Model of Ganymede’s internal layers (annotated).
We don’t hear a lot about Ganymede these days, not with Titan, Enceladus and Europa often stealing the science spotlight. Which is unfortunate because one: Ganymede is really fascinating in its own right, and two: it’s the biggest moon in the solar system – even bigger than the planet Mercury! So even though giant Ganymede may not have a Titanic atmosphere or impressive jets like Enceladus, it does have surprisingly complex subsurface layers “like a club sandwich,” according to new NASA-funded research.
Or, if you prefer my titular reference to a certain animated ogre, like an onion. (Or maybe even a parfait?) Except at the Ganymede deli, the sandwich (or onion) is made up of alternating layers of ice and liquid water, increasing in saltiness with depth.
“This is good news for Ganymede,” said Steve Vance of NASA’s Jet Propulsion Laboratory in Pasadena. “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.”
The idea of an internal ocean on Ganymede isn’t new; previous models of its interior suggested a single layer of water between an icy crust and an ice-covered, rocky core.
While that would have meant a considerable amount of liquid water inside Ganymede, it also isolated the water from the moon’s rocky layer — eliminating much of the surface area where water/rock chemical processes could occur and potentially create environments conducive for life.
This new model, based on the complex behavior of salt — in Ganymede’s case, magnesium sulfate — in water under increasing pressures puts a layer of liquid water just above the moon’s rocky seafloor, increasing the chances that primitive life could have evolved there.
Of course, that would require not only the right conditions but also the conditions to have been present for a considerable amount of time.
“We don’t know how long the Dagwood-sandwich structure would exist,” said Christophe Sotin of JPL, who along with Vance is a member of the Icy Worlds team at JPL. ”This structure represents a stable state, but various factors could mean the moon doesn’t reach this stable state.”
Still, even if this icy sandwich structure hasn’t remained stable on Ganymede, it’s a viable model for conditions on other similarly icy moons… or even on other planets outside our solar system.