If a material is thicker, it should be less transparent, right? It turns out that in Saturn’s rings, that’s not always the case. A new examination of the B ring shows that even though it’s the most opaque of Saturn’s rings, it’s not the densest one.

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The puzzling result is not just isolated to this ring, either. Scientists have found similar results in other studies when looking at the gas giant’s other rings, NASA said. This latest research comes from using the Cassini spacecraft, which is slowly wrapping up investigations at Saturn since arriving there in 2004.

“Appearances can be deceiving,” said research co-author and Cassini co-investigator Phil Nicholson, at Cornell University in New York, in a NASA statement. “A good analogy is how a foggy meadow is much more opaque than a swimming pool, even though the pool is denser and contains a lot more water.”

The research team looked at the ring’s mass density by studying spiral density waves. These features appear when ring particles move under the influence of gravity — gravity from Saturn’s moons as well as the huge gas giant planet itself. Each wave’s structure depends on how dense it is, and the effect of gravity. Scientists now know that the B ring is less dense than it appears, but the full reason is still a mystery.

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“It could be something associated with the size or density of individual particles, or it could have something to do with the structure of the rings,” stated Matthew Hedman, the study’s lead author and a Cassini participating scientist at the University of Idaho.

While Saturn’s rings are arguably the most spectacular in the solar system, it’s not the only planet with rings. Jupiter, Uranus and Neptune also have faint ring systems of their own. Maybe this implies different origin stories, but more study will be needed to figure this out for sure. NASA says Saturn’s rings — made up of billions of particles and pieces — are likely fragments of broken-up moons, comets or asteroids that were pulled apart under Saturn’s gravity.

The major features in Saturn’s rings are labelled in this image, which is cropped from a panoramic view of the planet.NASA/JPL-Caltech/Space Science Institute

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Saturn’s B ring was studied before using Cassini’s composite infrared spectrometer, but this study took a bit of a different approach. Using visible and infrared light, the team looked at a bright star in between the rings. They also combined the results of multiple observations, making it possible to see even more subtle ring waves than before.

By looking at how dense the rings are, this could help scientists pin down the age of Saturn’s rings, which are considered relatively young compared with the more than 4.5 billion years the solar system has been around.

“A less massive ring would evolve faster than a ring containing more material, becoming darkened by dust from meteorites and other cosmic sources more quickly,” NASA wrote in the same statement. “Thus, the less massive the B ring is, the younger it might be — perhaps a few hundred million years instead of a few billion.”

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The study was published in the journal Icarus. The Cassini mission will complete its work next year when, low on fuel, the spacecraft will deliberately crash into Saturn to avoid contaminating icy moons in the region that could be hospitable to life, such as Enceladus.