Waves in Saturn's rings give precise measurement of planet's rotation rate

January 18, 2019, University of California - Santa Cruz
This image of Saturn's rings was taken by NASA's Cassini spacecraft on Sept. 13, 2017. It is among the last images Cassini sent back to Earth. Credit: NASA/JPL-Caltech/Space Science Institute

Saturn's distinctive rings were observed in unprecedented detail by NASA's Cassini spacecraft, and scientists have now used those observations to probe the interior of the giant planet and obtain the first precise determination of its rotation rate. The length of a day on Saturn, according to their calculations, is 10 hours 33 minutes and 38 seconds.

The researchers studied created within Saturn's rings by the planet's internal vibrations. In effect, the rings act as an extremely sensitive seismograph by responding to vibrations within the planet itself.

Similar to Earth's vibrations from an earthquake, Saturn responds to perturbations by vibrating at frequencies determined by its internal structure. Heat-driven convection in the interior is the most likely source of the vibrations. These internal oscillations cause the density at any particular place within the planet to fluctuate, which makes the gravitational field outside the planet oscillate at the same frequencies.

"Particles in the rings feel this oscillation in the . At places where this oscillation resonates with orbits, energy builds up and gets carried away as a wave," explained Christopher Mankovich, a graduate student in astronomy and astrophysics at UC Santa Cruz.

Mankovich is lead author of a paper, published January 17 in the Astrophysical Journal, comparing the wave patterns in the rings with models of Saturn's interior structure.

Saturn's C ring is home to a surprisingly rich array of structures and textures. Much of the structure seen in the outer portions of Saturn's rings is the result of gravitational perturbations on ring particles by moons of Saturn, but some of the features in the rings are due to the vibrations of the planet itself. Credit: NASA/JPL-Caltech/Space Science Institute

Most of the waves observed in Saturn's rings are due to the gravitational effects of the moons orbiting outside the rings, said coauthor Jonathan Fortney, professor of astronomy and astrophysics at UC Santa Cruz. "But some of the features in the rings are due to the oscillations of the planet itself, and we can use those to understand the planet's internal oscillations and internal structure," he said.

Mankovich developed a set of models of the of Saturn, used them to predict the frequency spectrum of Saturn's internal vibrations, and compared those predictions with the waves observed by Cassini in Saturn's C ring. One of the main results of his analysis is the new calculation of Saturn's , which has been surprisingly difficult to measure.

As a gas giant planet, Saturn has no solid surface with landmarks that could be tracked as it rotates. Saturn is also unusual in having its nearly perfectly aligned with its rotational axis. Jupiter's magnetic axis, like Earth's, is not aligned with its rotational axis, which means the magnetic pole swings around as the planet rotates, enabling astronomers to measure a periodic signal in radio waves and calculate the rotation rate.

The rotation rate of 10:33:38 determined by Mankovich's analysis is several minutes faster than previous estimates based on radiometry from the Voyager and Cassini spacecraft.

This Cassini image shows in unprecedented detail a density wave in Saturn's B ring, most likely caused by a small moon systematically perturbing the orbits of ring particles. Credit: NASA/JPL-Caltech/Space Science Institute
"We now have the length of Saturn's day, when we thought we wouldn't be able to find it," said Cassini Project Scientist Linda Spilker. "They used the rings to peer into Saturn's interior, and out popped this long-sought, fundamental quality of the planet. And it's a really solid result. The rings held the answer."

The idea that Saturn's rings could be used to study the seismology of the planet was first suggested in 1982, long before the necessary observations were possible. Coauthor Mark Marley, now at NASA's Ames Research Center in Silicon Valley, subsequently fleshed out the idea for his Ph.D. thesis in 1990, showed how the calculations could be done, and predicted where features in Saturn's rings would be. He also noted that the Cassini mission, then in the planning stages, would be able to make the observations needed to test the idea.

"Two decades later, people looked at the Cassini data and found ring features at the locations of Mark's predictions," Fortney said.

