The icy moons of our outer solar system are considered among the best chances for finding alien life close to Earth, and new research is now revealing how the subsurface oceans on these distant moons may have driven their geological development.
Moons like Saturn’s Titan and Enceladus, along with Jupiter’s Europa, fascinate astronomers interested in extraterrestrial life. Now, thanks to new simulations of these moons’ tidal forces by researchers at the University of California, Davis, their research, published in Nature Astronomy, provides even deeper insights into their formation.
Subsurface Oceans
“Not all of these satellites are known to have oceans, but we know that some do,” said lead author Max Rudolph, associate professor of earth and planetary sciences at the University of California, Davis. “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.”
Despite their liquid water and potential for habitability, these moons are very different environments from Earth. On Earth, the surface geology stems from underground rock activity: the hot core melts rock into lava, and tectonic plates shift. Icy moons, however, are shaped by the actions of ice and water rather than hot rocks.
The tidal forces from the planets they orbit provide the moons with subsurface heat, which keeps their oceans liquid. Since a planet like Saturn has many moons, those bodies can interact as they orbit, increasing or decreasing heating for a period. As the heat increases, the thick layer of ice covering the moon may begin to melt and thin, then thicken again as the heating subsides.
The Geology of Icy Moons
The researchers investigated the effects of this activity over time. Typically, the ice shell would thin or thicken from underneath. Since ice has a greater volume than liquid water, more water would be squeezed into a smaller space as the ice thickened. This means that the shell would be under increased pressure to contain the water beneath it, which may be a potential explanation for the “tiger stripes” seen on Enceladus.
However, the researchers also considered the opposite scenario, when the shell begins to melt. Surprisingly, this could cause the ocean to boil, the team says. With an increase in relatively low-density liquid water, the pressure beneath the shell would drop. On some smaller moons of Uranus’s Miranda and Saturn’s Mimas and Enceladus, that pressure drop could reach such an extreme that ice, liquid water, and water vapor could all co-exist at the same time.
The ridges and cliffs on Miranda, observed by the Voyager 2 space probe, may be the result of such ocean boiling, the researchers suggest. At a mere 250 miles wide, Mimas is a tiny moon nicknamed “Death Star” for a large, prominent crater that makes it reminiscent of the space station from the Star Wars films. Although observations suggest that Mimas is geologically dead, a wobble in the moon’s orbit is indicative of a subsurface ocean. The strange contradiction of a liquid ocean and geologically frozen surface is reconciled by the fact that its ice shell is estimated to be strong enough that tanning would not crack it.
Cracking the Shell
Larger moons fare far differently. The icy covering of Uranus’s moon Titania would likely experience cracks well before ice melt would allow water to reach the triple point, according to the team’s calculations. Therefore, the team believes that the surface geology on Titania is likely driven by the thickening and thinning of its ice shell.
The new paper shows how even within a relatively small category of objects within our local solar system, such as icy moons, very different conditions may be present. Geology explains the surface features of planets like our own as they develop over tremendous periods of time. In the new research, it is revealed that geology influenced by factors very different from our own can also explain an object’s surface evolution.
The paper, “Boiling Oceans and Compressional Tectonics on Emerging Ocean Worlds,” appeared in Nature Astronomy on November 24, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.