Saturn’s rotation period from its atmospheric planetary-wave configuration

Abstract

Saturn's bulk rotation period is not expected to have changed appreciably over the past few decades, so a good surrogate of the rotation period should also have remained fairly constant. Recent results from the Cassini mission raise doubts over the conventional approach towards the measurement of the interior rotation of Saturn, the period of rotation of features in the magnetosphere, since they produce a period that is approximately 7 minutes longer than 10 h 39 m 24 s measured 28 years ago by Voyager. Read et al. report a new determination of Saturn's rotation period of 10 h 34 m 13 s ± 20 s based on an analysis of potential atmospheric vorticity. This shifted reference frame is consistent with an alternating jet pattern, suggesting that Saturn's winds are more like Jupiter's than was previously thought. The rotation period of a gas giant's magnetic field (the System III reference frame) is commonly used to infer its bulk rotation, but this approach cannot be used for Saturn because its dipole magnetic field is not tilted relative to its rotation axis. Consequently, the surrogate measure of long-wavelength radiation is used to fix the System III rotation period. The period as recently measured by the Cassini spacecraft is up to 7 minutes longer than the value measured 28 years ago by Voyager. Here, a determination of Saturn's rotation period is reported, based on an analysis of potential vorticity. The rotation period of a gas giant's magnetic field (called the System III reference frame) is commonly used to infer its bulk rotation1. Saturn's dipole magnetic field is not tilted relative to its rotation axis (unlike Jupiter, Uranus and Neptune), so the surrogate measure of its long-wavelength (kilometric) radiation is currently used to fix the System III rotation period2. The period as measured now by the Cassini spacecraft is up to ∼7 min longer3 than the value of 10 h 39 min 24 s measured 28 years ago by Voyager2. Here we report a determination of Saturn's rotation period based on an analysis of potential vorticity. The resulting reference frame (which we call System IIIw) rotates with a period of 10 h 34 min 13 ± 20 s. This shifted reference frame is consistent with a pattern of alternating jets on Saturn that is more symmetrical between eastward and westward flow. This suggests that Saturn's winds are much more like those of Jupiter than hitherto believed4.