Sen— NASA's Cassini spacecraft has found more evidence that Saturn's moon Dione was geologically active in the past and that it could still be active now.
Cassini'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, another of Saturn's moons, that currently spray water ice and organic particles.
"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 California, 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."
The Cassini spacecraft looks down, almost directly at the north pole of Dione. The feature just left of the terminator at bottom is Janiculum Dorsa, a long, roughly north-south trending ridge. Image credit: NASA/JPL/Space Science Institute
The mountain on Dione examined in the latest paper, published in March in the journal Icarus, is called Janiculum Dorsa and ranges in height from about 0.6 to 1.2 miles (1 to 2 kilometres). The moon's crust appears to pucker under this mountain as much as about 0.3 mile (0.5 kilometre).
"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.
Dione is heated 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.
Dione Anaglyph. Image credit: NASA/JPL/Space Science Institute
Why Enceladus became more active than Dione is unknown. 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 icy satellites, fueling the hope that other icy worlds, 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.
Cassini was launched in 1997 and entered Saturn's orbit in 2004. Cassini continues to provide much information and stunning images of Saturn, its rings and its moons.