If you try to apply simple common sense to how Saturn’s rings really work you’re going to be sorely mistaken: the giant planet’s signature features run circles around average Earthly intuition. This has been the case for centuries and is still true today after recent news from Cassini that the most opaque sections of rings aren’t necessarily the densest; with Saturn looks literally are deceiving.
While rings are features shared by all of the gas and ice giant planets in our Solar System, Saturn is by far the “ring king” in terms of sheer magnitude, complexity, and utter beauty of its ring system. First observed through a telescope by Galileo in 1610, Saturn’s brilliant rings were initially thought to be additional “stars,” or perhaps solid protrusions on either side of the planet like the handles of a cup. It wasn’t until 1659 that Dutch astronomer Christiaan Huygens determined that the handles were actually an encircling ring not attached to Saturn “but was separated from it the same distance all around.” (Source) And just sixteen years later Giovanni Cassini observed the largest gap in Saturn’s rings (which became his namesake feature) and correctly determined that they’re divided into sections.
While Galileo, Huygens, and Cassini all made invaluable contributions to the study of Saturn, they all assumed the rings to be solid. It was French astronomer Jean Chapelain who suggested in 1660 the rings must be made of tiny particles traveling in orbit around Saturn; however this wasn’t confirmed until the late 1850s when famed physicist James Clerk Maxwell calculated that, due to the forces of gravity and dynamical stability on such a large solid object, “the rings must consist of disconnected particles; these may be either solid or liquid, but they must be independent.” (Source)
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Maxwell’s On the Stability of the Motion of Saturn’s Rings was published 100 years before Pioneer 11 gave us our first close-up views of Saturn. See a timeline of observations of Saturn’s rings here.
Fast forward to the present day.
In addition to countless observations from ground-based telescopes and space observatories like Hubble, Saturn has been flown past after Pioneer 11 by both Voyagers 1 and 2 and is currently being investigated by NASA’s Cassini spacecraft, which will soon enter its twelfth year of orbital exploration. Via the Cassini mission we’ve learned more about Saturn, its rings, and family of moons in the last decade than we had in the 394 years since Galileo first peered at it through his homemade telescope. And when it comes to its rings, researchers are learning what looks like more really is less.
Saturn’s incredibly complex ring system is divided up into distinct regions, named alphabetically outwards from the planet D, C, B, A, F, G, and E. (There’s also an enormous and diffuse infrared-visible ring surrounding Saturn at distance but we won’t get into that here.) All together these sections — each made up of bands of orbiting icy particles ranging from house-sized to finer than candle smoke — stretch a distance of 464,000 km (over 288,000 miles) from the top of Saturn’s atmosphere.. yet they’re only about ten meters (30 feet) thick.
One of Saturn’s ring sections — B — is much more visually opaque than the others, demonstrated both by its reflectivity and how it prevents light from background stars from easily passing through it as readily as in other rings. One might quickly conclude that such an apparently dense ring would therefore contain considerably more mass. Yet, as it turns out, that’s not the case.
Cassini measurements of density waves moving across the B ring in reaction to the gravitational pulls from nearby moons have given researchers the first accurate “weigh-in” of the ring’s core. What they found was a fairly consistent distribution of mass, regardless of any changes in opacity within the ring.
“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 in a Feb. 2 press release. Hedman is the recent study’s lead author and a Cassini participating scientist at the University of Idaho, Moscow.
And although the 25,500-kilometer-wide B ring likely does contain the most mass of all Saturn’s rings, it’s only by about a factor of two or three, despite being ten times more opaque than its outward A ring neighbor.
“Appearances can be deceiving,” noted co-investigator Phil Nicholson of Cornell University in Ithaca, New York. “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.”
Makes sense. (Then again, your senses can fool you in astronomy!)
The actual mass of sections of Saturn’s rings is important in determining their age and evolution. Less material in the B ring than suspected may indicate a young age, since a “lighter” ring would evolve — and thus grow darker via meteorite impacts — quicker than a “heavier” one.
“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, Cassini project scientist at JPL.
Further measurements will be taken as Cassini goes into the final phase of its mission, eventually passing through the rings in 2017. The data it gathers along the way will help scientists more accurately calculate the mass and true age of the planet’s most iconic — and most enigmatic — features.