Multiple moons may hold layer of liquid beneath their outer shells
Jupiter and Saturn aren’t really heat sources, but they are helping liquefy portions of their moons. Both Jupiter’s moon Io and Saturn’s Enceladus are yanked and pulled by their respective planet’s gravity, and that gravity causes friction inside each moon’s layers of rock and ice. Recent modeling has predicted that the relatively small amount of heat from this internal friction is likely enough to not only melt some of the moons’ composition, but create layers of liquid spanning the whole satellite.
A layer of sludgy magma
In the case of Io, gravitationally induced friction has helped create the most volcanically active body in the solar system. The moon exhibits frequent eruptions, lava flows and plumes of sulfur, sulfur dioxide and silicate. Despite this activity, the surface of the moon is considered very dry, and the was assumed to be solid iron with some pockets of liquid magma. To test this assumed composition, researchers created a variety of simulations of Io, each with a different combination of liquid and solid layers. They then ran these simulations against known gravitational effects from Jupiter to see what version best matched the actual moon. While a solid core could possibly exhibit similar volcanic behavior, they found that a layer of interconnected slush below the surface of the moon was the closest match of the real thing.
An ice shell over a single sea
Enceladus offers a slightly different example of friction’s effects within a moon. The surface of the moon has long been known to be frozen ice, with some liquid beneath it, but it wasn’t clear how far that liquid sea extended. Craters were selected and tracked as reference points to figure out the tidally locked moon’s motion. Simulations then showed that the small shifts in position in relation to Saturn only make sense if the layer of liquid water covers the entire satellite. The icy crust was then determined to be nine to twelve miles thick, floating on an ocean 15 to 18 miles deep.
The data on Enceladus was gathered over seven years by the Cassini spacecraft, which is likely as close as we’ll be to that moon for some time. This won’t be the last potential “shell” moon we’ll be studying though, as a new mission is slated to send an orbiter to Jupiter’s moon, Europa. Europa is yet another possible home of a subsurface ocean in our solar system, complete with liquid salt water like Enceladus.