Two-Timing Saturn

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information below was derived from the following publication: “Discovery of a north-south asymmetry in Saturn’s radio rotation period,” Geophysical Review Letters (Aug. 20, 2009), D.A. Gurnett (University of Iowa); A. Lecacheux (Observatory of Paris); W.S. Kurth, A.M. Persoon, J.B. Groene (University of Iowa); L. Lamy (Imperial College of London); P. Zarka, Observatory of Paris); J.F. Carbary (Applied Physics Laboratory, Johns Hopkins University, Baltimore, Md.).

Saturn’s spin-rate – the length of its day – has long eluded precise measurement because the distinctive radio burst emitted with each rotation has varied by seconds to minutes almost every time it was clocked. Now, to confound scientists further, it appears that Saturn’s northern and southern hemispheres have different rates of rotation, with not one, but two distinctly different radio emissions coming from each of the polar auroral zones. Asking the length of Saturn’s day seems like a simple question. But the answer still eludes scientists as they try to pinpoint Saturn’s rotation, or at least explain why it appears to vary.

It has long been known that with each rotation, Saturn emits intense radio energy with kilometer-long wavelengths. Early on, scientists looked to this Saturn kilometric radiation, or SKR, to set the time of a Saturnian day, but almost every time it has been measured since 2000, SKR occurs at a slightly different time.

The Cassini spacecraft’s growing number of orbits near Saturn’s northern and southern poles has given scientists a closer listen to SKR, producing new clues to the source of this perplexing phenomenon. Recently, scientists on the Radio and Plasma Wave Science team found that SKR is not a solo, but a duet, and its singers were inexplicably out of synch.

The scientists decided to test a long-shot idea that this asymmetry could be due to different rates of rotation between the northern and southern hemispheres “even though we thought such an asymmetry was unlikely,” they write. Their thoroughness was instantly rewarded, however.

Two spectrograms were produced, one for the northern hemisphere and the other for the southern hemisphere. To the team’s surprise, the data showed clearly that the first component of the radio burst originates from the southern hemisphere, and the second component originates from the northern hemisphere. The southern hemisphere’s longer- period component has a rotation rate of about 10.8 hours and the shorter period component, with a rotation rate of about 10.6 hours, originates in the northern hemisphere. In Cassini data from multiple near-polar orbits from January 2008 to April 2009, the asymmetry is particularly clear. “What is surprising is that the plasma feature responsible for the radio emission could rotate at different rates in the two hemispheres.

“Just what causes the SKR to blink on and off with near clock-like precision is not known precisely but is almost certainly related to the rotation of plasma and magnetic fields in Saturn’s magnetosphere,” the authors write. “The usual assumption is that the plasma is frozen to the magnetic field lines; if the northern and southern auroral zones are on the same closed magnetic field lines, then any rotational effect transmitted to the two auroral plasmas from the rotating equatorial plasma should be locked together at the same rotation rate.” But, apparently, the connection between these plasmas and field lines slipped enough to let one hemisphere get slightly behind the other.

The reason may lie in the fact that the Saturnian system, despite looks to the contrary, is not completely symmetric. Due to the tilt of Saturn’s rotational axis relative to its orbital plane, there is a north-south asymmetry caused by the difference in the solar illumination of the two hemispheres. During the first five years of Cassini’s tour (2004 to the present) the south polar region has been in more sunlight than the north. This difference increases the atmospheric conductivity of the southern polar region causing differences in the magnetic field and plasma environment of that hemisphere relative to the northern hemisphere.

One intriguing prediction could be borne out in the near future. If the timing difference is related to different amounts of sunlight falling on each hemisphere, then the two SKR components should get closer to singing at the same tempo when Saturn’s equinox occurs Aug 11, 2009, and sunlight falls evenly on the two hemispheres. The latest data, the authors say, show a tendency of the rotation rates to approach each other.