A team of planetary researchers from the United States has found surprisingly little correlation between how dense a Saturn’s ring might appear to be and the amount of material it contains.
Saturn’s B ring is the brightest and most opaque of the gas giant’s 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.
The results concern Saturn’s B ring and are consistent with previous studies that found similar results for Saturn’s other main rings.
The authors – Dr. Matthew Hedman of the University of Idaho and Dr. Phil Nicholson of Cornell University – analyzed data from NASA’s Cassini mission and found that, while the opacity of Saturn’s B ring varied by a large amount across its width, the mass did not vary much from place to place.
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.
The structure of each wave depends directly on the amount of mass in the part of the rings where the wave is located.
“Using a wavelet-based analyses of 17 occultations of the star gamma Crucis observed by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft, we are able to examine five density waves in the B ring,” the team wrote in a paper in the journal Icarus.
“Two of these waves are generated by the Janus 2:1 and Mimas 5:2 Inner Lindblad Resonances at 96,427 km and 101,311 km from Saturn’s center, respectively. The other three wave signatures are associated with the Janus 3:2, Enceladus 3:1 and Pandora 3:2 Inner Lindblad Resonances at 115,959 km, 115,207 km and 108,546 km.”
The researchers found that the overall mass of the B ring is unexpectedly low.
“Estimates of the ring’s surface mass density derived from these five waves fall between 40 and 140 g/cm2, even though the ring’s optical depth in these regions ranges from 1.5 to almost 5,” they wrote in the study. “This suggests that the total mass of the B ring is most likely between one-third and two-thirds the mass of Saturn’s moon Mimas.”
“It was surprising, because some parts of the 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,” Dr. Hedman said.
Despite the low mass, the B ring is still thought to contain the bulk of material in Saturn’s ring system.
And although this study leaves some uncertainty about the B ring’s mass, a more precise measurement of the total mass of Saturn’s rings is on the way.
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M.M. Hedman & P.D. Nicholson. The B-ring’s surface mass density from hidden density waves: Less than meets the eye? Icarus, published online January 22, 2016; doi: 10.1016/j.icarus.2016.01.007
