≡ Menu

Saturn’s Rings: Puzzling Mass Density Findings

Given that we have four planets in our Solar System with rings, it’s a natural thought that if so-called Planet Nine does exist, it might likewise show a system of rings. After all, Caltech’s Konstantin Batygin and Mike Brown are talking about a planet with a mass on the order of ten times that of the Earth. Neptune is about 17 Earth masses, while Uranus is 14.5 as massive. If Planet Nine is an ejected ice giant, perhaps it joins Uranus, Neptune, Jupiter and Saturn in having a ring system of its own, along with a thick atmosphere of hydrogen and helium.

Of course, we have to discover Planet Nine first, a process that may take some time if, indeed, it is successful. Meanwhile, we have interesting developments in the Solar System’s most intriguing ring system. As compared with those of other planets, Saturn’s rings are visually stunning. The B ring is the brightest and most opaque of the planet’s rings, but now we’re finding out that brightness and opacity have little correlation with how dense a ring may actually be. In fact, more opaque areas in the rings do not necessarily contain more material.


Image: Even Saturn’s rings appear to dwarf Tethys (1,062 kilometers across), which is in the upper left of the image, although scientists believe the moon to be many times more massive than the entire ring system combined. This view looks toward the unilluminated side of the rings from about 18 degrees below the ringplane. The image was taken in green light with the Cassini spacecraft wide-angle camera on Aug. 19, 2012. The view was acquired at a distance of approximately 2.4 million kilometers from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 63 degrees. Image scale is 138 kilometers per pixel. Credit: NASA/JPL-Caltech/Space Science Institute.

This is work using Cassini data that goes about the complicated process of ‘weighing’ the opaque center of the B ring, which involves analyzing spiral density waves within the structure. These fine-scale ring features are pulled into existence by Saturn’s gravity and that of individual moons tugging on ring particles, their structure reflecting the amount of mass in given parts of the rings. While different levels of opacity are obvious, the results on mass are puzzling.

Matthew Hedman (University of Idaho), who is lead author on the work and a participating scientist in the Cassini mission that collected the needed data, frames the problem this way:

“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.”

So we have a prominent B ring whose opacity varies markedly across its width, while the amount of material in it does not seem to vary greatly from one part of the ring to another. Cassini co-investigator Phil Nicholson (Cornell University) notes that appearances aren’t always what we think. “A good analogy is how a foggy meadow is much more opaque than a swimming pool,” says Nicholson, “even though the pool is denser and contains a lot more water.”

Just what it is about Saturn’s rings that sets them so far apart from the other rings in our Solar System is an open question. As this JPL news release points out, an earlier study by Cassini’s composite infrared spectrometer team has suggested that there was less material in the B ring than researchers once thought. This new work measures the density of mass in the ring for the first time to corroborate the idea. The researchers were able to use data from Cassini’s visible and infrared mapping spectrometer as the craft looked through the rings toward a bright star. The method helped to identify spiral density waves that were otherwise not apparent.

From the paper in press:

… this new estimate of the ring’s total mass is based on just a few locations in the B ring, and so one could argue that most of the B-ring’s mass is hidden in the truly opaque parts of the rings. However, the measurements considered here include regions with optical depths between 3.5 and 4.5, so these massive regions would have to be those with opacities above 4 or 5. Note that only about 27% of the ring’s surface area has an optical depth greater than 4, and less than 16% has an optical depth above 5, so there is not much space to hide a large amount of mass.

Understanding how the rings are constituted is a step toward understanding their age and origin. While the B ring is thought to contain the bulk of material in the ring system, it’s still surprising that the B ring may have only two to three times the mass of the A ring. Parts of the B ring are, after all, up to ten times more opaque than the A ring. We can look to one of Cassini’s last acts, in 2017, to help extend our data. The spacecraft, enroute to an ultimate descent into Saturn’s clouds, will fly just inside the rings, offering measurements that will help us separate the mass of the rings in the aggregate from the previously measured combined mass of planet and rings.

The paper is Hedman and Nicholson, “The B-ring’s surface mass density from hidden density waves: Less than meets the eye?” in press at Icarus and available online 22 January 2016 (abstract).


Comments on this entry are closed.

  • ljk February 4, 2016, 13:59

    As this very recent article about the planet Uranus attests, the darker rings of that world are just as dynamic and interesting as those of their brighter and far more famous neighbor’s:


    • Michael February 20, 2016, 14:42

      Ah Uranus the butt of a lot of jokes, but I would be worried about holding a candle anywhere near it!

      ‘The best planet is Uranus—Uranus the bizarre. Uranus the unique. Saturn may be flashy and pretty, and Jupiter may be huge and dramatic, but they can’t hold a candle to Uranus’s intrigue…’

      It is time for an orbiter but the distance is so huge, I would think we would be better spending the money developing the infrastructure for our expansion into space.

