Uranus’ moon Miranda is known as a world of amazing icy cliffs. New research has led scientists to believe that deep beneath them may lie an ocean.
Exploration of Uranus’ moon Miranda
A new study suggests that Uranus’ moon Miranda may contain a water ocean beneath its surface. This conclusion challenges many assumptions about the history and composition of this celestial body and may put it on par with a few selected worlds in our Solar System with potentially habitable environments.
“To find evidence of an ocean inside a small object like Miranda is incredibly surprising,” said Tom Nordheim, a planetologist at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and co-author of the study published in The Planetary Science Journal.
This study helps develop the theory that some of Uranus’ moons may be really interesting. Several ocean worlds may exist around one of the most distant planets in our Solar System, which is both fascinating and bizarre.
Among the moons of the Solar System, Miranda is a standout. Several images taken by Voyager-2 in 1986 show that its southern hemisphere (the only part we’ve seen) is a mishmash of icy cliffs forming a grooved topography riddled with uneven ledges and craters like squares on a quilt. Most researchers suspect that these bizarre structures are the result of tidal forces and heating of 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, reviewed the Voyager-2 images. The team decided to explain Miranda’s mysterious geology by reverse engineering the surface features, working backwards to figure out what the moon’s internal structure must have been to shape the moon’s geology in response to tidal forces.
Subsurface ocean and structure of Uranus’ moon
After first mapping various surface features such as fractures, ridges, and Miranda’s unique trapezoidal coronae, the team developed a computer model to test several possible structures of the moon’s interior by comparing predicted stress patterns to the real surface geology.
The setup that provided the best match between predicted stress patterns and observed surface features required the existence of a vast ocean beneath Miranda’s icy surface about 100-500 million years ago. According to the study, the depth of this subsurface ocean was at least 100 kilometers, and the thickness of the ice crust didn’t exceed 30 kilometers.
Given that Miranda has a radius of only 235 kilometers, the ocean would fill almost half of the moon’s body. “That result was a big surprise to the team,” Strom said.
Researchers believe that tidal forces between Miranda and neighboring moons were a key factor in the creation of this ocean. These regular gravitational tugs can be enhanced by orbital resonances, a configuration where each moon’s period of orbit around a planet is an accurate integer of the periods of the other moons.
Orbital impact on Miranda’s subsurface
For example, Jupiter’s moons Io and Europa have a 2:1 resonance: For every two orbits Io makes around Jupiter, Europa makes exactly one, resulting in tidal forces that are known to support the 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 keep the heat inside. This also creates stresses that crack the surface, creating a rich tapestry of geologic features. Numerical simulations suggest that Miranda and its neighboring moons probably had such a resonance in the past, suggesting a potential mechanism that could have heated Miranda’s interior to create and maintain a subsurface ocean.
By some point, the orbital ballet of moons desynchronized, slowing the heating process so that the moon’s interior began to cool and solidify. But the team believes Miranda’s interior hasn’t completely frozen over yet. If the ocean froze completely, it would expand and cause certain noticeable cracks in the surface that do not exist there.
Similar cases in the Solar System
No ocean was thought to exist on Miranda. Given its small size and antiquity, scientists thought it was most likely a frozen ball of ice. The residual heat from its formation was thought to have dissipated long ago. But, as Patthoff notes, predictions regarding icy moons can be wrong, as evidenced by Saturn’s moon Enceladus.
Until the arrival of the Cassini spacecraft in 2004, many scientists thought Enceladus was a frozen ball of ice and rock. But it actually hid a global ocean and active geologic processes within it. “Few scientists expected Enceladus to be geologically active,” Patthoff said. Enceladus is now a major target for the search for life beyond Earth.
Miranda may be a similar case. It is comparable in size and composition to Enceladus, and according to a 2023 study led by APL’s Ian Cohen, it may be actively ejecting material into space. If Miranda has an ocean, it could be a future target for a livability study. However, Nordheim cautions that we still don’t know too much about Miranda and Uranus’ moons to make assumptions about the existence of 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.”
Provided by phys.org
