The outer planets are surrounded by dozens of moons locked in thick shells of ice. A few 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 crucial for all times as we understand it, these moons rank amongst probably the most promising places within the solar system to look for extraterrestrial life.
A study published in Nature Astronomy explores what could also be happening far below their frozen surfaces and offers latest explanations for the weird landscapes seen on several of those moons.
“Not all of those satellites are known to have oceans, but we all know that some do,” said Max Rudolph, associate professor of earth and planetary sciences on the University of California, Davis and lead creator on the paper. “We’re all in favour of the processes that shape their evolution over thousands and thousands of years and this enables us to take into consideration what the surface expression of an ocean world can be.”
How Tidal Heating Shapes Icy Worlds
On Earth, features akin to mountains and earthquakes are driven by heat and moving rock deep underground. On icy moons, water and ice play that very same role.
These moons are warmed by tidal forces generated by the huge planets they orbit. Gravitational interactions between neighboring moons could cause heating levels to rise and fall over time. When heating intensifies, the ice shell can melt and turn into 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 room than liquid water, freezing increases pressure on the encircling shell. That pressure may help create dramatic surface features akin to the long fractures referred to as the “tiger stripes” on Enceladus.
When Hidden Oceans Begin to Boil
The brand new study examines the other scenario. What happens when the ice shell melts from below and becomes thinner?
In accordance with the researchers, that process could cause the underlying ocean to boil.
As ice turns into less-dense liquid water, pressure contained in the moon decreases. The team calculated that on smaller icy moons, including Saturn’s Mimas and Enceladus, in addition to Miranda orbiting Uranus, the pressure drop might be significant enough to achieve 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 referred to as coronae. The researchers suggest that ocean boiling beneath the surface may explain how these striking features formed.
Why Moon Size Matters
Mimas is lower than 250 miles wide and heavily cratered, including one massive impact crater that offers it the nickname “Death Star.” Even though it appears geologically inactive, Rudolph noted that a subtle wobble in its motion hints at a hidden ocean below. Since the ice shell on Mimas isn’t expected to fracture because it thins, it is feasible for the moon to have an ocean while still looking inactive on the surface.
Size plays a critical role in how these processes unfold. On larger icy moons akin to Titania, one other moon of Uranus, the drop in pressure attributable to melting would likely crack the ice shell before reaching the triple point for water, the team found. Titania’s surface features may due to this fact reflect a cycle during 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 inner activity of icy moons offers clues to why their surfaces look the way in which 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 partially by NASA.