Explore further: Image: Saturn's inside-out rings

More information: Christopher Mankovich et al, Cassini Ring Seismology as a Probe of Saturn's Interior. I. Rigid Rotation, The Astrophysical Journal (2019). DOI: 10.3847/1538-4357/aaf798

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22 comments

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cantdrive85
1.4 / 5 (10) Jan 18, 2019
They are detecting the electric currents driving Saturn's rotation, the waves and oscillations are evidence of these currents.
Phyllis Harmonic
5 / 5 (10) Jan 18, 2019
They are detecting the electric currents driving Saturn's rotation, the waves and oscillations are evidence of these currents.


No where in the article was interactions with electric currents mentioned.

"Particles in the rings feel this oscillation in the gravitational field. At places where this oscillation resonates with ring orbits, energy builds up and gets carried away as a wave," explained Christopher Mankovich, a graduate student in astronomy and astrophysics at UC Santa Cruz.


But I have a strong feeling your DK-powered character will prevent you from accepting actual research as anything other than conspiratorial clap-trap.
jonesdave
4.1 / 5 (13) Jan 18, 2019
They are detecting the electric currents driving Saturn's rotation, the waves and oscillations are evidence of these currents.


Lol. How the hell are electric currents making a bloody great planet rotate? Sheer idiocy.
Mark Thomas
4.8 / 5 (4) Jan 18, 2019
Great result that makes Saturn's rotation rate even closer to Jupiter's.

One might wonder if something more than simple random chance has caused the planets' solar day lengths to come in pairs?

Mercury (176 days) and Venus (117 days)
Earth (24 hours) and Mars (24 hours 40 minutes)
Jupiter (9 hours 55 minutes) and Saturn (10 hours 33 minutes)
Uranus (17 hours 15 minutes) and Neptune (16 hours, 7 minutes)

https://www.unive...planets/
Solon
1 / 5 (1) Jan 18, 2019
"No where in the article was interactions with electric currents mentioned."

Maybe not but low-frequency electrostatic perturbations, moving ions and charged dust are discussed it's hard to dismiss electrical involvement.

Excitation of dust acoustic modes in dusty plasmas of Saturn's rings
https://www.resea...'s_rings
jonesdave
4.1 / 5 (9) Jan 18, 2019
"No where in the article was interactions with electric currents mentioned."

Maybe not but low-frequency electrostatic perturbations, moving ions and charged dust are discussed it's hard to dismiss electrical involvement.

Excitation of dust acoustic modes in dusty plasmas of Saturn's rings
https://www.resea...'s_rings


And....? You think plasma physicists have never considered such things before? That is standard stuff.
TuringTest
5 / 5 (3) Jan 18, 2019
I would think Venus and Uranus would be complete anomalies there because of their retrograde rotation, and wouldn't the similarities of the other pairs have to do with their similar masses (relitivily speaking of course) and their relatively similar distances from the sun? Just a guess.
jonesdave
4.1 / 5 (9) Jan 18, 2019
Maybe not but low-frequency electrostatic perturbations, moving ions and charged dust are discussed it's hard to dismiss electrical involvement.


What is it with the EU loons that makes them think that any finding of 'electrical involvement' is a win for their silly cult?

https://scholar.g...y+plasma

Copy/ paste that into Google. Look at the dates of some of those papers. When was the electric lightning bolt cult founded?

Mark Thomas
5 / 5 (1) Jan 18, 2019
TuringTest, sounds like as good as guess as any. Could this be some subtle effect related to the formation of orbital resonances? Probably not, but I don't know.

https://en.wikipe...esonance
Phyllis Harmonic
5 / 5 (4) Jan 18, 2019
Maybe not but low-frequency electrostatic perturbations, moving ions and charged dust are discussed it's hard to dismiss electrical involvement.


In fluid dynamics, "gravity waves" (not *gravitational waves*- they are two different things) are generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. I think a related phenomenon is more likely the cause of these density waves in the rings, than any electrostatic forces.
cantdrive85
2.6 / 5 (5) Jan 19, 2019
How the hell are electric currents making a bloody great planet rotate?

