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- Titania and Oberon are two of Uranus\u2019 largest moons.<\/strong> Scientists expected them to be darker in color on their trailing hemispheres, due to being hit by radioactive particles.<\/li>\n
- New observations by the Hubble Space Telescope,<\/strong> however, show they are darker on their leading hemispheres instead. Why?<\/li>\n
- Dust from smaller \u201cirregular\u201d moons<\/strong> is coating the moons\u2019 leading hemispheres and making them darker, a new study suggested.<\/li>\n<\/ul>\n
Dark and bright hemispheres of Uranus\u2019 largest moons<\/h3>\n
Astronomers used the Hubble Space Telescope for a new study of four of Uranus\u2019 largest moons: Ariel, Titania, Oberon and Umbriel. They wanted to see how the planet\u2019s magnetosphere \u2013 a bubble around the planet filled with charged particles \u2013 affected the moons\u2019 surfaces. They had predicted the leading hemispheres of the moons would be brighter and the trailing hemispheres would be darker. But the researchers from the Space Telescope Science Institute (STScI) and John Hopkins Applied Physics Laboratory (JHUAPL) in Maryland said on June 10, 2025, that they found the opposite, with darkening on the leading hemispheres of the two outer moons, Titania and Oberon.<\/p>\n
For Ariel and Umbriel, the two hemispheres of both moons were almost the same brightness. What could explain all this?<\/p>\n
All four moons are tidally locked, meaning the same hemisphere is always facing toward Uranus. This is similar to how the same side of the moon always faces Earth. Uranus has 28 known moons altogether.<\/p>\n
Researcher Christian Soto at STScI presented their results on June 10, 2025, at the 246th Meeting of the American Astronomical Society (AAS) in Anchorage, Alaska.<\/p>\n
![\"Bluish](\"https:\/\/earthsky.org\/upl\/2025\/06\/Uranus-Titania-Oberon-Umbriel-Miranda-Ariel-moons-NASA-June-10-2025.jpg\")
View larger. | Uranus\u2019 5 largest moons: Ariel, Miranda, Titania, Oberon and Umbriel. Image via NASA\/ ESA\/ STScI\/ Christian Soto (STScI).<\/figcaption><\/figure>\n Hemispheres opposite of what scientists expected<\/h3>\n
The researchers expected Uranus\u2019 magnetosphere \u2013 the region where electrically charged particles are affected by Uranus\u2019 magnetic field \u2013 would interact with the surfaces of the four large moons. Radiation from charged particles in the magnetosphere would cause the trailing hemispheres of the moons to darken. Or, at least, that was the assumption. But that\u2019s not what the research team found. The hemispheres of the two inner moons, Ariel and Umbriel, were basically the same brightness.<\/p>\n
The hemispheres of the two outer moons, Titania and Oberon, did<\/em> show a difference. But it was the opposite of what the scientists expected to find. Instead of the trailing hemispheres being darker, it was the leading hemispheres.<\/p>\n
Could Uranus\u2019 extreme axial tilt have something to do with it? Principal investigator Richard Cartwright at Johns Hopkins Applied Physics Laboratory explained:<\/p>\n
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Uranus is weird, so it\u2019s always been uncertain how much the magnetic field actually interacts with its satellites. For starters, it is tilted by 98 degrees relative to the ecliptic. At the time of the Voyager 2 flyby, the magnetosphere of Uranus was tilted by about 59 degrees from the orbital plane of the satellites. So, there\u2019s an additional tilt to the magnetic field.<\/p>\n<\/blockquote>\n
The reasoning was that the magnetic field lines should hit the moons as they rotate around Uranus. That\u2019s because they rotate faster than the moons move in their orbits. As a result, the charged particles should preferentially hit the trailing hemispheres. But that seems not to be the case.<\/p>\n
![\"Smiling](\"https:\/\/earthsky.org\/upl\/2025\/06\/Richard-Cartwright-Johns-Hopkins-Applied-Physics-Laboratory.jpg\")
Richard Cartwright at the Johns Hopkins Applied Physics Laboratory is the principal investigator of the study about Uranus\u2019 unusual moons. Image via Johns Hopkins Applied Physics Laboratory.<\/figcaption><\/figure>\n A dusty explanation?<\/h3>\n
So why are Titania and Oberon darker on the \u201cwrong\u201d side? The researchers suspect it might be due to dust. That dust might come from some of Uranus\u2019 smaller, irregular moons. Those are moons with large, eccentric and inclined orbits. When tiny micrometeorites \u2013 submillimeter in size \u2013 impact the moons, they eject small bits of material and dust that then also orbit Uranus. As the dust gradually moves closer to Uranus, it hits the leading hemispheres of Titania and Oberon. The researchers liken it to bugs hitting the windshield of your car.<\/p>\n
As the dust accumulates, the leading hemispheres become darker. In addition, these moons shield the inner moons Ariel and Umbriel from the dust. Consequently, their hemispheres remain relatively unchanged. Co-investigator Bryan Holler of the Space Telescope Science Institute said:<\/p>\n
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This is some of the first evidence we\u2019re seeing of a similar material exchange among the Uranian satellites.<\/p>\n<\/blockquote>\n
Cartwright added:<\/p>\n
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So that supports a different explanation. That\u2019s dust collection. I didn\u2019t even expect to get into that hypothesis, but you know, data always surprise you.<\/p>\n<\/blockquote>\n
Bottom line: Scientists thought four of Uranus\u2019 largest moons would have darker trailing hemispheres and lighter leading hemispheres. But for Titania and Oberon, it\u2019s the opposite. Is dust the reason?<\/p>\n
Source: Revealing the classical Uranian satellites in UV<\/p>\n
Via STScI<\/p>\n
Read more: New moons! Uranus now has 28 and Neptune 16<\/p>\n
Read more: Evidence for ocean on Uranus moon Miranda is a surprise<\/p>\n
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\n - New observations by the Hubble Space Telescope,<\/strong> however, show they are darker on their leading hemispheres instead. Why?<\/li>\n