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Shorter days on Saturn
Posted: October 07, 2009

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Saturn has been found to be spinning slightly faster than previously believed, changing what we know about the planet’s mysterious interior.

As bizarre as it sounds, the period of Saturn’s rotation, i.e. the exact length of its day, has been something of a puzzle. Measuring the length of time a gas giant planet takes to make one rotation isn’t as simple as for a rocky planet like Mars where the surface stays still. Features in the atmospheres of gaseous planets are constantly moving and swirling, making it impossible to pin any of them down and measure their exact rotation. However, astronomer’s can use a trick: a planet’s magnetic field rotates along with the rest of the planet, and by measuring the rotation of the magnetic field of a gas giant via its radio emissions, it is possible to infer the length of day. This has been successfully achieved for Jupiter, Uranus and Neptune, wherein each case the magnetic field is tilted with respect to the rotational axis of the planet, meaning we can see it swivel as it rotates.

Saturn's day has been measured at ten hours, 34 minutes and 13 seconds. Image: NASA/JPL/Space Science Institute.

Things are not as simple for Saturn. The ringed planet’s magnetic field is perfectly aligned with the rotational axis – we can’t see it swivel as it rotates, so rotation measurements have always been a bit fuzzy. In 1980 and 1981 the Voyager spacecraft measured Saturn’s day as ten hours, 39 minutes and 24 seconds. But when the Cassini spacecraft arrived in 2004, it measured the length of day as ten hours, 47 minutes and six seconds.

Planets don’t just slow down in such a short space of time, and so it was evident something was wrong with the measurements. Now Dr Tim Dowling of the University of Louisville, USA, and Professor Peter Read from the University of Oxford, have discovered a new and accurate way of measuring the length of Saturn’s day. They found standing Rossby waves in Saturn’s atmosphere that are ‘still’ relative to disturbances deep within Saturn (Dowling discovered similar waves in Jupiter’s atmosphere in 1989). Dowling likens the phenomenon to a swimmer heading upstream and just keeping up with the current.

“We don’t know the exact nature of such disturbances, but given that the interiors of the gas giants are convecting, it is likely that there are plenty of pressure anomalies in the interior that the atmosphere can latch on to,” says Dowling. “A good analogy is a pottery wheel – the craftsman can feel the subtle imperfections of his newly forming pot as it spins under his finger tips.”

Using these waves as a reference point that is standing still with respect to the rotation of Saturn, Dowling and Read measured the length of Saturn’s day as ten hours, 34 minutes and 13 seconds – almost five minutes shorter than previously believed. Amazingly, this five minutes affects what we understand about the interior structure of Saturn.

“We know Saturn is an oblate shape, and the fact that it is rotating faster with the same shape means that more of its mass must be concentrated in its centre than previously believed,” says Dowling. If the mass was more evenly distributed around Saturn then the planet would be made even more oblate by the same outwards centrifugal ‘force’ that you feel on a merry-go-round. This implies that Saturn has a larger rocky/metallic core, which would have aided its formation 4.6 billion years ago as it attracted gas with its gravity. Furthermore, because Saturn is spinning five minutes faster, it equates to a reduction in measurements of wind speed of 275 kilometres per hour – though as Saturn’s winds are well over 1,000 kilometres an hour, it is still blowing quite a gale!