Uranus' Moon Miranda May Have an Ocean Beneath Its Surface, Study Finds

A new study suggests Uranus' moon Miranda may harbor a vast subsurface ocean, challenging previous assumptions about its frozen state and positioning it as a potential ocean world alongside other icy moons. Phys.Org reports: Among the moons in the solar system, Miranda stands out. The few images Voyager 2 captured in 1986 show Miranda's southern hemisphere (the only part we've 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's enigmatic geology by reverse engineering the surface features, working backward to uncover what the moon's interior structure must have been to shape the moon's geology in response to tidal forcing.
 
After first mapping the various surface features like cracks, ridges and Miranda's unique trapezoidal coronae, the team developed a computer model to test several possible structures of the moon's 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's icy surface some 100-500 million years ago. This subsurface ocean was at least 62 miles (100 kilometers) deep, according to the study, 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's body. "That result was a big surprise to the team," 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 -- a configuration where each moon's period around a planet is an exact integer of the others' periods. Jupiter's 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's 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's interior to produce and maintain a subsurface ocean.
 
At some point, the moons' orbital ballet desynchronized, slowing the heating process so that the moon's insides started to cool and solidify. But the team doesn't think Miranda's 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't there. This suggests that Miranda is still cooling -- and may have an ocean beneath its surface even now. Miranda's modern-day ocean is probably relatively thin, Strom noted. "But the suggestion of an ocean inside one of the most distant moons in the solar system is remarkable," he said. Miranda wasn't 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's moon Enceladus. The study has been published in The Planetary Science Journal.
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