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Growing evidence suggests that a subsurface ocean lurks beneath the icy surface of Uranus' moon Ariel, but new , published in Icarus, characterizes the possible evolution of this ocean, and found that it may have once been over 100 miles (170 kilometers) deep. For perspective, the Pacific Ocean averages 2.5 miles (4 kilometers) deep.

"Ariel is pretty unique in terms of icy moons," said paper co-author Alex Patthoff, a Planetary Science Institute senior scientist.

Ariel is Uranus' brightest and second-closest moon, and at just 720 miles (1,159 km) across (the distance from PSI in Tucson to Salt Lake City, Utah), it is the fourth-largest in the Uranian system. It has very old geological features, like craters, next to very young ones, like smooth terrain possibly created by cryovolcanism, said paper first author Caleb Strom, a recent graduate of the University of North Dakota. It has fractures, ridges and grabens—crust that has dropped lower than its surroundings—at scales larger than almost anywhere else in the solar system.

It's this dramatic surface that prompted the study. The research team wanted to understand the past interior structure and eccentricity—how much a body's orbit deviates from circular—that would be needed to produce the features seen on Ariel's surface today. Both characteristics can contribute to how much stress can be applied to the surface causing it to fracture under the push and pull of gravity as the small moon orbits its gas giant.

"First, we mapped out the larger structures that we see on the surface, then we used a computer program to model the tidal stresses on the surface, which result from distortion of Ariel from soccer ball-shaped to slight football-shaped and back as it moves closer and farther from Uranus during its orbit," Patthoff said. "By combining the model with what we see on the surface, we can make inferences about Ariel's past eccentricity and how thick the ocean might have been."

The team found that, in the past, Ariel needed to have an eccentricity of about 0.04. This is about 40 times larger than its current value. While 0.04 may not sound dramatic, eccentricity can strengthen the effects of tidal stresses, and Ariel's orbit would have been four times more eccentric than that of Jupiter's moon Europa, which is wracked by the tidal forces that push and pull it to create its cracked and broken surface. Yet, to the eye, the orbit will still resemble a circle.

"In order to create those fractures, you have to have either a really thin ice on a really big ocean, or a higher eccentricity and a smaller ocean," Patthoff said. "But either way, we need an ocean to be able to create the fractures that we are seeing on Ariel's surface."

What's more, this is the second in a series of papers investigating the past subsurface of Uranus' moons. Last year, this same team published a paper on Miranda with .

"We are finding evidence that the Uranus system may harbor twin ocean worlds," said co-author, Tom Nordheim of Johns Hopkins University Applied Â鶹ÒùÔºics Laboratory and principal investigator.

"Unfortunately, we've only seen the southern hemispheres of Ariel and Miranda. But our results can give us predictions of what a future spacecraft might see on the moons' unimaged northern hemispheres, such as the location of fractures and ridges there. Ultimately, we just need to go back to the Uranus system and see for ourselves."

Researchers still aren't sure exactly how long ago this deep ocean might have existed. However, this work will provide an important input for future research that investigates the behavior of the outer solar system oceans over time.