Boiling Oceans Hidden Beneath Icy Moons’ Surfaces
In a fascinating new revelation, scientists have discovered that small, icy moons in our solar system may harbor subsurface oceans that are literally boiling, despite their frigid environments. This surprising finding, reported in Nature Astronomy on November 24, challenges our understanding of these distant worlds and could help explain unusual geological features observed on their surfaces. The research suggests that as the thickness of a moon’s icy crust changes over time, pressure conditions in the underlying ocean can reach a critical point where water boils at unexpectedly low temperatures—around zero degrees Celsius.
“The idea that a subsurface ocean on an icy world could actually reach boiling conditions is a really remarkable scenario,” explains Max Rudolph, a planetary geophysicist from the University of California, Davis who led the research. The process begins with the constant gravitational squeezing these moons experience from their parent planets. This gravitational kneading generates heat within the moons—a process that can intensify over time as orbits evolve, particularly in systems with multiple moons exerting gravitational influences on each other. When this heating increases, the researchers found that a moon’s icy shell may begin melting from beneath, gradually thinning the crust and decreasing pressure on the ocean below.
This pressure reduction is particularly significant for smaller moons. According to the new computer simulations, when a small moon’s ice shell thins enough, the decreased pressure can cause the subsurface ocean to reach what scientists call its “triple point”—a unique condition where ice, liquid water, and water vapor can coexist. At this point, despite the extremely cold temperatures, the ocean begins to boil. The escaping gases from this boiling process could potentially create cracks and other distinctive geological features visible on the moon’s surface. This mechanism might explain formations like the strange ridges called coronae observed on Miranda, one of Uranus’ moons, which has puzzled scientists since their discovery.
The size of the moon plays a crucial role in determining whether this boiling phenomenon can occur. The research team discovered that only moons smaller than approximately 600 kilometers in diameter can experience these boiling ocean conditions. For larger moons with stronger gravitational fields, such as Uranus’ moon Titania, the stresses on the icy surface would cause it to crack before the ocean could reach its triple point. “These cracking patterns may account for features observed in images taken by the Voyager 2 spacecraft during its 1986 flyby,” Rudolph notes. Meanwhile, the absence of visible geological features on some moons thought to contain subsurface oceans, such as Saturn’s moon Mimas, might be explained by a thicker ice shell that successfully contains the pressurized ocean below.
Not all planetary scientists are fully convinced by these findings. Bill McKinnon from Washington University in St. Louis acknowledges the importance of studying ice melting effects but questions whether such large-scale ocean boiling would actually occur in small moons. He points to what he considers contradictory geological evidence, such as the lack of expected features on Mimas. However, Paul Byrne, also from Washington University but not involved in the study, finds value in the research: “The overlying ice shell thickening and thinning over time might provide a means to explain how water could get through the ice shell from the ocean beneath to the surface,” he says, adding that the release of gases from an ocean can help explain the appearance of these icy moons.
Beyond enhancing our understanding of celestial bodies in our solar system, these findings have intriguing implications for the search for extraterrestrial life. “Looking at when these landforms developed could tell you something about when the ocean developed,” Rudolph explains. “And knowing that these icy worlds possess or have possessed a subsurface ocean through much of their geologic histories is really important in terms of thinking about the potential of those worlds to host life-forms.” This research opens new avenues for exploration and raises the possibility that these small, seemingly frozen moons might be more dynamic and potentially habitable than previously thought—adding to the growing list of places in our solar system where we might one day find evidence of life beyond Earth.
