NASA's Cassini mission has collected a treasure trove of data about Saturn. Among these, three-dimensional maps of the planet's atmosphere have allowed researchers to apply a novel approach to determining the still uncertain period of Saturn's rotation. Peter Read and his colleagues set this at 10 hours, 34 minutes and 13 seconds (see page 608).

Rotation rates of planets that have a solid surface, such as Earth and Mars, can be easily determined by tracking landforms. But for the 'gas giants' like Jupiter and Saturn, such measurements are trickier. Saturn's outer layer, for example, is primarily made up of hydrogen gas. Deeper down, the gas becomes a hot liquid. “The deep interior of the planet probably does rotate almost like a solid body, even though it's actually made of a fluid,” says Read, a physicist at the University of Oxford, UK.

But how do you measure the rotation of this hidden interior? Jupiter's rotation can be easily established by tracking the relative rotation of the planet's two north poles — magnetic north and geographic north. But Saturn's magnetic north is too close to its geographic north to be used as a marker, so scientists have had to work with less direct tracers of magnetic fields. However, rotation rates calculated using such methods were found to change drastically over time. “That tells us that the measurement is not directly related to the rotation of the deep interior,” says Read.

So Read started work on a different approach. He applied fundamental principles of fluid dynamics to an analysis of the wind and thermal data that the Cassini team was collecting, even before the spacecraft began orbiting Saturn in 2004. He then teamed up with Timothy Dowling, a physicist at the University of Louisville in Kentucky, who had been working for more than a decade on the idea that a planet's atmospheric circulation could be determined by tracking cloud movements. With the three-dimensional data from Cassini, the researchers could better describe waves in the upper level of the atmosphere and relate that information to what is happening deep inside the planet. This is because slow-moving waves penetrate far into the planet's interior, and may be affected by its internal rotation. Using this method, says Read, “We end up with a rotation period for Saturn that differs by more than 5 minutes from that which our colleagues in the magnetic fields teams calculated and have been trying to verify.”

The rotation rate agrees with that previously determined by the third member of the team, geophysicist Gerald Schubert of the University of California, Los Angeles. He had tried to deduce Saturn's interior rotation rate by a completely different method — using gravity fields and the shape of constant pressure surfaces in the atmosphere — which gave Saturn a day length of 10 hours, 32 minutes and 35 seconds (J. D. Anderson & G. Schubert Science 317, 1384–1387; 2007). “That was greeted with some interest, but quite a bit of scepticism, because it was so different,” Read says.

Given that something is amiss with the rotation rate calculated from magnetic-field data, independent derivation of similar rates by two very different methods makes for a strong case, although it awaits further confirmation. Read says that this change in bulk rotation period has significant implications for models of the deep interior structure and evolution of Saturn.