From a distance, most of Saturn’s moon Dione resembles a bland cueball. Thanks to close-up images of a 500-mile-long (800-kilometer-long) mountain on the moon from NASA’s Cassini spacecraft, scientists have found more evidence for the idea that Dione was likely active in the past. It could still be active now.
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. Lit terrain seen here is on the anti-Saturn and trailing sides of Dione (1,126 kilometers, or 700 miles across). The image was taken with the Cassini spacecraft narrow-angle camera on March 22, 2008 using a spectral filter sensitive to wavelengths of ultraviolet light centered at 338 nanometers. The view was acquired at a distance of approximately 650,000 kilometers (404,000 miles) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 99 degrees. Image scale is 4 kilometers (2 miles) per pixel. Image Credit: NASA/JPL/Space Science Institute
“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.”
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.
The Cassini spacecraft swooped in for a close-up of the cratered, fractured surface of Saturn’s moon Dione in this image taken during the spacecraft’s Jan. 27, 2010, non-targeted flyby. Cassini came within about 45,000 kilometers (28,000 miles) of the moon during this flyby, and this image was acquired at a distance of approximately 46,000 kilometers (29,000 miles). This view looks toward the leading hemisphere of Dione (1,123 kilometers, or 698 miles across). North on Dione is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera. The view was acquired at a Sun-Dione-spacecraft, or phase, angle of 108 degrees. Image scale is 270 meters (886 feet) per pixel. Image Credit: NASA/JPL/Space Science Institute
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.
The mountain 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 kilometers). 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, the paper’s lead author, who is based at Brown University, Providence, R.I.
This image, which is composed of data obtained by NASA’s Cassini spacecraft on Oct. 11, 2005 and Oct. 17, 2010, shows the topography of a mountain known as Janiculum Dorsa on Saturn’s moon Dione. Color denotes elevation, with red as the highest area and blue as the lowest. Janiculum Dorsa is a 500-mile-long (800-kilometer-long) mountain and ranges in height from 0.6 to 1.2 miles (1 to 2 kilometers). An analysis of the topography data suggests the crust under the mountain puckers as much as 0.3 mile (0.5 kilometer). This bending of the crust suggests the icy crust was warm at some time in recent history. The best explanation for such behavior is the existence of a subsurface ocean under the mountain when it formed. Image Credit: NASA/JPL-Caltech/SSI/Brown
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.
Source: Jet Propulsion Laboratory (JPL)