Saturn's B ring, which is the widest, most massive and most opaque in its ring system, appears more opaque in some areas than others. That made scientists think it was also more dense in those areas. Imagine a muddy, opaque puddle: It has more matter in it than a crystal-clear puddle of the same size. But it turns out this might not hold true in the rings of the gas giant.

The B ring is probably still the most dense in the system, but parts of it could be as little as one-seventh as dense as scientists had assumed. Other work using Cassini had made scientists suspect that the B ring's opacity might be misleading, but this is the first experiment to confirm those suspicions.

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"At present it's far from clear how regions with the same amount of material can have such different opacities. 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," the University of Idaho's Matthew Hedman, the study's lead author, said in a statement.

Weighing Saturn's rings can help us figure out how old they are. Scientists think that more massive rings are older than smaller ones. The B ring could be just a few hundred million years old, instead of the billions of years previously thought.

NASA's Cassini will provide even more answers before its mission ends. In 2017, the orbiter will move inside the ring system to measure the mass of Saturn without its rings. Since it's already measured the total mass of the system, the difference will tell NASA scientists just how massive the ring system is on its own.