  • J. Jason Wentworth February 6, 2016, 5:42

    This B ring opacity vs. mass problem is just one more of many reasons why a Saturn ring system probe (which could also investigate the strangely-smooth nearby satellite Methone, perhaps via a small “daughter” probe) should be dispatched to Saturn. Getting a spaccraft into the ring system would permit direct examination of the initial building blocks of a satellite, one which never formed because the rings are within Saturn’s Roche limit. Also:

    Is it possible that sunlight pressure (and/or Saturn’s magnetic field), acting in concert with Saturn’s gravity and the gravitational tugs of the moons, might account for the differing opacities (and colors) of the A, B, and C rings? If different types of ring particles have different optical and/or magnetic properties (as well as different masses), they might be so segregated into different orbital zones under the influences of such forces (even solar sails will work at Saturn’s distance from the Sun, just with much lower accelerations, and smaller ring particles could be more easily pushed around by the feeble pressure). Also, might the Yarkovsky Effect (the anisotropic emission of thermal photons from rotating objects warmed by sunlight, which affects small asteroids’ orbits over long time scales) have some influence, particularly on larger, darker ring particles?

  • Brett Bellmore February 6, 2016, 9:05

    Don’t you wish they’d just send a probe with a good camera and maneuvering jets, to actually enter the rings, and look at them up close? Seems to me that a spiral trajectory, displaced from the plane of the rings by only a few km, would be sustainable with such little thrust that even ion propulsion would be enough.

    • J. Jason Wentworth February 7, 2016, 7:22

      Indeed–ever since I saw the interplanetary freighter “Valley Forge” plow through Saturn’s rings in the movie “Silent Running” (with the botanist-turned-murderer Freeman Lowell [played magnificently by Bruce Dern] at the helm), I’ve been fascinated by what the rings *really* look like up close, as well as from within (the film depicted them–incorrectly–as gaseous, and no moons were depicted around Saturn, either). After I saw the ‘kinked’ F ring (which is shepherded by two ‘orbit-swapping’ satellites) that Pioneer 11 discovered, and the “spokes” and other unexpected ring features that the Voyagers saw, I wanted even more to see the rings from close range. Also:

      In Arthur C. Clarke’s and Chesley Bonestell’s 1972 book “Beyond Jupiter,” which covered the then-planned Grand Tour missions (I heartily recommend this work [AbeBooks.com http://www.abebooks.com booksellers carry it]–Bonestell’s paintings correctly depict the gloom of the outer solar system as the eyes of human explorers would see it out there), Clarke proposed a Saturn ring probe and described how it could ease its way into and through the rings safely, much as you described. Such a probe couldn’t “spoil the rings” even if it bumped into ring particles, because countless meteoroids have passed through the rings at far higher velocities since the rings were formed. In addition:

      A ring probe (with a possible secondary objective of studying Methone) would, as well as capturing the public’s imagination with its stunning photography, enable solar system dynamicists to study the initial stages of how natural satellites and (on a smaller scale) planets accrete. (The rings also provide a fairly good working model of the asteroid belt, whose bodies are prevented from accreting into one or more larger terrestrial-type planets by Jupiter’s gravity, which makes them collide at velocities too high to “stick together” to form larger worlds.) Studying the electrostatic effects that are at work in Saturn’s ring system (the levitated “spokes” appear to be caused by like-charge repulsion), and their interactions with the gravitational resonances in the rings, would also make a ring probe mission a scientifically rewarding one.

      • ljk February 9, 2016, 11:32

        Astronomers have known about the true dimensions of the ring particles of Saturn ever since they did radar studies in 1970, just before Silent Running premiered. The ring material averages a yard across and ranges from dust particles to ice boulders the size of a house.

        Not sure if the SR filmmakers were unaware of this or just ignored it to make a “cool” scene of the Valley Forge plowing through the rings like a trailer truck through a terrestrial snowstorm. In reality the ship would have been torn to shreds, especially the precious domes. But Trumbull wanted to FX Saturn because he could not do it in 1968 with 2001: A Space Odyssey. This also explains why they would have ships full of plants circling 800 million miles from Sol rather than much closer in.

        And it sounds crazy now but there was a serious debate about having the Pioneer 11 probe fly THROUGH the Cassini “Division”, which at the time was thought to be a clear zone in the rings. Thankfully the probe was guided outside the rings and lived to tell the tale.



  • Mark Zambelli February 8, 2016, 6:43

    I’ve always loved the cgi opening credits for Star Trek: Voyager… especially the brief moment where the camera p.o.v. soars upwards through the icy particles of a ring-system.

    The BBCs ‘Wonders of the Solar System’ with Prof Cox, did a stirling effort when portraying Saturns rings… https://en.m.wikipedia.org/wiki/Wonders_of_the_Solar_System … as was their depiction in the mini series ‘Space Odyssey’…
    https://en.m.wikipedia.org/wiki/Space_Odyssey_(TV_series) .

    I’ve often daydreamed of floating amongst those rings, especially with Cassini’s Equinox imagery… mindblowing.

    • Mark Zambelli February 8, 2016, 6:50

      And I couldn’t let an image slip by, especially one from my all-time personal favourite space artist, the late, great, Peter Elson… http://www.peterelson.co.uk/gallery/image.php?cat=11&id=218 … entitled ‘The Rings of Saturn’ (his brimming website is well worth browsing).