February 4, 2016 | David F. Coppedge

Density Can’t Keep Saturn’s Rings Old

Saturn’s B ring is much less dense than previously thought,
challenging attempts to keep it billions of years old.


Scientists have always been worried about the apparent youthfulness of planetary rings (3/19/10, 4/30/13). Back in 2007, Cassini scientist and planetary ring specialist Larry Esposito posited that Saturn’s rings might be billions of years old (see 12/13/07 and a JPL press release from Dec. 2007). He based it on the fact that the B-ring appeared dense enough to allow for repeated recycling of the material. This, he proposed, would maintain the ring’s brightness despite continuous infalling of dark meteoritic dust. (The B-ring is the densest part of the whole ring system.)

That explanation is now challenged by new measurements that find the B-ring to be one-half to one-seventh the mass of previous estimates. “Saturn’s Ring Puffs Itself Up With Optical Illusion,” a headline on National Geographic says.

With a magic trick that’s more smoke than mirrors, one of Saturn’s most mysterious rings has deceived astronomers for decades with an optical illusion.

As described by astronomers Matt Hedman and Phillip Nicholson, the B ring—Saturn’s brightest—has been deceptively opaque, making astronomers think that it contains up to seven times more mass than it actually does.

The open-access paper in Icarus used stellar occultations from multiple Cassini orbits to re-estimate the mass of the B-ring. Instead of equaling twice the mass of moon Mimas, it is probably between 1/3 to 2/3 of that. If the mass is so low, it puts pressure on theorists to imagine a special event in more recent times that created the rings (e.g., 10/07/10). The problem is that ad hoc events that cannot be observed can be criticized as special pleading. The paper in Icarus dances around the age problem in the introduction, but fails to address it in the body of the paper:

The large uncertainties in the B-ring’s mass and its typical surface mass density not only hamper efforts to understand the structure and dynamics of this ring, but also complicate efforts to ascertain the age and history of Saturn’s ring system.

A JPL press release on the paper notes the connection between density and age:

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 beperhaps a few hundred million years instead of a few billion.

That this represents a problem is clear from a quick calculation. A hundred million years is 1/45th the assumed age of the solar system (4.5 billion years). What happened 44/45ths into the lifetime of the planet to form rings? Why did it happen so recently that we can observe the rings today? The word “perhaps” indicates that the statement lacks empirical foundation. It could be far younger than a few hundred million years old.

Besides, there are other forces at work to hasten the disappearance of rings besides infalling meteoritic dust: among them, sputtering (erosion at the atomic scale), collisional spreading, and sunlight pressure (the Poynting-Robertson effect). And if the densest ring is no more than a few hundred million years old, what of the more tenuous D, C, A, F, G, and E rings? One must recall that planetary scientists were similarly astonished to find Jupiter, Uranus and Neptune ringed by orbiting material with apparently short lifetimes. It’s getting harder to keep rings old.

Further refinements of the B-ring’s mass will become possible in 2017 when the orbiter goes into highly-inclined orbits over the rings prior to its grand-finale descent into Saturn.

Sorry to pull that pedestal out from the moyboys. We know they were already running out of props (see, for instance, 12/03/07 and many other of our entries on the solar system, exacerbated by the latest discoveries at Pluto, e.g. 1/26/16).

Dr. Larry Esposito is an excellent scientist and a likable fellow, upbeat and pleasant in conversation. (The same can be said of Jeff Cuzzi, another ringmaster.)  Esposito is the chief scientist of the Cassini UVIS instrument, and has made many important scientific discoveries all the way back to Voyager days, when he discovered the F-ring. I had a few occasions to talk with him when I was at JPL. One time I asked him why he was proposing a theory to keep Saturn’s rings old when they looked so young. I remember him answering that it was for “philosophical reasons” – implying that it would be unpleasant to his world view to consider something special about our observing the rings now, if they were not billions of years old.* I hope he and the other Cassini scientists will have the courage to follow the evidence where it leads, even if it runs afoul of “received wisdom”.

Larry Esposito 7/01/2004

Dr. Larry Esposito enjoying Cassini’s first view of Saturn’s rings, July 1, 2004 (photo by David Coppedge)

*In an email written to one of his colleagues at the University of Colorado (dated 11/06/08), Dr. Esposito responded to a critique I had published on my own time at ICR about the proposal of denser B-rings. The colleague, Stuart J. Robbins, was upset and wanted to get Esposito’s response to use in a blog Robbins was proposing called “Exposing Pseudo Astronomy.” To his credit, Dr. Esposito CC’d me on his reply. He admitted to Robbins, “It is true that my major impetus for my recycling proposal is philosophical: I am not comfortable with the low probability of recent ring creation.

From what I see at Robbins’ website, he never wrote up his complaint on this topic, though he had a lot of glee writing about my firing two years later. By contrast, Esposito could express his view calmly and honestly, and was always pleasant in my experience.

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  • Jon Saboe says:

    What a miracle it is that we just happen to be alive at the precise time of Saturn’s 4-billion-year-old age when rings suddenly formed in the near past.

    What are the odds???

    Funny how, no matter what the billions of years folk say — design STILL sneaks up on us.

    The simple fact that an orbit is practically impossible (for something to fall into orbit is like a coin landing on its edge) makes rings even more impossible.

    The mechanics of rings forming is mind-boggling. And if they formed just so we could see them? Even more so…

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