Earth may display its seas on its surface for all the Universe to see, but further out in the Solar System liquid oceans are kept discreetly under wraps, hidden beneath cratered surfaces of ice and rock. And while Saturn’s moon Enceladus sprays its salty subsurface ocean out into space, other moons are less ostentatious — Europa, Ganymede, Titan… all are thought to have considerable underground oceans of liquid water, based on measurements of their mass, density, and shape.
Now, scientists are suggesting that Saturn’s 700-mile-wide moon Dione may also have a subsurface ocean… and may have even once exhibited icy geysers like its smaller sibling Enceladus.
“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 [of] 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.”
Hints of Dione’s activity have recently come from Cassini, which has been exploring the Saturn system since 2004. 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.
Over the course of 7 years the region around a long mountain on Dione called Janiculum Dorsa which ranges in height from about 0.6 to 1.2 miles (1 to 2 kilometers) was imaged by the Cassini spacecraft. The moon’s crust appears to pucker under this mountain as much as about 0.3 mile (0.5 kilometer).
“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, based at Brown University in 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.
“There may turn out to be many more active worlds with water out there than we previously thought.”
– Bonnie Buratti, JPL
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.
The presence of a subsurface ocean at Dione would boost the astrobiological potential of this once-boring iceball.