Saturn’s ring mystery: Why are opacity and density a mismatch?

With the help of a spacecraft exploring Saturn, scientists have “weighed” the most massive ring of the huge planet, first time ever, finding something unexpected.

The scientists 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. In fact, it’s the most opaque ring in the solar system, according to NASA.

“Appearances can be deceiving”, added study co-author Phil Nicholson who likened the deception to a meadow and a swimming pool. 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.

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.

“At present, it’s far from clear how regions with the same amount of material can have such different opacities”, said lead author and University of Idaho planetary scientist Matthew Hedman.

They determined the mass density of the center of the B ring by analyzing the so-called spiral density waves, which are fine-scale ring features created by gravity tugging on ring particles from Saturn’s moons, and the gas giant’s own gravity.

Saturn’s B ring is the most opaque of the main rings, appearing nearly black in this Cassini image taken from the unlit side of the ringplane.

As a ring develops, it accrues a significant amount of dust from meteorites and other sources, which acts to darken the ring and increase its mass. Therefore, since Saturn’s B ring has a lower-than-expected mass, it is possible that it is much younger than had previously been believed.

Saturn’s rings were first spotted by Galileo Galilei in 1610.

When it comes to Saturn’s rings, astronomers assumed that the brightest, densest sections (B ring) are the largest while the least transparent ones are the smallest.

Hedman and Nicholson used a new technique to analyze data from a series of observations by Cassini’s visible and infrared mapping spectrometer as it peered through the rings toward a bright star.

By figuring out how much material each section of the rings contain will enable scientists to tell what kind of physical process was responsible for their formation.

But researchers have to wait until 2017 to exactly see behind the B-ring’s curtain. It has been deceiving astronomers for decades with a trick, which makes it look like it contains more mass than it actually does.

Typically, when we look at something that is clear, we expect that it contains less matter than an object that is opaque, but a new study using data from NASA’s Cassini spacecraft has discovered that this rule of thumb does not necessarily apply to the rings of Saturn. In this case, the less massive the B ring is, the younger it might, perhaps a few hundred million years instead of few billion. “The rings are so magnificent and awe-inspiring, it’s impossible for us to resist the mystery of how they came to be”.

The rings’ estimated ages were also derived from their perceived size.

The analysis also found that the overall mass of the B ring is unexpectedly low. Cassini will finish its mission at Saturn with a “grand finale” next year, giving astronomers a chance to narrow down the ring’s mass, once and for all.