometers from Enceladus orbits another icy moon, Dione, which is wice as big and like Enceladus is probably an ocean world. From a distance, Dione resembles a bland cueball. But thanks to close-up images of the Janiculum Dorsa, a 500-mile-long (800-kilometer-long) mountain from NASA's Cassini spacecraft, scientists found more evidence for the idea that Dione was likely active in the past, and could still be active now.

 

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Evidence from Cassini’s instruments hinted at plume activity, but much weaker than at Enceladus. Buratti suspects that Janiculum Dorsa, one of the moon’s few mountains, may be somehow responsible, but the scientists were unable to gather definitive proof before Cassini’s fiery finale. The spacecraft's magnetometer has detected a faint particle stream coming from the moon, and images showed evidence for a possible liquid or slushy layer under its rock-hard ice crust. Other Cassini images have also revealed ancient, inactive fractures at Dione similar to those seen at Enceladus that currently spray water ice and organic particles.

Janiculum Dorsa may be Dione’s mirror of Enceladus’s south pole, where water sprays more than 60 kilometers into spac, fed by an underwater ocean. There is circumstantial evidence for Janiculum Dorsa as the source of similar “cryovolcanic” plume activity on Dione.

Other bodies in the solar system thought to have a subsurface ocean – including Saturn's moons Enceladus and Titan and Jupiter's moon Europa – are among the most geologically active worlds in our solar system. They have been intriguing targets for geologists and scientists looking for the building blocks of life elsewhere in the solar system. The presence of a subsurface ocean at Dione would boost the astrobiological potential of this once-boring iceball.

Janiculum Dorsa ranges in height from about 0.6 to 1.2 miles (1 to 2 kilometers). The moon's crust appears to pucker under this mountain as much as about 0.3 mile (0.5 kilometer).

 

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"The bending of the crust under Janiculum Dorsa suggests the icy crust was warm, and the best way to get that heat is if Dione had a subsurface ocean when the ridge formed," said Noah Hammond, the paper's lead author, who is based at Brown University, Providence, R.I.

Dione gets heated up by being stretched and squeezed as it gets closer to and farther from Saturn in its orbit. With an icy crust that can slide around independently of the moon's core, the gravitational pulls of Saturn get exaggerated and create 10 times more heat, Hammond explained. Other possible explanations, such as a local hotspot or a wild orbit, seemed unlikely.

Scientists are still trying to figure out why Enceladus became so active while Dione just seems to have sputtered along. Perhaps the tidal forces were stronger on Enceladus, or maybe the larger fraction of rock in the core of Enceladus provided more radioactive heating from heavy elements. In any case, liquid subsurface oceans seem to be common on these once-boring icy satellites, fueling the hope that other icy worlds soon to be explored – like the dwarf planets Ceres and Pluto – could have oceans underneath their crusts. NASA's Dawn and New Horizons missions reach those dwarf planets in 2015.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, Pasadena, Calif., a division of the California Institute of Technology, Pasadena, manages the Cassini-Huygens mission for

The Daily Galaxy via JPL  and Scientific American 

Image credit: Top of page Dione By Drell 

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