Scientists using NASA’s Hubble Space Telescope set out to study how Uranus’s magnetic field interacts with its largest moons—but instead, they stumbled upon a surprising discovery.
They focused on four of Uranus’s biggest moons—Ariel, Umbriel, Titania, and Oberon. These moons are “tidally locked,” which means they always show the same face to Uranus as they orbit it.
The scientists expected that Uranus’s magnetic field would mostly affect the sides of the moons that face backward as they move, called the trailing sides.
Based on previous data, they thought charged particles in the planet’s magnetosphere would hit these trailing sides, darkening them over time. But what they actually found was the opposite.
Instead of the trailing sides being darker, the leading sides—those facing forward in the moons’ orbits—were darker, especially on the two outer moons, Titania and Oberon.
The inner moons, Ariel and Umbriel, showed almost no difference in brightness between their leading and trailing sides. This completely contradicted what scientists expected to see.
So what’s going on?
The research team believes they’ve found a different explanation.
They think dust from Uranus’s irregular moons—small satellites with unusual, tilted orbits—is slowly drifting inward over time. Tiny meteorites regularly hit these irregular moons, kicking up dust that floats through space.
As this dust moves toward Uranus, it crosses paths with Titania and Oberon. These two outer moons, traveling through the dust on their leading sides, collect it—just like bugs hitting the front windshield of a car.
This dust buildup makes their forward-facing sides darker and redder in color. Meanwhile, the inner moons are shielded by the outer ones and don’t gather as much dust, so they look more evenly bright.
This dusty coating theory has been seen before in the Saturn system and possibly around Jupiter as well. But it’s the first time scientists have spotted this kind of effect around Uranus, adding a new layer of mystery to this already unusual planet.
Uranus is a strange world to begin with. It’s tilted almost completely on its side, with its magnetic field tilted at a different angle again. Because of this, its magnetic field rotates in a bizarre way that’s still not fully understood.
Scientists had hoped Hubble could help clarify how that magnetic field affects the moons. But now they’re wondering whether Uranus’s magnetosphere is much weaker or more complex than they thought. Maybe it doesn’t affect the moons in the way earlier research predicted—or maybe it does, but the signs are being hidden by something else, like the dust.
Hubble’s ultraviolet abilities were crucial for this research. Earth’s atmosphere blocks ultraviolet light, so this kind of observation can’t be done from the ground.
Hubble is the only space telescope with both the ultraviolet vision and sharp resolution needed to study the moons in this detail. Data from the James Webb Space Telescope may also help fill in the rest of the picture, offering a broader understanding of how these icy moons interact with Uranus and its magnetic environment.
As lead scientist Richard Cartwright put it, “Data always surprises you”—especially when it comes from a planet as weird as Uranus.
