Each of the satellites in the solar system are pummeled by...

Each of the satellites in the solar system are pummeled by interplanetary dust particles, or meteoroids which are marble-sized or millimeter-sized objects, ... When those hit a satellite, or moon, at high speeds, they slam into the surface and blast a crater, and all sorts of particles fly off. That's what is happening at Ganymede, and we think that's what's happening at all the satellites in the solar system.