The outer planet is surrounded by dozens of moons trapped in a thick icy shell. Some of these frozen worlds, such as Saturn’s moon Enceladus, are thought to have vast oceans of liquid water trapped between their icy crusts and rocky interiors. Because liquid water is essential to life as we know it, these moons rank among the most promising places in the solar system to search for extraterrestrial life.
Research published in natural astronomy It investigates what’s happening far below the frozen surface and provides new explanations for the unusual landscapes seen on some of these moons.
“While not all of these moons are known to have oceans, we do know that some have oceans,” said Max Rudolph, associate professor of Earth and planetary sciences at the University of California, Davis, and lead author of the paper. “We’re interested in the processes that shape its evolution over millions of years, and this allows us to think about what the surface representation of the oceanic world might look like.”
How tidal heating shapes the icy world
On Earth, features such as mountains and earthquakes are caused by heat and the movement of rocks deep underground. On an icy moon, water and ice play the same role.
These moons are heated by tidal forces generated by the giant planets they orbit. Gravitational interactions between neighboring satellites can cause heating levels to rise and fall over time. Intense heating can cause the ice shell to melt and become thinner. When the heat is reduced, the water refreezes and the shell thickens again.
In previous research, Rudolph and his colleagues studied what happens when the ice shell thickens. Ice takes up more space than liquid water, so when it freezes it increases the pressure on the surrounding shell. This pressure may help create dramatic surface features on Enceladus, such as the long cracks known as “tiger stripes.”
When the hidden sea begins to boil
A new study considers the opposite scenario. What happens when the ice shell melts from the bottom and becomes thinner?
The process could cause the underlying ocean to boil, researchers say.
As the ice changes to less dense liquid water, the pressure inside the moon decreases. The researchers calculated that on small icy moons such as Saturn’s Mimas and Enceladus and Uranus’s Miranda, the pressure drop could be large enough to reach the triple point (a condition in which ice, liquid water, and water vapor can coexist).
Miranda images taken by the Voyager 2 spacecraft reveal massive ridges and steep cliffs known as coronae. Researchers suggest that a boiling ocean beneath the surface could explain how these remarkable landforms formed.
Why the size of the moon is important
Mimas is less than 400 miles wide and has many craters, including one massive impact crater that gave it its nickname “Death Star.” Although it appears geologically inactive, Rudolph noted that subtle wobbles in its movement suggest an ocean hidden beneath. Mimas’ ice shell is not expected to crack as it thins, so the moon may have an ocean even though the moon’s surface still appears inactive.
Size plays an important role in how these processes unfold. For large icy moons like Uranus’ other moon, Titania, the researchers found that the drop in pressure caused by melting is likely to cause the ice shells to crack before reaching the triple point of water. Titania’s surface features may therefore reflect a cycle in which the ice shell first thins and then thickens again.
Just as studying Earth’s geology helps scientists understand how our planet has evolved over billions of years, studying the internal activity of icy moons provides clues to why its surface is the way it is today, Rudolph said.
The co-authors of this paper are: Michael Manga, University of California, Berkeley. Alyssa Roden, Southwest Research Institute, Boulder. and Matthew Walker of the Planetary Science Institute in Tucson. This research was supported in part by NASA.
