Saturn's nominal rotation period is timed by a 'radio clock' that counts bursts of emissions controlled by the planet's magnetic field. Buffeting by the solar wind may explain the clock's irregularities.
Einstein showed us that measurements of time made in systems moving at different speeds will not agree. What would he have thought of a clock whose ticking depends on how fast the solar wind — a gas of charged particles, or plasma, constantly flowing outwards from the Sun — is blowing? Yet that is what, on page 265 of this issue, Zarka and colleagues1 tell us is the case for the radio-emission clock that has been used to infer Saturn's rotation period.
Time-keeping on planets is linked to their orbital periods ('years') and their rotation periods ('days'). Strangely, it is not straightforward to determine the rotation periods of the gas-giant planets: Jupiter, Saturn, Neptune and Uranus. These planets lack solid surfaces with features to track as the planet rotates. Images of the gas giants allow one to track clouds, but their motions are not precisely tied to the rotation of the interior. How, then, do we establish rotation rates?
A particularly fruitful approach has been to monitor the intensity of radio-frequency emissions from sources close to the planet. Such emissions arise in a planet's magnetosphere, the region of space dominated by its magnetic field (Fig. 1), and their intensity depends on the angle between the observer and the magnetic field at the source. If the planet's internally generated magnetic field is asymmetric about its spin axis, the direction of the field at the source will seem to nod up and down as the planet spins, and the intensity of the observed emissions will vary with the rotation period. The planetary rotation rates of Jupiter2, Neptune and Uranus have been identified in this way.
Saturn emits radio signals modulated at a period of about 10.75 hours. This period has been used to define the period of rotation of the interior3, but it has proved hard to understand why the radio power varies periodically because the best available measurements fail to detect any asymmetry of the internal magnetic field4. Possibly even more puzzling is the recognition that the period of the modulation is not fixed. The first hints of that, initially greeted with some scepticism, came from measurements made by the Ulysses spacecraft5. More recently, observations by the Cassini orbiter have confirmed that the period changes by as much as 1% or so in months to years6.
So, today, we know that the radio power is periodically modulated but we do not understand why. And we know that the radio period drifts too rapidly to be consistent with changes in the rotation period of the deep interior of a massive spinning planet. Other types of analysis give an estimate of the rotation rate of the deep interior that is distinctly shorter than the shortest period inferred from the radio clock7. Thus, it seems likely that the radio clock responds to processes in the planet's upper atmosphere and magnetosphere. Explanations of the varying period of the radio clock have appealed to changing conditions that are either external to Saturn's magnetosphere (such as the speed of the solar wind8) or internal to it (such as the mass injected from the vapour plume of Saturn's small moon, Enceladus9). But evidence that such effects cause the observed drift in the period has been sketchy.
Zarka et al.1 use roughly three years of Cassini radio-wave data to provide compelling support for the hypothesis that external effects contribute to the modulation of the radio period. They find that the total power within a defined range of radio frequencies integrated over a full Saturn rotation period of 10.75 hours fluctuates on timescales of about 20–30 days. The properties of the solar wind are known to fluctuate at the solar rotation period of 25 days, and also to show trends over longer timescales. Zarka et al. find that cross-correlations with the speed of the solar wind are high, especially when Cassini's colatitude (the difference between its latitude and 90°) remains relatively constant, relative to Saturn's spin axis. The correlations with other properties of the solar wind (such as dynamic pressure) are weak.
Given evidence that the source of the radio emissions seems to be localized in the morning to noon sector8, it was previously proposed9 that changes in the period of the radio clock would occur if the source location shifts with changing solar-wind velocity. Such shifts could arise (and vary systematically with solar-wind velocity) if the emissions are triggered where the magnetospheric boundary becomes unstable through the growth of a phenomenon known as Kelvin–Helmholtz waves (the equivalent for magnetized plasma of wave-breaking when high winds blow over water). This interpretation remains speculative: the new results do not establish a mechanism for the changing periodicity. But the knowledge that the radio period is modulated by the speed of the solar wind should help in the quest for a more complete understanding.
Many planetary scientists expected Saturn's magnetosphere to be a bloated but rather boring analogue of Earth's. The data being collected by Cassini continue to belie this expectation. Ten years after its launch from Earth, the mission continues its fruitful exploration of a planetary system that is dramatically different from any previously investigated.