"To find evidence of an ocean inside a small object like Miranda is incredibly surprising," said Tom Nordheim, a planetary scientist at Johns Hopkins Applied Physics Laboratory (APL) in Maryland and the study's co-author. Nordheim, who led the project's funding, added, "It helps build on the story that some of these moons at Uranus may be really interesting - that there may be several ocean worlds around one of the most distant planets in our solar system, which is both exciting and bizarre."
Miranda is one of the solar system's most geologically unique moons. Captured images from Voyager 2's 1986 flyby show that its southern hemisphere is a patchwork of varied terrains, including grooved areas intersected by rugged scarps and cratered fields. Many scientists believe that these unusual structures result from intense tidal forces and heat within the moon itself.
Caleb Strom, a graduate student at the University of North Dakota working with Nordheim and Alex Patthoff of the Planetary Science Institute in Arizona, re-examined these Voyager images to understand Miranda's geology better. By analyzing its surface features, Strom's team aimed to deduce Miranda's past internal structure, influenced by tidal forces.
The researchers first mapped surface characteristics such as cracks, ridges, and the moon's distinctive trapezoidal coronae. Then, they created a computer model to simulate different internal structures, comparing predicted stress patterns to the observed surface features. Their findings indicated that Miranda likely had a subsurface ocean between 100 and 500 million years ago. This ancient ocean was estimated to be around 62 miles (100 kilometers) deep and shielded beneath an icy crust approximately 19 miles (30 kilometers) thick. Given Miranda's radius of only 146 miles (235 kilometers), this ocean would have occupied nearly half of the moon's volume. "That result was a big surprise to the team," Strom said.
The study suggests that gravitational interactions with nearby moons may have triggered this subsurface ocean. Orbital resonances, a condition in which celestial bodies complete their orbits in precise ratios, amplify tidal forces that can create the necessary internal friction and heat to sustain an ocean. This process occurs in Jupiter's moons Io and Europa, where a 2:1 orbital resonance leads to enough tidal heating to maintain Europa's own hidden ocean.
Such resonances likely affected Miranda in the past, warming its core to support an ocean beneath its crust. However, over time, these orbital interactions weakened, allowing Miranda's interior to cool and start freezing. Yet the study team suspects that the freezing process remains incomplete. "If the ocean had completely frozen," Nordheim explained, "it would have expanded and caused certain telltale cracks on the surface, which aren't there." This incomplete freezing hints that a modern, albeit thinner, ocean may still lie beneath Miranda's surface. "But the suggestion of an ocean inside one of the most distant moons in the solar system is remarkable," Strom said.
Historically, Miranda's small size and estimated age led scientists to assume it would be a frozen sphere of ice, having lost its heat long ago. But, as Alex Patthoff noted, assumptions about icy moons have often been challenged, such as in the case of Saturn's moon Enceladus. Before the arrival of the Cassini spacecraft in 2004, Enceladus was thought to be a frozen mass. Cassini's data revealed a global ocean and active geology. "Few scientists expected Enceladus to be geologically active," Patthoff commented. "However, it's shooting water vapor and ice out of its southern hemisphere as we speak." Enceladus is now a priority for researchers studying potential habitats for extraterrestrial life.
Miranda could offer similar opportunities. It shares many characteristics with Enceladus, and, according to a 2023 study by APL's Ian Cohen, might even be releasing materials into space. If it has an ocean, it could one day be studied for its potential habitability. However, Nordheim emphasized that knowledge of Miranda and Uranus' moons is still limited, making it premature to speculate on life.
"We won't know for sure that it even has an ocean until we go back and collect more data," he said. "We're squeezing the last bit of science we can from Voyager 2's images. For now, we're excited by the possibilities and eager to return to study Uranus and its potential ocean moons in depth."
Research Report:Constraining Ocean and Ice Shell Thickness on Miranda from Surface Geological Structures and Stress Modeling
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