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. Credit: NASA/JPL-Caltech/Space Science Institute
It seems intuitive opaque material should contain more stuff than a more translucent substance. But this does not always apply, according to a recent study of the rings using data from NASA’s Cassini mission. Saturn’s B ring is the brightest and most opaque of Saturn’s rings, and are consistent with previous studies that found similar results for Saturn’s other main rings.
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. Credit: NASA/JPL-Caltech/Space Science Institute
While opacity of B ring varied by a large amount across its width, the mass – or amount of material – did not vary much from place to place. They “weighed” the nearly opaque center of the B ring for the first time—they determined its mass density in several places—by analyzing spiral density waves. These are fine-scale ring features created by gravity tugging on ring particles from Saturn’s moons, and planet’s own gravity. The structure of each wave depends directly on amount of mass in the part of the rings where the wave is located.
“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,” said Matthew Hedman.
This image is cropped from a panoramic view of Saturn’s main ring system, and includes the names of major ring features. Credit: NASA/JPL-Caltech/Space Science Institute
Research on the mass of Saturn’s rings has important implications for their age. 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.
“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,” said Linda Spilker, JPL.
While all the giant planets in our solar system (Jupiter, Saturn, Uranus and Neptune) have ring systems of their own, Saturn’s are clearly different. Explaining why Saturn’s rings are so bright and vast is an important challenge in understanding their formation and history. An earlier study by members of Cassini’s composite infrared spectrometer team suggested there might be less material in the B ring than thought. New analysis is the first to directly measure the density of mass in the ring and demonstrate that this is the case.
The B ring is the brightest of Saturn’s rings when viewed in reflected sunlight. Credit: NASA/JPL-Caltech/Space Science Institute
Hedman and Nicholson used a new technique to analyze data from a series of observations by Cassini’s visible and infrared mapping spectrometer . By combining multiple observations, they identified spiral density waves in the rings that aren’t obvious in individual measurements. The analysis also found that the overall mass of the B ring is unexpectedly low. It was surprising, said Hedman, because some parts of the B ring are up to 10X more opaque than the neighboring A ring, but the B ring may weigh in at only 2 to 3 times the A ring’s mass.
Despite the low mass, B ring is still thought to contain the bulk of material in Saturn’s ring system. A more precise measurement of the total mass of Saturn’s rings is on the way. Previously, Cassini measured Saturn’s gravity field, showing 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 2 measurements is expected to finally reveal the rings’ true mass.
http://www.jpl.nasa.gov/news/news.php?feature=4886
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