Ariel, Uranus’ Brightest Moon, May Have Once Hidden an Ocean Over 100 Miles Deep

New research published in Icarus suggests that Ariel, one of Uranus’ major moons, may once have harbored a vast subsurface ocean over 100 miles (170 kilometers) deep.

This icy world, known for its fractured terrain and contrasting young and ancient geological features, is increasingly emerging as a potential “ocean world.” By modeling tidal stresses and orbital eccentricity, scientists have reconstructed how Ariel’s dramatic surface features may have formed — and what they reveal about its hidden past.

phys.org

1. Evidence of a past ocean: Ariel likely hosted a subsurface ocean more than 100 miles deep, far deeper than Earth’s average ocean depth of 2.5 miles.
2. Unique characteristics: Ariel is Uranus’ brightest and second-closest moon, measuring 720 miles (1,159 km) across, the fourth largest in the Uranian system.
3. Surface diversity: The moon’s surface combines very old geological features like craters with much younger terrains, possibly shaped by cryovolcanism.
4. Tectonic structures: Fractures, ridges, and grabens cover Ariel, some at scales larger than anywhere else in the solar system.
5. Tidal stress modeling: Researchers simulated how Ariel’s shape shifts slightly during its orbit — from nearly spherical to slightly elongated — causing stress and fracturing.
6. Orbital eccentricity: The team determined Ariel once had an eccentricity of about 0.04, forty times greater than its current value, intensifying tidal stresses.
7. Comparison with Europa: Ariel’s past orbit would have been four times more eccentric than Jupiter’s moon Europa, a known ocean world.
8. Ocean essential for fractures: The observed fractures on Ariel’s surface could only have formed if an ocean existed beneath the ice, regardless of its exact thickness.
9. Parallel with Miranda: This study is the second in a series; a previous paper found similar evidence of a subsurface ocean on Uranus’ moon Miranda.
10. Future exploration needed: Researchers stress the importance of future missions to Uranus to directly observe the northern hemispheres of Ariel and Miranda, which remain unmapped.