The new study uses data from Cassini to reconstruct Enceladus's shape in unprecedented detail. The shape of the moon is controlled by both the forces acting on it and its internal properties. The new shape, constrained from images obtained during the final close flybys of Cassini before it plunged into Saturn in September 2017, shows small deviations from a perfect ellipsoid that are most likely due to slightly different amounts of flattening around the poles and the equator.
"We are particularly sensitive to these shape variations because we use very accurate spacecraft ranging measurements, which basically consist in bouncing a radio signal between Cassini and Enceladus and precisely timing its travel time," says Luciano Iess of Sapienza University of Rome, Italy, who led the analysis.
As Enceladus orbits Saturn, its varying gravity field — due to the differences in density between the core and the icy crust — exerts a small pull on the spacecraft, causing Cassini's speed and trajectory to change slightly. Because the spacecraft was in a nearly polar orbit, it repeatedly crossed those regions where the gravity field has a maximum effect, close to the moon's equator, about every 2 hours over a period of 20 months, providing detailed information that enabled the recovery of the moon's shape.
"A rigid body such as a rocky moon would have shown up in our reconstruction with a nearly perfect ellipsoidal shape," says co-author Dennis Matson of the Jet Propulsion Laboratory (JPL) in Pasadena, California. "The differences we see instead — small but clearly visible — are almost certainly caused by the presence of a global internal ocean, which decoupled the ice shell from the rocky core."
Enceladus's ocean is covered by a layer of ice known to be from several kilometres to at most a few tens of kilometres thick, through which jets of salted water and gas spew out from cracks in the south polar region, forming the jets that feed Saturn's magnificent E ring. The team now knows that the entire outer icy shell is thick but not rigid, and floats and moves independently of the core.
The thickness of the icy shell is related to the temperature at the core–shell boundary, which determines how much ice can be melted by the hot core. Models of the evolution of icy satellites with a subsurface ocean predict that the thickness of the shell should increase as the moon cools and the ocean freezes over time, causing more of the internal heat to be trapped inside. This process of freezing provides a mechanism to keep Enceladus's subsurface ocean liquid, even though the moon's internal heat production is now expected to be small.
"The ability of Enceladus to sustain an ocean throughout geological time is a key requirement to maintain a habitable environment below the ice crust, making Enceladus the prime candidate for future exploration targeting the search for life in our Solar System," says Iess.
