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Saturn's rings may look smooth – even when viewed through a telescope – but they are in fact made up of clumps of particles and may be much denser than realized, scientists have found.

Measurements taken from the joint NASA, Italian and European Space Agency Cassini spacecraft show that the particles in Saturn's B ring are constantly colliding, which surprised scientists.

"The rings are different from the picture we had in our minds," said Larry Esposito of the University of Colorado at Boulder, who led the study done using ultraviolet imaging.

"We originally thought we would see a uniform cloud of particles. Instead we find that the particles are clumped together with empty spaces in between."

That means scientists have underestimated the mass of Saturn's rings. They may be two to three times denser than previous estimates, the researchers said.

Physicist Josh Colwell of the University of Central Florida in Orlando said Tuesday that the Cassini instrument used a time-honored method of measuring far-away objects – by watching them pass in front of stars, in a phenomenon known as occulting.

"By studying the brightness of stars as the rings pass in front of them, we are able to map the ring structure in 3-D and learn more about the shape, spacing and orientation of clusters of particles," Colwell said in a statement.

Writing in the journal Icarus, the researchers said the gravitational attraction of ring particles to each other creates clumps, or "self-gravity wakes."

If the clumps were farther from Saturn, they might aggregate even more to make a moon but because they are so close to the massive planet that they get stretched apart.