Cassini captures an image as it approaches Saturn.
Credit: NASA
SYDNEY: Astronomers have struggled to determine the length of a day on Saturn because the gas giant has no fixed surface point to clock its rotation rate, but now they think they have a solution.
In a study reported today in the U.K. journal Nature, researchers used clues from the planet's weather to derive a new figure for its rotation rate, estimating Saturn's day at 10 hours, 34 minutes and 13 seconds.
The gas giants Jupiter, Saturn, Uranus and Neptune spin on their axes with well-defined rotation periods, but the lack of a solid surface has made determining the rotation rates of their interiors difficult.
Difficult to determine
"Since Saturn has no solid surface, but is believed to be fluid - mostly hydrogen and helium - throughout, its bulk rotation rate is quite difficult to define or determine," said study lead author Peter Read, a physicist at the University of Oxford, in England.
The rotation rates of the other gas giants are determined using their magnetic field but according to Read, Saturn's magnetic north is too close to its geographic north to use as a marker to assess rotation.
"In the past we were duped by the magnetic field measurements of Saturn. But these do not give a proper measure of Saturn's internal rotation since the magnetic field is slipping relative to the planet," said Andrew Prentice, a mathematician from the Centre for Stellar and Planetary Astrophysics at Monash University in Melbourne, Australia.
"If you rely on magnetic fields then you are in for a rude shock since the period seems to have lengthened by seven to eight minutes since the time of the Voyager missions," commented Prentice, who was not involved with the study.
Magnetic field observations
The Voyager missions, which flew by Saturn in the early 1980s, measured the period of rotation as 10 hours and 39 minutes. More recently the Cassini-Huygens mission found the rotation period to be 10 hours and 47 minutes. Both these measurements were based on observations of the magnetic field.
Instead, Read and his co-workers guessed that Saturn's rotation rate could be determined by considering the infrared data from NASA's Cassini probe along with measurements of jet streams, currents and vortices in the planet's atmosphere.
"We realised that we could combine information on what was visible on the surface of Saturn with Cassini's infrared data about the planet's deep interior and build a three-dimensional map of Saturn's winds," Read said.
"With this map, we were able to track how large waves and eddies develop in the atmosphere and from this come up with a new estimate for the underlying rotation of the planet."
The results suggest that the Voyager and Cassini measurements were out by more than five minutes.
"While five minutes might not sound much, if our alternative rotation period for Saturn is accepted as the correct one, this would imply a change to the profile of east-west winds on Saturn by more than 80 m/s at the equator," said Read, meaning that some measurements of wind speed could be out by as much as 250 km/h.
Subtle change in the shape of the planet
"It also means that the weather patterns on Saturn are much more like those we observe on Jupiter, suggesting that, despite their differences, these two giant planets have more in common than previously thought," he added.
The researchers predict this change in rotation rate will have significant implications for models of the deep interior structure and evolution of Saturn. "It implies a subtle change in the shape of the entire planet and its inner distribution of mass," said Read.
His team plans to extend their analysis to cover the full three-dimensional structure of the atmospheres of Jupiter and Saturn, including the many waves and eddies that can be seen in images of the surfaces.
"Cousins rather than twins"
"The revised rotation period gives Saturn a more Jupiter-like countenance than previously appreciated," commented Adam Showman of the University of Arizona in an accompanying commentary also published in Nature.
"Nevertheless, Saturn's winds are stronger than Jupiter's, its banded cloud patterns and populations of hurricane-like vortices differ considerably, and its magnetic field, which is almost symmetrical… contrasts with Jupiter's tilted dipole," he said.
"These contrasts indicate that the planets are cousins rather than twins, whose intriguing mix of similarities as well as differences will keep planetary scientists engaged for years to come."
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