what causes planet rings to hold their shape
Emily Lakdawalla • Dec 15, 2011
What do we know about planetary rings? Quite a lot, actually!
Short version of this blog entry: for everything you lot ever wanted to know almost planetary ring systems, read a new review article by Matt Tiscareno titled, laconically, "Planetary Rings," now available on arXiv. (Thanks to Luke Dones for the pointer.)
And now for the longer version. I most commonly write about papers from Science and Nature for the perhaps non very good reason that those papers are short. Their brevity ways that it's pretty easy to wrap my caput around an commodity and write a meaningful mail service almost it in the space of a few hours. Sometimes I write about scientific papers published in Icarus or the Journal of Geophysical Research. These papers are invariably more nuanced and based on a much longer bridge of research work (and so are far more likely to stand up up to scrutiny than anything published in Science or Nature), merely they also take more than time for me to understand and synthesize, usually half a day to a day.
And then at that place are review papers. By their very nature, it's impossible to summarize review papers, because they are themselves summaries of decades -- or even centuries -- of work past numerous scientists. And so I never write nearly them. But I'm moved to write about Tiscareno'south new paper because information technology contains answers to so many questions I've attempted to inquiry on the Spider web while writing stories, and failed to find useful resources. In simply the commencement few pages I've learned a lot of things that fabricated me get "hmm" or "aha!"
Early in the paper there's stuff about Roche limits. The Roche limit, as Tiscareno defines information technology, "is the distance from a planet inside which its tides can pull apart a compact object....However, the Roche limit does not actually have a single value, but depends particularly on the density and internal material strength of the moon that may or may not get pulled autonomously."
This much I knew, merely in the following paragraphs he discusses Saturn's small-scale ringmoons as fiddling probes of the ring's physical properties. Ringmoons are chunks of solid material that are resistant to being torn apart by tidal forces. Over time they accrue a surface dusting of very fluffy fabric, merely that stops when the density of the whole thing (solid cadre plus fluffy coating) reaches the Roche critical density. The unusual persistence and surface area of Saturn's rings results from the fact that their Roche critical density is lower than for whatsoever other planet, budgeted a density only xl% that of water ice. This, in plough, results from the fact that Saturn's rings have a much higher proportion of water ice to stone than any other planet's, which lowers their overall density. (Tiscareno doesn't become into why Saturn's rings are more icy than rocky; a contempo paper by Robin Canup proposes that Saturn ate the cadre of a Titan-sized moon and left its icy mantle in orbit.)
In that location'south some fascinating discussion of the other giant planets' Roche limits and what they imply near the densities of their moons. For instance, if you lot smashed upward Uranus' "Portia group" of moons (Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, Perdita, and Puck) and spread their mass over the area of your orbits, you'd get something with roughly the same density as Saturn's A ring. Then why are they moons instead of an A ring? Likely considering they're denser (rockier) than Saturn's A band: "This moon organization may be very similar in origin to the known ring systems, except that the natural density of accreted objects is larger than the Roche critical density...so that whatever moon that gets disrupted by a collision (which ought to have happened many times over the age of the solar system) will merely re-accrete."
Planetary rings are often described equally being modest versions of the kinds of disks from which planets course. The parallel is useful but it'south interesting that the 2 systems, both of which consist of rotating disks of particles orbiting a large central mass, are apartment disks for entirely different reasons. Ultimately, planetary ring systems are apartment considering of the oblate (equatorially bulging) shapes of planets, which creates an asymmetric gravity field effectually the planets. Stellar debris disks don't accept these asymmetric gravity fields. They are flat, ultimately, considering of the large angular momentum of the disk itself. While the two systems have different causes, they both wind upward with particles orbiting in a preferred plane considering collisions amidst particles clammy out any motion perpendicular to that aeroplane. Saturn, uniquely, has one prepare of rings aligned with the planet's rotation plane (due to Saturn's equatorial bulge) and a much larger, more distant ring aligned with its orbital plane (the Phoebe ring, which is too far from Saturn for the equatorial bulge to accept an important contribution to the orbital development of its particles).
The paper reviews the reasoning for why there should be rings at Mars and Pluto and why they have been difficult to detect (if they exist at all). It reviews the prove for rings at Saturn's moon Rhea and why they likely do not exist. It reviews the detectability of exoplanetary rings.
Afterward considering all these cases of known or theorized actual rings, the paper goes on to a more than theoretical treatment of the types of rings and the phenomena found within them. Information technology explains the difference between spiral density waves (which are compressional and propagate outward) and spiral angle waves (which are transverse and propagate inward) and why they're cool: "Spiral waves, especially weak ones, are useful structures that tin can be idea of as in situ scientific instruments placed in the rings." You can get masses of small moons from screw density waves, and deduce properties of the rings themselves from bending waves, under the right lighting conditions, near the equinoxes.
There'south a fair amount of mathematics in the article, which I instinctively skip while reading. Merely when I force myself to go dorsum and read the equations I find they're of a general form that even someone who hasn't written down an equation for 12 years can read. Nosotros learn about Keplerian shear, and how the scalloped edges of the Encke gap combined with relatively elementary mathematics enabled Mark Showalter and his coworkers to predict the location of, and and then discover, the embedded moon Pan in archived Voyager images. We learn virtually propellers and spokes.
There's more than, but like I said, information technology's folly to attempt to summarize a paper that is itself a summary of the results of four centuries of scientific research. Have you ever looked at Cassini images and tried to figure out what makes those absurd structures in Saturn's rings? Take you always wondered what the departure is between a bending wave and a density wave and a ringmoon wake? Take yous always asked questions like "why doesn't Mars have rings" or "practise extrasolar planets accept rings?" or "what are they talking about when they hash out 'propellers' in Saturn's rings?" Give this newspaper a endeavor! Yous'll also learn about "Propeller Belts" and "frog resonances" and the fact that "The F ring is the grandfather of narrow dusty ringlets" and it contains "an unseen chugalug of kilometer-sized moonlets."
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Source: https://www.planetary.org/articles/3302
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