Why are Jupiter’s rings so thin? Sky and telescope

Jupiter in the infrared
An infrared image of the James Webb Space Telescope clearly reveals Jupiter’s thin ring. (The moons Thebe, Europa, Metis, and Adrastea, left to right, are also visible.)
NASA / European Space Agency / Canadian Space Agency / Judy Schmidt

Nature has given the largest planet in the solar system a set of anemic episodes. Saturn’s rings have been marked as one of the masterpieces of the solar system since the invention of the telescope, but no one even noticed that Jupiter had a much smaller cluster until the Voyager 1 spacecraft flew by on March 5, 1979.

Why should Saturn’s rings, which have a third of Jupiter’s mass, outperform the weak rings around the larger, more massive planet?

Ring locations vs satellites
This graph shows the relative sizes and distances between the moons of Jupiter and Saturn, respectively. The red dashed lines indicate the Roche boundaries of the planets, where gravitational tidal forces prevent the moons from forming or disintegrating any very close drift.
Ken and Lee 2022

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

“I’ve always wondered why these wonderful rings are on Saturn,” says Stephen Kane (University of California, Riverside). To find out why Jupiter’s rings aren’t measured, he and his student Zhexing Li (also at UCR) developed a computer model to see the effect of Jupiter’s four Galilean moons on a thick dust ring around the planet.

Rings are dynamic, and their shape and size at a particular time depends on the mass of the planet it orbits and the date of the moons orbiting it. inside a point called roach limit, the planet’s gravity dominates, preventing the moons from forming and ripping off any moons that come too close; Rings are formed there instead. Saturn’s brightest rings lie within the Roche boundary, as do most of Jupiter’s rings.

But rings can form outside the limits of the Roche as well, as happens around both Saturn and Jupiter. Here’s where the moon’s gravity plays a role, Kane’s team confirms.

The orbits of Jupiter’s three innermost moons – Io, Europa and Ganymede – resonate in a strong periodic resonance: Io orbits Jupiter four times and Europa twice each time Ganymede orbits the planet once. (The Galilean’s farthest moon, Callisto, is getting stuck in a different echo: It’s gradually closing so that it always faces the same side of Jupiter as it rotates, 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 boundary to 28 Jupiter radii from the planet’s cloud tops within a million years. Only a few small rings will remain within that region, some between the orbits of Ganymede and Callisto and some near the orbit of Callisto. When Kane and Lee extended their model to last 10 million years, the inner tones also disappeared.

An orbital resonance with Ganymede and Callisto would remove material that exceeds 29 Jupiter radii — beyond Callisto’s orbit — within tens of millions of years, the team reports in Planetary Science Journal.

Jupiter, if it has rings like Saturn's
This artist’s rendering shows what Jupiter would look like if its rings were as wide as those of Saturn. If a buyer has had rings like this before, they won’t last long.
Stephen Kane / UCR

While Kane notes that the moons have little effect within the Roche boundary, it’s possible that Jupiter’s magnetic field, which was not considered in the model, prevents material from gathering there.

For another point of comparison, Kane wants to study Uranus. While not close to the brightness of Saturn, its rings were the second to be observed among the giant planets and are thick enough to obscure distant stars. Observers can use this astrology to measure episodes.

“We don’t understand [Uranus’s] Dynamic history,” notes Kane; that’s because it tilts on its side, with its equator perpendicular to its orbit, possibly due to a massive collision long ago. However, studying the moons and rings of Uranus can help test their theory. And research into the planets can help. Outer Rings offers a more specific test, but only with live shooting, which is beyond current technology for all but the closest possible targets.


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