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Saturn ring density is an illusion

The most opaque parts of Saturn’s rings – the parts that look most dense – don’t always contain more material. Another great Saturn mystery!

Saturn, via the Cassini spacecraft.

Saturn and its rings, via the orbiting Cassini spacecraft.

Saturn’s rings look solid through telescopes and for, hundreds of years, many thought they were solid. But the solidity of Saturn’s rings turned out to be an illusion, and, since spacecraft have begun exploring the solar system, we’ve known Saturn’s rings as billions of separate moonlets. Now – using data from NASA’s Cassini mission – researchers who “weighed” the central parts of Saturn’s B ring for the first time showed that Saturn’s rings are presenting us with another optical illusion. Their results show that the denser-looking areas in the rings don’t necessarily contain more material.

In fact, they said, although some parts of the B ring are up to 10 times more opaque (less see-through) than the neighboring A ring, the B ring may have only two to three times the A ring’s mass.

The researchers said in a statement from JPL:

It seems intuitive that an opaque material should contain more stuff than a more translucent substance. For example, muddier water has more suspended particles of dirt in it than clearer water. Likewise, you might think that, in the rings of Saturn, more opaque areas contain a greater concentration of material than places where the rings seem more transparent.

But this intuition does not always apply…

[In this analysis, we found] surprisingly little correlation between how dense a ring might appear to be — in terms of its opacity and reflectiveness — and the amount of material it contains.

The study was published online by the journal Icarus in late January, 2016.

Saturn's B ring is the most opaque of the main rings, appearing almost black in this Cassini image taken from the unlit side of the ringplane. Image credit: NASA/JPL-Caltech/Space Science Institute

Saturn’s B ring is the most opaque of the main rings, appearing almost black in this Cassini image taken from the unlit side of the ringplane. Yet, a new study shows, the B ring does not contain more material. Image via NASA/JPL-Caltech/Space Science Institute

Some parts of Saturn's B ring are up to 10 times more opaque than the neighboring A ring, but the B ring may weigh in at only two to three times the A ring's mass. Image credit: NASA/JPL-Caltech/Space Science Institute

Some parts of Saturn’s B ring are up to 10 times more opaque than the neighboring A ring, but the B ring may weigh in at only two to three times the A ring’s mass. Image via NASA/JPL-Caltech/Space Science Institute

These scientists determined the mass density of the B ring – its mass per unit volume – in several places by analyzing spiral density waves. According to the JPL statement, these are:

… fine-scale ring features created by gravity tugging on ring particles from Saturn’s moons, and the planet’s own gravity. The structure of each wave depends directly on the amount of mass in the part of the rings where the wave is located.

They used a new technique to analyze data from the Cassini spacecraft, obtained when its instruments peered through the rings toward a bright star.

Although these scientists performed their unique analysis for Saturn’s B ring only, they say the results are consistent with previous studies that used different techniques to draw similar conclusions about Saturn’s other main rings.

They said they also found that, while the opacity of the B ring varied by a large amount across its width, the mass – or amount of material – did not vary much from place to place.

And they said that this new research on the mass of Saturn’s rings has important implications for the age of the rings, explaining:

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.

Thus, the less massive the B ring is, the younger it might be — perhaps a few hundred million years instead of a few billion.

The B ring is the brightest of Saturn's rings when viewed in reflected sunlight. Image credit: NASA/JPL-Caltech/Space Science Institute

The B ring is the brightest of Saturn’s rings when viewed in reflected sunlight. Image credit: NASA/JPL-Caltech/Space Science Institute

The lead author of the study is Matthew Hedman, a Cassini participating scientist at the University of Idaho, Moscow. Herman tried to explain the apparently counter-intuitive result of his study by saying:

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.

Cassini co-investigator Phil Nicholson of Cornell University, Ithaca, New York, added:

Appearances can be deceiving. 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.

Despite the low mass found by Hedman and Nicholson, the B ring is still thought to contain the bulk of material in Saturn’s ring system, these scientists said.

They said a more precise measurement of the total mass of Saturn’s rings is on the way. Previously, Cassini had measured Saturn’s gravity field, telling scientists the total mass of Saturn and its rings. In 2017, Cassini will determine the mass of Saturn alone by flying just inside the rings during the final phase of its mission.

The difference between the two measurements is expected to finally reveal the rings’ true mass.

Learn more about Cassini’s daring plunge through Saturn’s rings on EarthSky News on video

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Bottom line: Researchers who “weighed” the central parts of Saturn’s B ring for the first time showed that the denser-looking areas don’t necessarily contain more material.

Read more from NASA/JPL

Deborah Byrd

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