A day on Saturn lasts 10 hours 32 minutes 45 seconds (+/- 46 seconds), says a group of astronomers led by Dr Ravit Helled of Tel Aviv University, Israel.
This image was obtained on 10 September 2007 by NASA’s Cassini spacecraft, at a distance of approximately 3.3 million km from Saturn. Rhea – Saturn’s second largest moon – is visible against the bluish backdrop of the planet’s northern hemisphere. Mimas – Saturn’s seventh largest moon – appears as a speck against the ring shadows on planet’s western limb. Image credit: NASA / JPL / Space Science Institute.
To measure the rotation period of Saturn – the sixth planet from the Sun, Dr Helled and his colleagues from the Weizmann Institute of Science in Rehovot, Israel, have created a novel method, which is based on Saturn’s measured gravitational field and the unique fact that its east-west axis is shorter than its north-south axis.
“In the last two decades, the standard rotation period of Saturn was accepted as that measured by Voyager 2 in the 1980s: 10 hours, 39 minutes, and 22 seconds,” explained Dr Helled, who is the lead author on the paper published in the journal Nature.
“But when the Cassini spacecraft arrived at the planet three decades later, the rotation period was measured as 8 min longer. It was then understood that Saturn’s rotation period could not be inferred from the fluctuations in radio radiation measurements linked to the magnetic field of the planet, and was in fact still unknown.”
Cassini had measured a signal linked to Saturn’s magnetic field with a periodicity of 10 hours, 47 minutes and 6 seconds long – slower than previous recordings.
“Since then, there has been this big open question concerning Saturn’s rotation period. In the last few years, there have been different theoretical attempts to pin down an answer. 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,” Dr Helled said.
The new method is based on a statistical optimization method that involved several solutions. First, the solutions had to reproduce Saturn’s observed properties: its mass and gravitational field. Then the astronomers 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.
“We cannot fully understand Saturn’s internal structure without an accurate determination of its rotation period,” Dr Helled said.
Knowledge of Saturn’s composition provides information on the formation of gas giants in general and on the physical and chemical properties of the protosolar nebula.
Dr Helled and his colleagues hope to apply their new method to other Solar System’s gas giants such as Uranus and Neptune. The method could also be applied in the future to study extrasolar gaseous planets.
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Ravit Helled et al. Saturn’s fast spin determined from its gravitational field and oblateness. Nature, published online March 25, 2015; doi: 10.1038/nature14278
