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The most recent Hubble Space Telescope observations indicate a twist in the narrative of Uranus' moons. Rather than the anticipated radiation "sunburn," the moons Ariel, Umbriel, Titania, and Oberon appear to be actually collecting cosmic dust. It turns out that the planet's unusual tilt isn't roasting their backsides as expected, but rather covering the front ends of the two outer moons with a type of space-grime. Astronomers are perplexed by this finding since it contradicts their expectations given Uranus' twisted magnetic field.
Dust, not Radiation
According to evidence from NASA's Voyager 2 flyby in 1986 and decades of modeling, astronomers concluded Uranus' sideways rotation caused its magnetic field to bombard each moon's trailing side (the "back window") with charged particles, darkening it. The back portions were meant to seem dreary and gloomy. Instead, Hubble's ultraviolet measurements reveal a different story: Titania and Oberon (the distant pair) are actually darker on their leading faces, contrary to what the radiation hypothesis anticipated. In other words, the result is no radiation harm at all. Instead, it appears that Uranus' magnetosphere misses these moons.
Cosmic Windshield Effect
Uranus's far irregular moons generate space dust. Micrometeorites are continually bombarding those faraway spacecraft, pushing microscopic grit inside over millions of years. Titania and Oberon blast through the dust cloud, accumulating debris on their forward flanks like bugs on a car windshield. This cosmic "bug splatter" gives their leading faces a somewhat deeper, reddish tinge.
Meanwhile, Ariel and Umbriel ride in their larger brothers' dust shadows, seeming roughly the same brightness on both sides. Uranus's large moons have been subjected to a slow-motion cosmic car wash, leaving them with dusty fronts rather than UV burns. In other words, dirty windshields, not radiation, are painting these moons. It serves as a reminder that space may surprise us, even if it is only with dust.
