Little Enceladus disturbs Saturn’s magnetic field

Like a mouse reining in an elephant, Saturn’s tiny moon Enceladus is acting as a brake on the giant planet’s magnetic field, researchers assert in the April 20 Science. Don Gurnett of the University of Iowa in Iowa City and his colleagues trace the effect to water vapor and ice particles shot from geysers at Enceladus’ south pole.

Bombarded by radiation, the ejected material acquires an electric charge and is captured by the magnetic field girdling Saturn. The particles’ total mass exerts a drag on the field, causing its rotation to lag slightly behind that of the planet. Bursts of radio waves emitted once by the magnetic field during each rotation show the same slowdown, detectors on the Cassini spacecraft have revealed.

The drag induced by little Enceladus is not only surprising but also poses a challenge for planetary scientists trying to discern Saturn’s rate of rotation, says Gurnett. Saturn’s high-altitude clouds prevent direct observation of the planet’s spin, but scientists had assumed that the radio wave bursts were constant and would indicate the planet’s rotational speed. The drift in timing, however, shows that the signals don’t trace the planet’s unvarying rotation.