A regular period for Saturn's magnetic field that may track its internal rotation

Abstract

The rotation rate of a planet is one of its fundamental properties. Saturn's rotation, however, is difficult to determine because there is no solid surface from which to time it, and the alternative `clock'-the magnetic field-is nearly symmetrically aligned with the rotation axis. Radio emissions, thought to provide a proxy measure of the rotation of the magnetic field, have yielded estimates of the rotation period between 10h 39min 22s and 10h 45min 45s (refs 8-10). Because the period determined from radio measurements exhibits large time variations, even on timescales of months, it has been uncertain whether the radio-emission periodicity coincides with the inner rotation rate of the planet. Here we report magnetic field measurements that revealed a time-stationary magnetic signal with a period of 10h 47min 6s +/- 40s. The signal appears to be stable in period, amplitude and phase over 14months of observations, pointing to a close connection with the conductive region inside the planet, although its interpretation as the `true' inner rotation period is still uncertain.