Saturnâs Rings: Less than Meets the Eye?
âș Researchers âweighedâ the central parts of Saturnâs most massive ring for the first time.
âș The results confirm that more opaque areas in the rings do not necessarily contain more material.
âș Research on the mass of Saturnâs rings has important implications for their age.
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, according to a recent study of the rings using data from NASAâs Cassini mission. In their analysis, scientists found surprisingly little correlation between how dense a ring might appear to be â in terms of its opacity and reflectiveness â and the amount of material it contains.
The new results concern Saturnâs B ring, the brightest and most opaque of Saturnâs rings, and are consistent with previous studies that found similar results for Saturnâs other main rings.
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. They âweighedâ the nearly opaque center of the B ring for the first time â technically, they determined its mass density in several places â by analyzing spiral density waves. These are fine-scale ring features created by gravity tugging on ring particles from Saturnâs moons, and the planetâs 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.
âAt present itâs 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,â said Matthew Hedman, the studyâs lead author and a Cassini participating scientist at the University of Idaho, Moscow. Cassini co-investigator Phil Nicholson of Cornell University, Ithaca, New York, co-authored the work with Hedman.
âAppearances can be deceiving,â said Nicholson. âA 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.â
Research on the mass of Saturnâs 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 â perhaps a few hundred million years instead of a few billion.
âBy âweighingâ 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 Saturnâs rings,â said Linda Spilker, Cassini project scientist at NASAâs Jet Propulsion Laboratory, Pasadena, California. âThe rings are so magnificent and awe-inspiring, itâs impossible for us to resist the mystery of how they came to be.â
While all the giant planets in our solar system (Jupiter, Saturn, Uranus and Neptune) have ring systems of their own, Saturnâs are clearly different. Explaining why Saturnâs rings are so bright and vast is an important challenge in understanding their formation and history. For scientists, the density of material packed into each section of the rings is a critical factor in ascribing their formation to a physical process.
An earlier study by members of Cassiniâs composite infrared spectrometer team had suggested the possibility that there might be less material in the B ring than researchers had thought. The new analysis is the first to directly measure the density of mass in the ring and demonstrate that this is the case.
Hedman and Nicholson used a new technique to analyze data from a series of observations by Cassiniâs visible and infrared mapping spectrometer as it peered through the rings toward a bright star. By combining multiple observations, they were able to identify spiral density waves in the rings that arenât 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 10 times more opaque than the neighboring A ring, but the B ring may weigh in at only two to three times the A ringâs mass.
Despite the low mass found by Hedman and Nicholson, the B ring is still thought to contain the bulk of material in Saturnâs ring system. And although this study leaves some uncertainty about the ringâs mass, a more precise measurement of the total mass of Saturnâs rings is on the way. Previously, Cassini had measured Saturnâs 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 ringsâ true mass.
The study was published online by the journal Icarus.
The Cassini-Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASAâs Science Mission Directorate in Washington.
IMAGEâŠ.This magnificent view looks down upon, and partially through, Saturnâs rings from their unlit side.
The densest part of the rings occults the bright globe of Saturn. Scientists can use observations like this to determine precisely the concentration of ring particles.
When the bright source is the signals coming from the spacecraft, the technique is called a 'radio occultation.â In a radio occultation measurement, a signal is beamed toward Earth from Cassiniâs 4-meter-wide (13-foot) high-gain antenna. Researchers on Earth receive the signal as the spacecraft passes behind the rings. The reduction in Cassiniâs radio signal tells researchers how densely packed the ring particles are. Scientists can also learn about the size distributions of the particles from occultations.
As an added (but tiny) bonus, Saturnâs moon Atlas (32 kilometers, or 20 miles across) is visible as a dark speck against the planet, just outside the A ring.
The image was taken in visible red light with the Cassini spacecraft wide-angle camera on Aug. 2, 2005, at a distance of approximately 617,000 kilometers (383,000 miles) from Saturn. The image scale is 37 kilometers (23 miles) per pixel.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASAâs Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.
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