Uranus is a dangerous place for its moons

In a few million years, things are going to get ugly.
By | Published: September 11, 2017
Uranus is one of the most distant worlds in our solar system — and one of the most dangerous for the moons orbiting it.
Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory
Discovered in 1781, Uranus is an ice giant orbiting our Sun once every 84 Earth years. This mysterious world, which appears as just a tiny dot in most amateur telescopes, not only possesses a system of thin, faint rings, but also 27 moons (by our current count). However, at least one of those things is destined to change: new measurements indicate at least two likely collisions between four of the planet’s moons millions of years in the future.

Robert Chancia at the University of Idaho and his colleagues set out to better understand Uranus’ Eta ring. They discovered that the ring’s shape is not perfectly circular, but instead it is triangular — and the cause of the distortion is the tiny moon Cressida, just 51 miles (82 kilometers) across. Based on the size and shape of the distortion, the team was able to accurately measure Cressida’s mass and density; they used these qualities to determine that gravitational interactions between Cressida and other nearby moons will mean a collision between Cressida and another moon, most likely Desdemona, is imminent. Their work has been accepted for publication in the Astronomical Journal.

The collision between Cressida and Desdemona, currently orbiting just 560 miles (900km) apart, is likely to take place within the next million years. And they’re not the only two moons destined for doom: In 2012, SETI Institute researchers Robert French and Mark Showalter determined that the moons Cupid and Belinda were likely to collide between 10 and 1,000 million years from now.

These future collisions seem even more probable when viewed in light of two diffuse rings around the planet today that likely formed from the debris of previous collisions between now-long-gone moons.

The Hubble Space Telescope spotted eight of Uranus’ moons while tracking clouds in the ice giant’s atmosphere. Cressida and Desdemona are fated to collide in about a million years.
Determining Cressida’s fate wasn’t originally the goal of the study. The team initially set out to find the cause of the Eta ring’s distortion, which orbits the planet much faster than the individual particles in the rings. The speed of the distortion, they found, instead matched the orbital speed of Cressida, linking the two together as cause and effect.

As a result, Chancia and his colleagues made the first mass measurement of the moon, finding Cressida is about 1/300,000th the mass of Earth’s Moon and has a density 86 percent that of water. This actually makes Cressida denser than many of Saturn’s small moons, indicating that it likely contains a decent amount of rock as well as water ice.

Although several of the planet’s moons are slated to someday collide, the impending collisions might not completely destroy the moons involved. If that happens, Uranus could end up with “new” moons that future astronomers refer to as Cupbel and Cresdemona.

It seems that celebrity names are the only ones subjected to potential mash-ups.
SOURCE: New Scientist