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E-mail the story Making sense of Saturn's impossible rotation. According to the new method, Saturn's day is 10 hours, 32 minutes and 44 seconds long.
When the researchers applied their method to Jupiter, whose rotation period is already well known, the results were identical to the conventional measurement, reflecting the consistency and accuracy of the method.
For years, scientists have had difficulty coming up with a precise measurement of Saturn's rotation. It was then understood that Saturn's rotation period could not be inferred from the fluctuations in radio radiation measurements linked to Saturn's magnetic field, and was in fact still unknown. We came up with an answer based on the shape and gravitational field of the planet. We were able to look at the big picture, and harness the physical properties of the planet to determine its rotational period.
Helled's method is based on a statistical optimization method that involved several solutions. First, the solutions had to reproduce Saturn's observed properties within their uncertainties : its mass and gravitational field.
Then the researchers harnessed this information to search for the rotation period on which the most solutions converged. The derived mass of the planet's core and the mass of the heavy elements that make up its composition, such as rocks and water, are affected by the rotation period of the planet. Knowledge of Saturn's composition provides information on giant planet formation in general and on the physical and chemical properties of the solar nebula from which the solar system was formed.
If you improve your measurement of Saturn's gravitational field, you narrow the error margin. When Cassini got to Saturn, it also measured Saturn's rotation rate from its radio emissions.
That's Saturn didn't slow down between Voyager in and Cassini in ; there is no force in the solar system that could have made such a massive planet slow its rotation so much, so rapidly. One -- or both -- of the measurements was wrong. Later on in the Cassini mission, the team realized that their data showed different rotation rates in the northern and southern hemispheres! Clearly, the method of using radio emissions to measure Saturn's rotation rate did not work the way it did at other giant planets.
They are measuring something rotating, but it's not the bulk rotation of the planet. The Cassini magnetometer and radio and plasma wave science teams spent the rest of the mission trying to find a signal in their data that accurately represented the rotation rate of Saturn.
They were working on lots of other science too, and were very successful, but they failed to solve the problem of its rotation rate. Not for lack of trying and they're still trying , but just because the universe just doesn't work the way we thought it did, and the apparently simple question about the length of Saturn's day remains to be answered with other methods. It approaches Saturn's rotation rate in a very different way, using ring seismology.
Some wave structures in Saturn's rings are sensitive to the gravity field of Saturn. Gravity is a powerful way to investigate the deep interior of a planet. Their number, 10 hours, I will try to explain ring seismology, but it's not easy. There is a good explanation in this paper section 3. Feel free to skip this paragraph -- you don't need it to get the rest of the article! Saturn like other planets has internal vibrations -- it rings like a bell, on many different frequencies.
A very deep-toned bell, to be sure. These internal vibrations cause little perturbations in its gravity field in the space close in to the planet. Saturn, unusually among planets, has a lot of particles orbiting in the space close to the planet that form a structure that we can see and measure -- the C ring. Certain frequencies of Saturn's internal vibrations resonate with orbits of particles in the C ring. Gravitational resonances with ring particles sculpt waves and gaps in Saturn's rings.
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