A new estimate of Saturn’s rotation rate reveals days on the gas giant are five minutes shorter than previously believed — and that Saturn’s atmosphere has much in common with that of its planetary neighbor, Jupiter.
The new results appear today in the journal Nature.
(Image caption: Saturn as photographed by Cassini-Huygens. Credit: NASA)
For planets with solid surfaces, the spin rate can simply be determined by tracking the motion of landforms as they rotate across the surface.
Like the rocky planets, gas giant planets such as Jupiter and Saturn spin on their axes with well defined rotation periods. But, with no solid surface features to track, measuring the rotation period of a gas giant is a challenge. The approach that has worked for Jupiter, Uranus and Neptune — using the rotation of the planet’s magnetic field to infer its bulk rotation — gives results for Saturn that change with time, and implies a pattern of atmospheric winds that is very different from that seen on Jupiter.
Peter Read, of the University of Oxford in the UK, and his colleagues used atmospheric dynamics on Saturn to derive a rotation rate that is slightly faster than those inferred from magnetic measurements. When Saturn’s atmospheric winds are viewed relative to this new interior reference frame, they show a pattern of alternating eastward and westward jets similar to the pattern seen on Jupiter.
“This shifted reference frame is consistent with a pattern of alternating jets on Saturn that is more symmetrical between eastward and westward flow,”Read and his co-authors write. “This suggests that Saturn’s winds are much more like those of Jupiter than hitherto believed.”
The authors propose a new rotation rate of 10 hours and 34 minutes, as opposed to the previous estimate of 10 hours 39 minutes. The new rate also sheds light on Saturn’s interior structure, including its density and the mass of a possible rocky core. And it bears on the latitudinal gradient of temperatures below the clouds.
In a related editorial, Adam Showman of the University of Arizona in Tucson writes that there remain key differences between the atmospheres of Saturn and Jupiter: “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 about its axis — a puzzle in its own right — contrasts with Jupiter’s tilted dipole,” he notes. “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.”
Second image caption: An image of Saturn from NASA’s Cassini spacecraft, clearly showing the ‘geographic’ South Pole of the planet (at the center of the circle of clouds, lower left). The bulk rotation of the planet is around an axis passing through the South Pole and Saturn’s clouds (of ammonia ice) are organized into dark ‘belts’ and light ‘zones’ that are generally aligned with lines of latitude, indicating the influence of the planet’s rotation on its meteorology.