When the Cassini spacecraft visited Saturn, its measurements of the gas giant鈥檚 magnetic field showed that we may have been wrong about how fast the planet is spinning 鈥� it was 7 minutes slower than the last time we measured the magnetic field鈥檚 rotation with the Voyager probes.
But Cassini also gave us a deluge of images of Saturn鈥檚 rings, and buried within those pictures were clues to the giant planet鈥檚 true rotation rate.
鈥淚t turns out that Saturn鈥檚 rings are an extraordinarily sensitive way of detecting disturbances inside the planet,鈥� says David Stevenson of the California Institute of Technology in Pasadena, California.
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Reading the rings
In the early 1980s, the twin Voyager probes flew past Saturn, giving astronomers their first close-up shots of the giant planet鈥檚 ring system and some unexpected features that resembled record grooves.
Researchers quickly realised that Saturn鈥檚 many moons were gravitationally tugging on rock and ice particles in the rings. When the moons and ring particles orbited at simple ratios of one another, the moons would give the particles periodic kicks 鈥� an effect known as orbital resonance.
鈥淲ith resonances you can launch waves that propagate through the rings, kind of like dropping a rock in a pond,鈥� says Christopher Mankovich at the University of California, Santa Cruz.
While the moon-generated undulations moved outward in the rings, away from the planet, a handful of anomalous ones were headed in the direction of Saturn itself. Researchers soon pointed out that earthquake-like events inside the gas giant could be moving huge amounts of mass around, tugging ring debris towards the planet and producing the 鈥榖ackwards鈥� waves. So, the ripples can reveal what鈥檚 going on deep in the planet.
Inner swirl
The Voyager missions also recorded data on Saturn鈥檚 magnetic field, finding that it was whipping around on its axis approximately every 10 hours and 40 minutes, a figure that was taken as the rotation rate of the entire planet itself. But when the Cassini spacecraft arrived at Saturn in the mid-2000s, it measured the magnetic field鈥檚 rotation rate as greater than 10 hours and 47 minutes. It was impossible that massive Saturn had slowed down that much in a few short decades.
Cassini catalogued many more inward-moving waves than the Voyager probes. Using data from Saturn鈥檚 second innermost C ring, Mankovich and his colleagues created computer models that simulated Saturn鈥檚 internal structure and could account for the patterns and positions of the inward-propagating perturbations.
The models suggested that Saturn is rotating about every 10 hours and 35 minutes, a finding that agrees fairly closely with other recent estimates that use measurements of the planet鈥檚 gravitational field from Cassini. Mankovich says that having two new techniques to attack the issue is helping astronomers build a more complete understanding of Saturn鈥檚 swirling and mysterious interior. For instance, the computer models also hint that different depths of Saturn鈥檚 interior may rotate at different rates.
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Read more: Saturn鈥檚 rings may be from the whirl of a passing icy rock
