On moons in the outer Solar System with oceans between an icy shell and rocky core, geology is driven by the action of water and ice, in some cases creating cracks, ridges, mountains and plains. A new study published in Nature Astronomy sheds light on how melting ice – and boiling oceans – can contribute to such diverse surface features.
“Water is pretty important for habitability, so there is a lot of effort in understanding the processes which modify the oceans and the types of histories these environments experience,” said paper coauthor Matthew Walker, a Planetary Science Institute associate research scientist.
“We’re interested in the processes that shape [an icy ocean world’s] evolution over millions of years and this allows us to think about what the surface expression of an ocean world would be,” said paper lead author Max Rudolph of the University of California, Davis.
Icy moons are heated by tidal forces – the gravitational push and pull on one celestial body from another. The moons orbiting a planet can interact, leading to periods of higher and lower heating. More heat can melt and thin the ice layer, while less heat thickens it.
The research team had previously looked at what happens when the ice shell thickens. They found that because ice has a greater volume than liquid water, freezing the bottom of the ice shell would put pressure on the ice shell, which could cause features such as the “tiger stripes” of Enceladus.
But what happens when the opposite happens and the ice shell melts from the bottom? That could actually cause the ocean to boil, the researchers conclude.
That’s because as ice melts into less-dense liquid water, pressure drops. Rudolph and colleagues calculated that at least on the smallest icy moons – such as Saturn’s Mimas and Enceladus, or Uranus’ Miranda – the pressure could drop low enough to reach the triple point at which ice, liquid water and water vapor can all co-exist.
“This can explain some of the compressive features noted on some smaller moons,” Walker said, Images of Miranda from the Voyager 2 space probe show distinct areas of ridges and cliffs called coronae. Ocean boiling could explain how these features formed. “It also bolsters the case that some of the more featureless looking places could host oceans.”
For example, Mimas is less than 250 miles across and pitted with craters, including a very large crater earning it the nickname “Death Star.” It appears to be geologically dead, Rudolph said, but a wobble in its movement suggests an ocean is present. This research shows that Mimas’ ice shell is not expected to break as a result of ice shell thinning, so the presence of an ocean can be reconciled with a geologically dead surface.
The size of these moons is important. On larger ice moons, such as Titania, another moon of Uranus, the drop in pressure from melting ice would cause the ice shell to crack before the triple point for water is reached, the team calculated. The authors find that Titania’s geology could be the product of a period of ice shell thinning followed by re-thickening.
“All in all, the two papers work together offer insight about the lifetimes of ocean worlds and the processes which drive their evolution,” Walker said.
Other coauthors on the paper include Michael Manga, UC Berkeley and Alyssa Rhoden, Southwest Research Institute, Boulder. The work was supported in part by NASA.