Cassini team weighs Saturn's B ring

Studies of Saturn’s B ring have shown little correlation between how opaque or dense certain areas appear to be and the amount of material present, adding to the mystery of how the ringed planet’s prominent features formed.

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An image of Saturn and its rings captured by Cassini from the unlit side of the ringplane
Image credit: NASA/JPL-Caltech/Space Science Institute

Astronomers have used data sent back by NASA’s Cassini mission to analyse and ‘weigh’ the apparently dense centre of Saturn’s B ring – its brightest and most opaque ring – for the first time. The results showed that, while the opacity of the B ring varies across its width, the mass does not vary significantly in relation.

"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," says Matthew Hedman, the study's lead author and a Cassini participating scientist at the University of Idaho, Moscow.

The team were able to weigh the centre of the B ring by analysing what are called spiral density waves. These are fine-scale ring features caused by gravity pulling at ring particles. The structure of the wave is related to the amount of mass in the section of the rings where it is located.

The results have found that there is less material in Saturn’s B ring than had been previously inferred through observations, and that more opaque areas of the ring do not necessarily mean denser mass. According to the team, some parts of the B ring are as much as 10 times more opaque than its A ring, but the B ring probably weighs only two to three times the A ring’s mass.

"By 'weighing' the core of the B ring for the first time, this study makes a meaningful step in our quest to piece together the age and origin of Saturn's rings," says Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory. "The rings are so magnificent and awe-inspiring, it's impossible for us to resist the mystery of how they came to be."

Further studies of Saturn and its rings are to be undertaken by Cassini before its mission comes to an end in 2017. The spacecraft will fly just inside the rings during the final pahse of its mission, and will collect data to enable the Cassini team to measure the mass of Saturn by itself. When combined with previous Cassini calculations that determined the mass of both the planet and its rings together, the results should reveal the rings’ mass once and for all.


Front image: Saturn captured by Cassini during its cruise around the planet in October 2004
Credit: NASA/JPL/Space Science Institute
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