Science 321, 1475–1478 (2008)

Credit: NASA-JPL

Before 13 June 2007, astronomers observed Saturn's moon Titan (pictured) only when it was inside the planet's magnetic field. But on that day the probe Cassini observed Titan outside the controlling influence of Saturn's magnetosphere, in a region permeated by the Sun's magnetic field.

Cesar Bertucci, now at the Institute for Astronomy and Space Physics in Buenos Aires, and his colleagues report that Titan, which lacks a major magnetic field of its own, retains an imprint of Saturn's magnetic fields after leaving the planet's magnetosphere. This occurs in the form of 'fossil' fields frozen into the plasma surrounding the moon. The solar magnetic field then overlaid these fossil fields, which persisted for between 20 minutes and three hours, they report.