A new study suggests Uranus鈥� moon Miranda may harbor a water ocean beneath its surface, a finding that would challenge many assumptions about the moon鈥檚 history and composition and could put it in the company of the few select worlds in our solar system with potentially life-sustaining environments.

鈥淭o find evidence of an ocean inside a small object like Miranda is incredibly surprising,鈥� said Tom Nordheim, a planetary scientist at the 秘密直播 Applied Physics Laboratory (APL) in Laurel, Maryland, a study co-author, and the principal investigator on the project that funded the study. 鈥淚t helps build on the story that some of these moons at Uranus may be really interesting 鈥� that there may be several ocean worlds around one of the most distant planets in our solar system, which is both exciting and bizarre.鈥�

Among the moons in the solar system, Miranda stands out. The few images Voyager 2 captured in 1986 show Miranda鈥檚 southern hemisphere (the only part we鈥檝e seen) is a Frankenstein-like hodgepodge of grooved terrain quartered off by rough scarps and cratered areas, like squares on a quilt. Most researchers suspect these bizarre structures are the result of tidal forces and heating within the moon.

Caleb Strom, a graduate student at the University of North Dakota who worked with Nordheim and Alex Patthoff of the Planetary Science Institute in Arizona, revisited the Voyager 2 images. The team set out to explain Miranda鈥檚 enigmatic geology by reverse engineering the surface features, working backward to uncover what the moon鈥檚 interior structure must have been to shape the moon鈥檚 geology in response to tidal forcing.

After first mapping the various surface features like cracks, ridges and Miranda鈥檚 unique trapezoidal coronae, the team developed a computer model to test several possible structures of the moon鈥檚 interior, matching the predicted stress patterns to the actual surface geology.

The setup that produced the best match between predicted stress patterns and observed surface features required the existence of vast ocean beneath Miranda鈥檚 icy surface some 100-500 million years ago. This subsurface ocean was at least 62 miles (100 kilometers) deep, according to the , and hidden beneath an icy crust no more than 19 miles (30 kilometers) thick. Given Miranda has a radius of just 146 miles (235 kilometers), the ocean would have filled almost half of the moon鈥檚 body. 鈥淭hat result was a big surprise to the team,鈥� Strom said.

Key to creating that ocean, the researchers believe, were tidal forces between Miranda and nearby moons. These regular gravitational tugs can be amplified by orbital resonances 鈥� a configuration where each moon鈥檚 period around a planet is an exact integer of the others鈥� periods. Jupiter鈥檚 moons Io and Europa, for example, have a 2:1 resonance: For every two orbits Io makes around Jupiter, Europa makes exactly one, leading to tidal forces that are known to sustain an ocean beneath Europa鈥檚 surface.

These orbital configurations and the resulting tidal forces deform the moons like rubber balls, leading to friction and heat that keeps interiors warm. This also creates stresses that crack the surface, creating a rich tapestry of geologic features. Numerical simulations have suggested that Miranda and its neighboring moons likely had such a resonance in the past, offering a potential mechanism that could have warmed Miranda鈥檚 interior to produce and maintain a subsurface ocean.

At some point, the moons鈥� orbital ballet desynchronized, slowing the heating process so that the moon鈥檚 insides started to cool and solidify. But the team doesn鈥檛 think Miranda鈥檚 interior has fully frozen yet. If the ocean had completely frozen, Nordheim explained, it would have expanded and caused certain telltale cracks on the surface, which aren鈥檛 there. This suggests that Miranda is still cooling 鈥� and may have an ocean beneath its surface even now. Miranda鈥檚 modern-day ocean is probably relatively thin, Strom noted. 鈥淏ut the suggestion of an ocean inside one of the most distant moons in the solar system is remarkable,鈥� he said.

Miranda wasn鈥檛 predicted to have an ocean. With its small size and old age, scientists thought it would likely be a frozen ball of ice. Any leftover heat from its formation was assumed to have dissipated long ago. But as Patthoff pointed out, predictions about ice moons can be wrong, as evidenced by Saturn鈥檚 moon Enceladus. Before the Cassini spacecraft arrived in 2004, many scientists thought Enceladus was a frozen ball of ice and rock. But it was actually harboring a global ocean and active geological processes. 鈥淔ew scientists expected Enceladus to be geologically active,鈥� Patthoff said. 鈥淗owever, it鈥檚 shooting water vapor and ice out of its southern hemisphere as we speak.鈥� Enceladus is now a prime target in the search for life beyond Earth.

Miranda might be a similar case. It鈥檚 comparable in size and composition to Enceladus, and according to a 2023 study led by APL鈥檚 Ian Cohen, it may be actively releasing material into space. If it has (or even had) an ocean, it could be a future target for studying habitability and life. However, Nordheim cautions that there鈥檚 still too much we don鈥檛 know about Miranda and the Uranian moons to speculate about the existence of life.

鈥淲e won鈥檛 know for sure that it even has an ocean until we go back and collect more data,鈥� he said. 鈥淲e鈥檙e squeezing the last bit of science we can from Voyager 2鈥檚 images. For now, we鈥檙e excited by the possibilities and eager to return to study Uranus and its potential ocean moons in depth.鈥