The outer planets are surrounded by dozens of moons locked in thick shells of ice. Some of these frozen worlds, including Saturn’s moon Enceladus, are believed to contain vast oceans of liquid water trapped between their icy crust and rocky interiors. Because liquid water is essential for life as we know it, these moons rank among the most promising places in the solar system to search for extraterrestrial life.
A study published in Nature Astronomy explores what may be happening far below their frozen surfaces and offers new explanations for the unusual landscapes seen on several of these moons.
“Not all of these satellites are known to have oceans, but we know that some do,” said Max Rudolph, associate professor of earth and planetary sciences at the University of California, Davis and lead author on the paper. “We’re interested in the processes that shape their evolution over millions of years and this allows us to think about what the surface expression of an ocean world would be.”
How Tidal Heating Shapes Icy Worlds
On Earth, features such as mountains and earthquakes are driven by heat and moving rock deep underground. On icy moons, water and ice play that same role.
These moons are warmed by tidal forces generated by the massive planets they orbit. Gravitational interactions between neighboring moons can cause heating levels to rise and fall over time. When heating intensifies, the ice shell can melt and become thinner. When heating eases, the shell thickens again as water refreezes.
In earlier work, Rudolph and his colleagues studied what happens when the ice shell grows thicker. Because ice takes up more space than liquid water, freezing increases pressure on the surrounding shell. That pressure may help create dramatic surface features such as the long fractures known as the “tiger stripes” on Enceladus.
When Hidden Oceans Begin to Boil
The new study examines the opposite scenario. What happens when the ice shell melts from below and becomes thinner?
According to the researchers, that process could cause the underlying ocean to boil.
As ice turns into less-dense liquid water, pressure inside the moon decreases. The team calculated that on smaller icy moons, including Saturn’s Mimas and Enceladus, as well as Miranda orbiting Uranus, the pressure drop could be significant enough to reach the triple point, the condition at which ice, liquid water, and water vapor can exist together.
Images of Miranda taken by the Voyager 2 spacecraft reveal enormous ridges and steep cliffs known as coronae. The researchers suggest that ocean boiling beneath the surface may explain how these striking features formed.
Why Moon Size Matters
Mimas is less than 250 miles wide and heavily cratered, including one massive impact crater that gives it the nickname “Death Star.” Although it appears geologically inactive, Rudolph noted that a subtle wobble in its motion hints at a hidden ocean below. Because the ice shell on Mimas is not expected to fracture as it thins, it is possible for the moon to have an ocean while still looking inactive at the surface.
Size plays a critical role in how these processes unfold. On larger icy moons such as Titania, another moon of Uranus, the drop in pressure caused by melting would likely crack the ice shell before reaching the triple point for water, the team found. Titania’s surface features may therefore reflect a cycle in which the ice shell first thinned and later thickened again.
Just as studying Earth’s geology helps scientists understand how our planet evolved over billions of years, examining the internal activity of icy moons offers clues to why their surfaces look the way they do today, Rudolph said.
Coauthors on the paper are: Michael Manga, UC Berkeley; Alyssa Rhoden, Southwest Research Institute, Boulder; and Matthew Walker, Planetary Science Institute, Tucson. The work was supported in part by NASA.