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A puzzle about our Solar System — how long is a day on Saturn? — has been solved with new data analysis. This figure was hard to calculate because the gas giant does not have a solid surface so there are no landmarks which can be tracked as the planet rotates. In addition, a magnetic field makes the rate of rotation hard to see.

But now NASA scientists have used data from the Cassini spacecraft to pin down a final answer: a day on Saturn is ten hours, 33 minutes, and 38 seconds long. The new day length of 10:33:38 is somewhat shorter than previous estimates, such as the 10:39:22 estimation from 1981 based on magnetic field data from Voyager.

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An image of Saturn captured by NASA’s Cassini spacecraft in 2016. The rings of Saturn have been used to work out the length of a day on the planet. NASA/JPL-Caltech/Space Science Institute

The new figure was calculated by looking at Saturn’s rings, about which Cassini gathered lots of detailed data during its mission from launch in 1997 to its eventual destruction in the planet’s atmosphere in 2017. During its orbit of Saturn from 2004 onward, the craft collected high resolution images of the planet and data on its icy, rocky rings. This data was then used by graduate student Christopher Mankovich to study wave patterns within the rings.

Mankovich found that the rings acted as a kind of seismometer, responding to vibrations the occurred within the planet. When the inside of the planet vibrates and earthquakes occur, the vibration frequencies cause variations in the planet’s gravitational field, and these variations are transmitted to the rings. “Particles throughout the rings can’t help but feel these oscillations in the gravity field,” Mankovich explained in a statement. “At specific locations in the rings these oscillations catch ring particles at just the right time in their orbits to gradually build up energy, and that energy gets carried away as an observable wave.”

This means that scientists can now track the movements of the planet’s interior, and from that tracking they can see the planet’s rotation. This is what allowed them to calculate the exact length of a day on Saturn. “The researchers used waves in the rings to peer into Saturn’s interior, and out popped this long-sought, fundamental characteristic of the planet. And it’s a really solid result,” said Cassini Project Scientist Linda Spilker in the same statement. “The rings held the answer.”

The paper is available at physics archive arXiv.

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