The unexplored secrets of Uranus are still far out of reach, and the latest observations of the icy planet’s moons have added yet another mystery to the fold. On the hunt for clues about the planet’s magnetic environment, a team of researchers encountered an unusual surprise related to the brightness of four moons of Uranus — Ariel, Umbriel, Titania, and Oberon.
These tidally locked moons each have a “leading” side that always faces the same direction when orbiting Uranus, and a “trailing” side that always faces away. Researchers believed that the trailing side of the moons would be darker than the leading side, but this assumption has been completely turned on its head thanks to findings from NASA's Hubble Space Telescope that illustrate the utter weirdness of Uranus.
The Mysteries of Uranus
The magnetosphere of Uranus is not fully understood quite yet, with new details continuing to emerge. NASA’s Voyager 2 spacecraft initially observed Uranus in 1986, finding odd characteristics exhibited by its magnetosphere (a region around a planet that is dominated by the planet’s magnetic field).
Namely, it found a lack of plasma and strong radiation belts, which confounded scientists at the time. However, a 2024 study confirmed that these conditions were the result of intense solar activity that had reached the planet during the Voyager 2 flyby.
Scientists continue to search for clarity on Uranus’ magnetosphere, hoping to understand the planet’s quirks.
“Uranus is weird, so it's always been uncertain how much the magnetic field actually interacts with its satellites,” said principal investigator Richard Cartwright of the Johns Hopkins University’s Applied Physics Laboratory, in a statement. “For starters, it is tilted by 98 degrees relative to the ecliptic.”
Read More: The Mysterious World of Uranus, the Ice Giant
Discovering a Darkened Surprise
Both Uranus and its magnetic field lines rotate faster than its moons can orbit the planet. The magnetic field lines constantly sweep past the moons as a result, hitting their trailing sides with charged particles. The researchers believed that this would make the trailing sides darker.
Relying on the Hubble Space Telescope’s ultraviolet vision to observe the moons’ brightness, the researchers uncovered something entirely unexpected: the two hemispheres of the inner moons (Ariel and Umbriel) were similar in brightness, and the two hemisphere of the outer moons (Titania and Oberon) presented an even more shocking feature. The leading hemispheres of these two moons were darker and redder than their trailing hemispheres, shattering previous assumptions.
Collecting Cosmic Dust
The researchers have a guess to explain this phenomenon; other irregular satellites with large orbits around Uranus are constantly struck with a barrage of micrometeorites, emitting dust that gradually moves inward toward the planet. This dust gets in the path of Titania and Oberon, which end up collecting it on their leading hemispheres like “bugs hitting the windshield of your car as you drive down a highway,” as the statement puts it.
The dust then darkens the leading hemispheres of the two outer moons, which bear the brunt of the debris and shield the inner moons, Ariel and Umbriel, from going through the same thing.
“We see the same thing happening in the Saturn system and probably the Jupiter system as well,” said co-investigator Bryan Holler of the Space Telescope Science Institute. “This is some of the first evidence we’re seeing of a similar material exchange among the Uranian satellites.”
“So that supports a different explanation,” said Cartwright. “That's dust collection. I didn't even expect to get into that hypothesis, but you know, data always surprise you.”
The identity of Uranus' magnetosphere remains elusive, and now the researchers are saying it could be even more complicated than previously thought. It may not be very active after all, or perhaps, interactions between the magnetosphere and Uranian moons could be happening, but they may not be affecting the moons' brightness as first predicted. Now that the mystery has deepened, the researchers say more studies are needed to find answers.
Read More: Astronomers Have A New Theory Why Uranus Spins On Its Side
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Nature Astronomy. The anomalous state of Uranus’s magnetosphere during the Voyager 2 flyby
NASA. Moons of Uranus
Jack Knudson is an assistant editor at Discover with a strong interest in environmental science and history. Before joining Discover in 2023, he studied journalism at the Scripps College of Communication at Ohio University and previously interned at Recycling Today magazine.
