John Timmer, for ArsTechnica:

The discovery is a masterpiece of indirect detection, using Hubble Space Telescopes observations of the aurora that lights up above the moon, then using that data to work out its magnetic properties and in turn using those measurements to work out its interior structure.

Ganymede is a very large moon, bigger than the planet Mercury. It has a magnetic field, which indicates that the moon hosts a magnetic core. Like the other large moons of Jupiter, it is subjected to tidal forces from the giant planet and its other moons; these forces heat the interior of the moon sufficiently to melt enough of its iron core to generate a magnetic field.

To me, NASA’s method of discovery is nearly as interesting as the discovery itself. I can’t wait to meet the Ganymates.