A study published in Nature Astronomy shows how the cyclical melting and condensation of ice shells on small moons like Mimas, Enceladus, and Miranda change the pressure above their hidden oceans – to the point of boiling at the triple point and creating distorted landscapes of ridges, cliffs, and coronae.
The outer planets of the solar system are accompanied by many moons covered in thick layers of ice. Some of them, including Enceladus, a moon of Saturn, contain oceans of liquid water trapped between the icy surface and the rocky interior. Because of these hidden seas, they are among the most promising places in our solar system to search for extraterrestrial life.
According to a study from Nature Astronomy, researchers are beginning to reveal what may be happening beneath the outer ice on these distant worlds and how the very diverse geological structures visible above the surface may have evolved.
"Not all of these moons are known to have oceans, but some of them are known to have them," said Max Rudolph, professor of Earth and planetary sciences. "We're interested in the processes that shape their evolution over millions of years, and that allows us to think about how an ocean world would manifest itself on the surface."
Ice and water drive geological forces
Mountains, earthquakes, and other geological activity on Earth result from the movement and melting of rocks deep beneath the surface. The geology of icy moons, however, is shaped by the behavior of water and frozen layers.
These distant worlds gain internal heat from tidal forces generated by the planets they orbit. Interactions between several moons can change the strength of this heating over time. As tidal heating increases, the ice shell melts and becomes thinner; as the heating decreases, the ice layer thickens again.
Rudolph and his colleagues have previously studied what happens when the ice shell thickens. They found that because ice has a greater volume than liquid water, freezing would put pressure on the ice shell, which could cause phenomena like the "tiger stripes" on Enceladus.
But what happens when the opposite happens and the ice sheet melts from the bottom? This could cause the ocean to boil, the researchers concluded.
That's because as ice melts into less dense liquid water, the pressure drops. Rudolph and his colleagues calculated that at least on the smallest icy moons, like Saturn's Mimas and Enceladus, or Uranus's Miranda, the pressure can drop low enough to reach the triple point where ice, liquid water, and water vapor can coexist.
Boiling oceans and strange topographies
Miranda's images show distinct areas of ridges and cliffs called coronae. Boiling oceans could explain how these structures formed.
Mims is less than 400 kilometers across and has craters, including a very large one that gave it the nickname "Death Star." It appears geologically dead, Rudolph says, but a wobble in its motion suggests the presence of an ocean. Since Mims' ice shell is unlikely to break up due to its decocking, the existence of an ocean could be consistent with a geologically dead surface.
The size of these moons matters. On larger icy moons, like Titania, another moon of Uranus, the team calculated that the pressure drop from melting ice would cause the ice shell to crack before reaching the triple point of water. They find that Titania's geology could be the result of ice shell decoupling followed by recondensation.
More of the topic in Hayadan:
