Why are Jupiter’s rings so thin? – Sky and Telescope

Nature has given the largest planet in the solar system an anemic set of rings. Saturn’s rings have been singled out as one of the solar system’s splendors since the invention of the telescope, but no one realized that Jupiter had its own much smaller set until the Voyager 1 spacecraft flew by on March 5, 2019. 1979.

Why should the rings of Saturn, which is one-third the mass of Jupiter, so dwarf the faint rings around the larger, more massive planet?

At least part of the answer may be that Jupiter’s three innermost moons got in the way. A computer model of the Galilean satellites shows that the same gravitational resonance that keeps Io, Europa, and Ganymede in a synchronized 4:2:1 orbital step also drains the equatorial orbits of dust that could form rings.

“I’ve always wondered why Saturn has these glorious rings,” says Stephen Kane (University of California, Riverside). To figure out why Jupiter’s rings don’t measure up, he and his student Zhexing Li (also at UCR) developed a computer model to see what effect Jupiter’s four Galilean moons would have on a thick dust ring around Jupiter. planet.

The rings are dynamic and their shape and size at any given time depends on the mass of the planet they orbit and the history of the moons that orbit it. Inside a point called Roche limit, the planet’s gravity dominates, preventing moons from forming and tearing apart moons that get too close; instead, rings form there. Saturn’s brightest rings lie within the Roche limit, as do most of Jupiter’s rings.

But rings can also form outside the Roche limit, as they do around Saturn and Jupiter. This is where the gravitational pulls of the moons play a role, Kane’s team argues.

The orbits of Jupiter’s three innermost large moons, Io, Europa, and Ganymede, are locked in a strong periodic resonance: Io orbits Jupiter four times, and Europa orbits twice each time Ganymede circles the planet once. (The outermost Galilean moon, Callisto, got caught in a different resonance: It’s tidally locked, so it always faces the same side of Jupiter as it orbits, just as the same side of Earth’s Moon always faces our planet).

Kane’s computer model showed that resonance between the inner moons would remove any dust from Jupiter’s Roche limit up to 28 Jupiter radii from the planet’s clouds within a million years. Only a few ripples would remain within that region, some between the orbits of Ganymede and Callisto and others near Callisto’s orbit. When Kane and Li extended their model to last 10 million years, the internal curls also disappeared.

Orbital resonances with Ganymede and Callisto would remove material beyond 29 Jupiter radii, well beyond Callisto’s orbit, within tens of millions of years, the team reports in the Planetary Science Magazine.

While Kane points out that the moons have little effect inside the Roche limit, it’s possible that Jupiter’s magnetic field, which isn’t considered in the model, could prevent material from accumulating there.

For another point of comparison, Kane wants to study Uranus. While not as bright as Saturn’s, its rings were the second observed among the giant planets and are thick enough to hide distant stars. Observers can use these occultations to measure the rings.

“We do not understand [Uranus’s] dynamic history,” Kane notes; that’s because it’s tilted to one side, with its equator perpendicular to its orbit, perhaps due to a massive collision long ago. Studying Uranus’ moons and rings could help, though. to test his theory.The search for exoplanets for rings could offer more definitive proof, but only with direct imaging, which is beyond current technology for all but the closest potential targets.

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