The part about Saturn’s deep rotation is very interesting. Voyager in the 80s measured Saturn’s rotation (based on radio signals) at 10h39m, while Cassini itself before the Grand Finale measured Saturn’s rotation (again based on radio signals) at 10h45m. Now with the Grand Finale data, Cassini has measured the “deep” (core) rotation at 10h33m.
All the measurements are regarded as correct. The explanation for the 6 minute disagreement between Cassini in 2004-05 (10h45m) and Voyager in 1980-81 (10h39m) was proposed to be slippage between Saturn’s magnetic field and Saturn’s core causing the magnetic field to be slowing down (and presumably speeding up) over time. The explanation for the difference between Cassini’s radio measurements of magnetic field rotation (10h45m) and Cassini’s Grand Finale measurements of core rotation (10h33m) is that Saturn likely has pretty extreme differential rotation: the core rotates once every 10 hours and 33 minutes, and the concentric layers of its atmosphere rotate progressively slower.
I’m guessing the core is throwing out a steady magnetic field but there is enough magnetically charged material in the slower-rotating layers to drag on that magnetic field. Likely that there’s some grand cycle going on: as the magnetic field rotation is slowed by charged atmosphere drag, the core begins to exert more force on the atmosphere through magnetism in addition to friction, speeding the atmosphere’s rotation back up. Because of the massive amounts of material involved, there is a massive amount of inertia, and the system overshoots equilibrium. It’s getting very speculative and well outside my layman’s expertise but I think this would predict there are periods during the cycle where the atmosphere of Saturn actually rotates faster than the core does! I think it’s also possible that the cycle goes from “significantly slower than core” to “slightly slower than core” and back?
At this point I am well into fantastical sci-fi scenarios but I can also imagine the differential rotation being too strongly determined by friction and other non-magnetic forces, so all of this charged drag is concentrating certain types of material (highly affected by the magnetic field, relatively free to move) into a thin shell some distance from the core, and that shell expands and contracts in diameter as it tries to find the specific height in the atmosphere that is currently rotating at the same speed as the magnetic field is. So like a marble inside a balloon, and the balloon is expanding and contracting, except there’s a bunch of gas inside and outside the balloon obscuring all of this from our sight.
I don’t know of a good term, no. “Differential rotation” is the broader concept but almost all of the information there is about bodies of fluids, with not much treatment of discrete objects. Even a single discrete object inside a body of fluid is hard to find information on; “object A inside shell B, all in a fluid C” is probably asking too much. Hopefully someone does, though! This is probably one of those cases where if you know the right term you can access a whole hidden field of literature on the topic.
Would fluid-structure interaction [1] with encapsulated systems fit the bill? It’s a subset of continuum mechanics which is probably the best field to model the layers of atmosphere as a mix of solid and fluid continuums and oscillatory systems are a defining feature of the field:
> Fluid–structure interaction (FSI) is the interaction of some movable or deformable structure with an internal or surrounding fluid flow. Fluid–structure interactions can be stable or oscillatory. In oscillatory interactions, the strain induced in the solid structure causes it to move such that the source of strain is reduced, and the structure returns to its former state only for the process to repeat.
IANAAstrophysicist, but I'm pretty sure that, with no external driving force, such an oscillation in differential rotation rates would die out quite quickly.
I would have thought so too, but losing 6 minutes out of 10.5 hours in just 25 years is a pretty major change on the timescales that planets operate on.
All the measurements are regarded as correct. The explanation for the 6 minute disagreement between Cassini in 2004-05 (10h45m) and Voyager in 1980-81 (10h39m) was proposed to be slippage between Saturn’s magnetic field and Saturn’s core causing the magnetic field to be slowing down (and presumably speeding up) over time. The explanation for the difference between Cassini’s radio measurements of magnetic field rotation (10h45m) and Cassini’s Grand Finale measurements of core rotation (10h33m) is that Saturn likely has pretty extreme differential rotation: the core rotates once every 10 hours and 33 minutes, and the concentric layers of its atmosphere rotate progressively slower.
I’m guessing the core is throwing out a steady magnetic field but there is enough magnetically charged material in the slower-rotating layers to drag on that magnetic field. Likely that there’s some grand cycle going on: as the magnetic field rotation is slowed by charged atmosphere drag, the core begins to exert more force on the atmosphere through magnetism in addition to friction, speeding the atmosphere’s rotation back up. Because of the massive amounts of material involved, there is a massive amount of inertia, and the system overshoots equilibrium. It’s getting very speculative and well outside my layman’s expertise but I think this would predict there are periods during the cycle where the atmosphere of Saturn actually rotates faster than the core does! I think it’s also possible that the cycle goes from “significantly slower than core” to “slightly slower than core” and back?
At this point I am well into fantastical sci-fi scenarios but I can also imagine the differential rotation being too strongly determined by friction and other non-magnetic forces, so all of this charged drag is concentrating certain types of material (highly affected by the magnetic field, relatively free to move) into a thin shell some distance from the core, and that shell expands and contracts in diameter as it tries to find the specific height in the atmosphere that is currently rotating at the same speed as the magnetic field is. So like a marble inside a balloon, and the balloon is expanding and contracting, except there’s a bunch of gas inside and outside the balloon obscuring all of this from our sight.
(Would love astrophysicists to chime in)