Saturn ring ripples explains how long a day lasts on planet

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Planet Saturn (Representational image)
Planet Saturn (Representational image)

New Delhi : We can hardly imagine, how long Saturn spins, just by watching the clouds swirling at its surface. But the ripples in the ring have all the explanations about how fast the planet rotates. Well, its day flies by in 10 hours, 33 minutes and 38 seconds.

“That’s a really fast clip,” says astronomer Christopher Mankovich of the University of California, Santa Cruz, who reports the rotation rate in the Astrophysical Journal on January 17. The planet is blessed with a radius of 58,000 kilometers and is about nine times the size of Earth yet its day is less than half as long.  

Earlier, scientists estimated possible lengths for a Saturnian day using radio measurements from the Voyager spacecraft in the 1980s and from the Cassini orbiter in the 2000s. But those estimates, while in the same ballpark as the new one, varied by about 20 minutes.

In 1993, astronomers Mark Marley and Carolyn Porco, realized that Saturn’s seismic activity changes the planet’s gravity enough to make its rings of tiny, orbiting particles shift in response. Because rotation stirs up the planet’s gas, the frequency of those inner oscillations could be used to estimate how fast the planet spins. Cassini’s 2004 arrival in Saturn’s orbit finally delivered ring images good enough to test the idea.

And, the images captured in 2017 by the NASA's orbiter's mission has helped Mankovich and colleagues to measure waves in the rings as the rings responded to Saturn’s vibrations. The scientists calculated precisely how fast the planet was rotating beneath the clouds.

“The amount we’ve learned about Saturn’s interior from the rings, of all places, is beyond almost anyone’s wildest imagination,” says Mankovich, whose research team included Marley, at NASA’s Ames Research Center in Moffett Field, Calif.

By knowing the rotation rate of Saturn, scientists can figure out the structure of this giant gas planet. Astronomers think Saturn has a solid core about 15 times the mass of all of Earth, topped by layers of different phases of fluid hydrogen. Mankovich and his colleagues assumed Saturn rotates as a rigid ball, but it could instead be layered like an onion, with each layer rotating at a different speed, Mankovich notes.