The Lorentz force.
wduckss
2.5 / 5 (2) Jan 19, 2019
Assemble two and two and assure yourself of the credibility of the article.
1. The average density of Saturn is 687,3 kg/m^3,
The average density: Pan 0,42 g/cm^3, Atlas 0,46 g/cm^3, Pandora 0,48 g/cm^3,
67P/Churymov-Garasimenko 0,533 g/cm^3 (its aggregate state is solid, so Philae could easily land on its surface. https://www.svemi...tructure )

2. "Strong 300 km/h (185 mph) winds at the cloud tops go around Venus about every four to five Earth days. Winds on Venus move at up to 60 times the speed of its rotation, whereas Earth's fastest winds are only 10–20% rotation speed."
prpuk
1 / 5 (1) Jan 20, 2019
From the article:
"Most of the waves observed in Saturn's rings are due to the gravitational effects of the moons orbiting outside the rings, said coauthor Jonathan Fortney"


OK, since this is the case and the earth has it's own equatorial ring in the form of the QBO of stratospheric winds, why aren't our QBO waves associated in some way with the gravitational effects of the moon (and of course the sun, which has less of a gravitational effect)?

Incidentally, the earth's QBO is invisible to the human eye, but if it did hold visibly-reflecting particles aloft, like Saturn, it would look similar to a Saturn ring.

jonesdave
3.9 / 5 (7) Jan 20, 2019
How the hell are electric currents making a bloody great planet rotate?

The Lorentz force.


Lol. What has that got to do with anything?
Da Schneib
4.2 / 5 (5) Jan 20, 2019
@jones, this person has just asserted that Saturn's rings are made of charged particles. Because only charged particles are acted upon by the Lorentz force.
cantdrive85
2 / 5 (4) Jan 20, 2019
Clearly Saturn's rings consist mostly of charged particles and electric dipole, that is the matter that will be affected. Sure there are neutrals as well, they will just be dragged along with the electric particles.
jonesdave
3.7 / 5 (6) Jan 20, 2019
Clearly Saturn's rings consist mostly of charged particles and electric dipole, that is the matter that will be affected. Sure there are neutrals as well, they will just be dragged along with the electric particles.


They consist of ice, you cretin!
cantdrive85
2.3 / 5 (3) Jan 20, 2019
Ice, which is water, is an electric dipole. And yes, there are charged particles as well. There are volumes of papers/observations that support this.
jonesdave
3.7 / 5 (6) Jan 20, 2019
Ice, which is water, is an electric dipole. And yes, there are charged particles as well. There are volumes of papers/observations that support this.


No, there are no papers suggesting that the orbits of particles in the rings is anything other than due to gravity. Nobody would be stupid enough to write such nonsense.
cantdrive85
2.3 / 5 (3) Jan 20, 2019
No, there are no papers suggesting that the orbits of particles in the rings is anything other than due to gravity

Did I suggest this? Nope, didn't say anything remotely close to that. What I did say;
And yes, there are charged particles as well. There are volumes of papers/observations that support this.

Which was in response to your low brow comment;
They consist of ice, you cretin!

jonesdave
3 / 5 (4) Jan 20, 2019
No, there are no papers suggesting that the orbits of particles in the rings is anything other than due to gravity

Did I suggest this? Nope, didn't say anything remotely close to that. What I did say;
And yes, there are charged particles as well. There are volumes of papers/observations that support this.

Which was in response to your low brow comment;
They consist of ice, you cretin!



Nope, you said;

They are detecting the electric currents driving Saturn's rotation, the waves and oscillations are evidence of these currents.


I asked how this was happening. You said the Lorentz force. I asked what that had to do with anything. Then you started babbling about charged particle nonsense.

rrwillsj
1 / 5 (1) Jan 20, 2019
Mark, I think it would have been better to compare Earth's rotation against Luna (as a planet) than against Mars? Which maybe should be compared against Ceres & the other Main Belt Asteroids?

That perhaps during this System's planetesmal rings bedlam period. As disorder coerced order out of the debris in a giant game of whack-a-mole.
The planetoids conglomerating from the same set of rings, fell into the same pattern of rotation?
With additional gravitational influences of Jupiter & Saturn.

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