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Saturn's day is shorter than once thought, scientists say

New study slices more than six minutes from Saturn's day

How long is a day on Saturn? This apparently simple question has dogged scientists for years, but a new study has revealed that the days on this ringed gas giant are shorter than we thought.

The findings, published in the journal Nature, help settle a long-standing mystery and could help researchers better understand the complex dynamics of giant gaseous planets.

Measuring a day on a rocky planet like Earth or Mars is a pretty simple exercise – as the planet spins, track a single feature on its surface to see how long it takes to complete one full rotation. That’s not so simple with gas giants, which don’t have a solid surface and whose mass is blocked by thick layers of atmosphere. For these planets, other tricks must be used, and they don’t always work.

When NASA’s Voyager spacecraft flew by Saturn roughly 35 years ago, radio measurements of its magnetic field pegged the planet's rotation at 10 hours, 39 minutes and 22.4 seconds. That method was also used to determine Jupiter’s rotation period (and thus, the length of its day), and it seemed to work just fine.

But when NASA’s Cassini spacecraft, which arrived at Saturn in 2004, used the same method to measure the planet’s rotation, it arrived at a day length of 10 hours, 47 minutes and 6 seconds – and the measurement changed over time. It turns out that because Saturn’s magnetic field is aligned with its spin axis, using radio-frequency measurements isn’t a reliable method to measure its rotation period. (The method does work for Jupiter, because the planet’s magnetic field is somewhat askew, which makes it easier for scientists to track as the planet spins.)

Other tactics, including tracking clouds to measure Saturn’s winds, were equally baffling.

“Estimates based upon Saturn’s measured wind fields have increased the uncertainty even more, giving numbers smaller than the Voyager rotation period,” the study authors wrote, “and at present Saturn’s rotation period is thought to be between 10h 32min and 10h 47min, which is unsatisfactory for such a fundamental property.”

A team led by Ravit Helled of Tel Aviv University decided to use another measurement to try and measure the spin: the planet’s gravitational field, which Cassini can measure by how much the planet tugs on the spacecraft’s orbit. The scientists also used the planet’s oblateness – how much it flattens and starts to bulge around the equator as it spins – to help determine the rotation rate.

After narrowing down the possibilities by modeling, the scientists found that Saturn completes one full rotation in 10 hours, 32 minutes and 45 seconds (give or take 46 seconds) – which means that a Saturnian day is more than 6 minutes shorter than the Voyager mission’s measurement.

To make sure their results for Saturn were accurate, they also successfully double-checked their method by measuring Jupiter, whose day length is well known.

The new method could help scientists refine their understanding of a host of different processes on Saturn, including wind patterns in the gas giant’s atmosphere. And in the future, it could also help scientists better understand the dynamics of more distant gas giants.

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