On the cover

The Jovian moon Ganymede is the only moon of the hundreds in the solar system that has liquid metal flowing in its core. The resulting dynamo generates an observable magnetic field. A new model of the moon’s slow-forming core shows that Ganymede’s dynamo may have developed at a lower temperature than previously thought. The model also offers clues as to why moons similar to Ganymede, including neighboring Callisto and Europa, lack dynamos. (Enhanced image from NASA/JPL-Caltech/SwRI/MSSS/Kalleheikki Kannisto © CC BY.)