The outer planets of the Solar System are swarmed by ice-wrapped moons. Some of these, such as Saturn's moon Enceladus, are known to have oceans of liquid water between the ice shell and the rocky core and could be the best places in our solar system to look for extraterrestrial life. A new study published Nov. 24 in Nature Astronomy sheds light on what could be going on beneath the surface of these worlds and provides insights into how their diverse geologic features may have formed.
"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."
From mountains to earthquakes, Earth's surface geology is powered by the movement and melting of rock deep inside the planet. On icy moons, geology is driven by the action of water and ice.
These worlds are heated by tidal forces from the planet they orbit. The moons orbiting a planet can interact, leading to periods of higher and lower heating. Higher heating can melt and thin the ice layer; when heating decreases, the ice gets thicker.
Rudolph and colleagues had previously looked at what happens when the ice shell gets thicker. They found that because ice has a greater volume than liquid water, freezing 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 Miranda, a moon of Uranus, the pressure could drop low enough to reach the triple point at which ice, liquid water and water vapor can all co-exist.
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.
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. Because Mimas' ice shell is not expected to break as a result of ice shell thinning, 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.
Just as geology on Earth helps us understand why our planet looks the way it does after billions of years of change, understanding geological processes on these moons can help us see why they have the features that they do, 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.
