The puzzle of the variable radio period of Saturn

Measurements of the rotation rate of giant planets using visible wavelengths are relatively inaccurate because they only give the combination of the internal rotation and the wind speeds, all unknown a priori. As planetary auroral radio emissions are produced by electrons moving along planetary magnetic field lines, they should be tied . via the magnetic field - to the planetary interior, and this is why they are used to measure the internal rotation of giant planets. For Jupiter, one obtains thus a stable period with an accuracy better than one part in one million. In the case of Saturn, Voyager radio measurements provided a period of 10h 39m 24s ±7s, adopted as Saturn.s rotation period. But subsequent observations by Ulysses and Cassini showed that this period actually fluctuates by ±6 min (thus ±1%) at a timescale of a few months to years. Similar variations seem to exist for the magnetic field measured in-situ in Saturn.s magnetosphere.

The origin of this huge variation (which would represent ±15 min. if compared to the duration of the Terrestrial day) obviously cannot be due to a change of the internal planetary rotation : contrary to an ice-skater, Saturn has no arms to fold in order to change its rotation rate while keeping its angular momentum constant, nor has it any efficient and rapid source or sink of angular momentum. These period variations are one of the main enigmas studied by the Cassini scientific community. Its solution is the key of the determination of atmospheric wind speeds, would constrain Saturn.s internal structure and shape (polar flattening), and is required to be able to define a longitude system on Saturn allowing to organize the observations over long time intervals.

The astronomers have concentrated their search on short-term period variations (a few days), in order to look for correlations with other phenomena more easily than for slow variations over several months. For that purpose, they have developed a method allowing us to measure these short-term variations with an accuracy better than 1% on a timescale of about 8 days. We have obtained the following two results [ref. 1] : (i) Saturn.s radio period varies at a timescale of 20-30 days, with an amplitude larger than that of long-term variations (which could therefore simply be the residual of the time averaging of short-term ones) ; (ii) these variations at 20-30 days are correlated with those of the solar wind speed around Saturn (especially the speed, and not the solar wind pressure or density). This proves that the origin of the variations of Saturn.s « radio clock » is . at least in part . external to the Saturn system, and that the solar wind speed is the key parameter of this external control.

One of the theories previously proposed for explaining the long-term variations of the radio period already suggested such an external control and a specific role for the solar wind speed [ref. 2]. This theory, which also applies to short-term variations, is strongly supported by the above recent results. It offers a frame in which we should eventually be able to subtract solar-wind induced variations in order to obtain the true internal rotation period of Saturn (if such one period does exist), thanks to the goniometric (i.e. « radio astrometry ») capabilities of the radioastronomy experiment (RPWS) onboard Cassini spacecraft.

[1] P. Zarka, L. Lamy, B. Cecconi, R. Prangé & H. O. Rucker, Modulation of Saturn’s radio clock by solar wind speed, Nature, 8 Nov. 2007. [2] B. Cecconi & P. Zarka, Model of a variable radio period for Saturn,, J. Geophys. Res. 110, A12203, 2005.

Reference

• Modulation of Saturn’s radio clock by solar wind speed Philippe Zarka(1), Laurent Lamy(1), Baptiste Cecconi(1), Renée Prangé(1) & Helmut O. Rucker(2) (1) LESIA, Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, Centre National de la Recherche Scientifique, Université Pierre et Marie Curie, Université Paris Diderot, 92190 Meudon, France (2)Space Research Institute, Austrian Academy of Sciences, A-8042 Graz, Austria Nature, 8 Novembre 2007