Uranus’ cold, icy world is surrounded by a pair of rings that orbit the planet at twice the distance as the main ring system. Each ring tells a different story. The outermost ring appears blue, while the inner ring is reddish. Astronomers have long wondered why the rings are so different and how each one formed. Deciphering the light from these rings may ultimately provide clues to its unique origin story.
A team of astronomers used the W.M. Keck Observatory in Hawaii to study the composition of Uranus’ outer ring and uncover its history. The findings, published in the Journal of Geophysical Research: Planets, show two very different formation processes. One ring is made of tiny ice grains, while the other may have formed from a history of violent collisions and shocks.
A pair like no other
Uranus’ first nine rings were discovered in 1977 by a group of astronomers observing Uranus’ atmosphere. Almost a decade later, the Voyager 2 mission discovered two other inner rings and 10 moons.
More recently, the Hubble Space Telescope photographed a pair of distant rings in 2004. Scientists have not noticed the outer ring in previous observations because it is very dark and is much further away from the planet than expected.
The outer ring is so far away that it is called Uranus’ second ring system. The μ ring, like Saturn’s E ring, is blue and exhibits very small particles. The ν ring, on the other hand, is reddish like other dusty rings commonly found in the solar system.
The difference in color suggested fundamental differences in the size of the particles making up the rings and their composition. However, Uranus’ outer ring is dark and narrow, making it difficult to observe. So far, data on planetary ring systems is very lacking.
Same but different
To uncover the details of each unique configuration, the team behind the new study analyzed how sunlight reflects off Uranus’ outer ring. “By deciphering the light from these rings, we can track both their particle size distribution and composition, thereby revealing their origins and providing new insights into how the Uranus system and its planets formed and evolved,” the study’s lead author Imke de Peyter, a professor at the University of California, Berkeley, said in a statement.
The researchers combined observations from the Keck Observatory with data from the Webb and Hubble Space Telescopes to compile a complete spectrum of the ring taken at different wavelengths. The μ ring closely matches the spectral signature of water ice, suggesting that the ring is composed of small ice particles thrown from Uranus’ small moon Mab in a series of collisions. The ν ring, on the other hand, is composed of rocky material mixed with about 10% to 15% of carbon-rich organic compounds commonly found in the outer solar system.
“The ν-ring material comes from the impact of micrometeorites on invisible rocky bodies rich in organic material, and from collisions between these bodies. These bodies must be orbiting between several known moons,” De Peyter explained.
The current results add to the mystery surrounding Uranus. The composition of the μ-ring confirms that the moon Mab is composed mostly of water ice, unlike the rest of Uranus’ inner moons, which have a rocky composition. “One of the interesting questions is why the compositions of the bodies sourcing these rings are so different,” De Peyter said.
Scientists involved in the study also noticed hints that the μ-ring’s brightness changes over time, but they don’t know what causes the change. They suggest that future missions to Uranus aimed at taking close-up images of this peculiar planet could help solve some of these mysteries surrounding its chaotic world of rings and moons.
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