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Thread: Planetary Rings

  1. #1
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    Planetary Rings

    Is there or would there be a qualitative difference between rings formed by the collection of small objects/particles and a ring system formed by the break up a larger object (moon, large asteroids, etc)?

    I see that there is a hypothesis that Saturn's rings were formed by a moon that shed its icy crust before plunging into Saturn itself. Would that ice be different than ices from a different sort like a series of comets?
    Solfe

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  2. #2
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    Lightbulb Origin of Saturn's Rings

    Quote Originally Posted by Solfe View Post
    Is there or would there be a qualitative difference between rings formed by the collection of small objects/particles and a ring system formed by the break up a larger object (moon, large asteroids, etc)?
    There certainly could be depending on the size of the disrupted object. Comets have lots of ice, or material from which ice can be made. Asteroids, on the other hand, will make far more rock than ice. Moons differ greatly depending on where & how they were formed (just compare Jupiter's moons Europa & Io; the former looks like an ice ball while the latter is volcanic and sulfurous). Larger objects tend to differentiate, meaning that the internal mass separates so that, in general, the denser stuff drops towards the core, while the less dense stuff floats towards the surface. That changes the chemistry and can change the ratio of isotopes as well. The isotope ratio in water ice is particularly sensitive to the freezing temperature, and that is used to determine where comets formed. So in principle, given a chemical constitution for the ice in the rings, we should be able to figure out quite a bit about where the ice came from.

    Quote Originally Posted by Solfe View Post
    I see that there is a hypothesis that Saturn's rings were formed by a moon that shed its icy crust before plunging into Saturn itself. Would that ice be different than ices from a different sort like a series of comets?
    That's Origin of Saturn's rings and inner moons by mass removal from a lost Titan-sized satellite by Robin Canup, Nature 468(7362): 943-926, December 2010.
    Abstract: "The origin of Saturn's rings has not been adequately explained. The current rings are more than 90 to 95 per cent water ice, which implies that initially they were almost pure ice because they are continually polluted by rocky meteoroids. In contrast, a half-rock, half-ice mixture (similar to the composition of many of the satellites in the outer Solar System) would generally be expected. Previous ring origin theories invoke the collisional disruption of a small moon, or the tidal disruption of a comet during a close passage by Saturn. These models are improbable and/or struggle to account for basic properties of the rings, including their icy composition. Saturn has only one large satellite, Titan, whereas Jupiter has four large satellites; additional large satellites probably existed originally but were lost as they spiralled into Saturn. Here I report numerical simulations of the tidal removal of mass from a differentiated, Titan-sized satellite as it migrates inward towards Saturn. Planetary tidal forces preferentially strip material from the satellite's outer icy layers, while its rocky core remains intact and is lost to collision with the planet. The result is a pure ice ring much more massive than Saturn's current rings. As the ring evolves, its mass decreases and icy moons are spawned from its outer edge with estimated masses consistent with Saturn's ice-rich moons interior to and including Tethys."

    I have not read the paper, just this abstract, so I don't know the details of her model. But the general idea is not out of the question. During the early stages of moon formation around a planet (and planet formation around a star) the forming moons (planets) are still embedded in a disk of dust out of which the moon (planet) forms. Friction between the moon (planet) and the disk causes it to lose energy and spiral inwards towards the central planet (star). So migrating into a regime of strong tidal forces is a reasonable thing to expect to happen early on in the process.

    I think this idea could be tested if we had a physical sample of the ice from Saturn's rings, especially the isotope ratio, and how much contamination there is from other material (dust, rock etc.). That should be able to differentiate between an ice mantle that has differentiated (and/or formed in a higher temperature region) or ice that is primordial.

    Robbins, et al., 2010 estimate the total mass of the Saturn ring system to be about 120% - 200% of the mass of Mimas (that would be about 7.6x1019 kg). That is a tiny fraction, about 0.0056% of the mass of Titan, so the basic numbers make sense; a Titan-sized icy moon would only have to shed a thin outer skin to do the job.

  3. #3
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    We don't need a physical sample of the rings. The spectra of the rings already point to them being made of extremely pure water ice. An explanation for the purity of the ice in Saturn's rings is one of the motivations behind the Canup model.

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