Ganymede, one of three ocean moons of Jupiter, could be hiding layers under its icy exterior. New experiments on the behaviour of saline water under pressure suggest a complex, layered ocean with a rocky sea floor, not a single ocean sandwiched between crusts of dense ice.
Until recently, the standard theory was that Ganymede had an icy shell protecting a liquid ocean, with another thick layer of ice coating the ocean floor. This broke astrobiologists' hearts, as rock-water boundaries are really interesting places to develop life. But those theories didn't account for how the combination of pressure and salinity affects phase state.
Steve Vance of the Icy Worlds team at Jet Propulsion Laboratory suggests that Ganymede may be more like one of the towering sandwiches in the Blondie legacy comic. "Ganymede's ocean might be organized like a Dagwood sandwich," explains Vance in what may be my new favourite quote from a formal science setting.
In a far less amusing quote, Vance explains, "This is good news for Ganymede. Its ocean is huge, with enormous pressures, so it was thought that dense ice had to form at the bottom of the ocean. When we added salts to our models, we came up with liquids dense enough to sink to the sea floor."
Instead of a seafloor coated in ice where rock and liquid water have no chance of interaction, the new model suggests that the lowermost layer may be dense, saline liquid water. Water in contact with rock is good news for astrobiologists. This isn't evidence of alien life by even the most exuberant overstatement, but it does bump Ganymede up the list of places to go looking for it.
We've suspected Ganymede had an ocean since the 1970s, then in the 1990s Galileo confirmed the ocean's existence. Galileo also found evidence that the ocean reaches hundreds of miles deep, and are salty seas, probably with magnesium sulfate.
The first round of models assumed that salt didn't change how water behaves under pressure, but the Icy Worlds decided to test the assumption. You can run a simple experiment at home by carefully measuring the level of a glass of water, then adding salt and watching the volume shrink. It sounds counterintuitive that adding material would decrease volume, but the salt ions attract water molecules, packing the liquid more densely.
Ganymede's oceans may be layered with water of variable salinity and ice phases. Image credit: NASA/JPL
It gets more complicated when considering different forms of ice. Ice I is the light, floating ice you're familiar with from your freezer. Under pressure, the ice molecules pack more closely, becoming more and more dense. On Ganymede, the pressure gets high enough to produce Ice VI, ice so dense that it sinks in water.
Mix those two processes when modelling the oceans of Ganymede, and things get downright complicated. Water sorts into layers by salinity, interspersed with different phases of ice as the pressure climbs. Here's where things get downright weird: the oceans of Ganymede might snow upwards. Cold plumes could trigger the formation of Ice III in the topmost layer. The freezing ice precipitates salts, floating up as the salt sinks. As the ice rises, the reduced pressure melts the ice, modifying local salinity and possibly even producing a layer of slush.
This fancy model isn't a sure thing, and even if it happens, it might be a temporary situation. Another member of the team, Christophe Sotin, explains, "This structure represents a stable state, but various factors could mean the moon doesn't reach this stable state."
Although applied specifically to Ganymede, the physics applies to any other icy ocean worlds we find. Ganymede, Europa and Callisto orbit Jupiter, while Titan and Enceladus orbit Saturn. Three of the moons are exploration targets for the European Space Agency in the 2030s, with the JUICE mission launching in 2022.