Ganymede is not only the largest satellite of Jupiter and the biggest moon in the Solar System, but it is the only moon with its own magnetic field and is also tidally locked, meaning it always shows Jupiter the same face just like our Moon does to us. On the far side of Ganymede, there is evidence of a colossal crater – evidence of an impact that changed the distant world forever. It all started about 4 billion years ago. An asteroid – about 20 times larger than the space rock that killed the dinosaurs, with a radius of 150 kilometers (93 miles) – created a transient crater with a radius of a…