Saturn’s rotation speed

Saturn, as captured by NASA's Cassini spacecraft in early October 2004.

DETERMINING the speed of rotation of solid planets, like the earth and Mars, is straightforward: measure the time it takes for a feature on the surface to return into view. But such methods cannot be used for giant gas planets like Saturn because they lack measurable solid surfaces and are covered by thick layers of clouds. Saturn has presented a greater challenge to scientists as different parts of its hot ball of hydrogen and helium rotate at different speeds, while its rotation axis and magnetic pole are aligned.

Three Israeli scientists, led by Ravit Helled of Tel Aviv University, have proposed a new method to determine the rotation speed of Saturn based on its gravity, oblateness and density profiles. The method also offers insights into the internal structure of the planet, its weather patterns, and the way it formed. The researchers made use of Saturn’s measured gravitational field and the unique fact that its east-west axis is shorter than its north-south axis. This work was recently published in Nature.

With their new method, the researchers determined Saturn’s day to be 10 hours, 32 minutes, and 44 seconds long. The currently accepted value of Saturn’s rotation period is based on Voyager 2’s measurements in 1980: 10 hours, 39 minutes, and 22 s. “But when the Cassini spacecraft arrived at Saturn 30 years later, the rotation period was measured as eight minutes longer. It was then understood that Saturn’s rotation period could not be inferred from the fluctuations in radio signal measurements linked to Saturn’s magnetic field, and was in fact still unknown…. 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,” said Helled. The new method is based on a statistical optimisation technique that involved several solutions. First, the solutions had to reproduce Saturn’s observed properties: its mass and gravitational field. Then, this information was used to match the rotation period on which 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,” said Helled. The better the planet’s gravitational field is known, the narrower the error margin. The researchers hope to apply their method to other gaseous planets in the solar system such as Uranus and Neptune. Their new technique could also be applied in the future to study gaseous planets orbiting other stars.