* Brightness is a combination of size and reflectivity.

* Io is the most volcanically active body in the solar system. This activity constantly reshapes its surface, creating fresh, relatively dark material.

* Io is the innermost of the four Galilean moons. This means it's closer to Jupiter, so its apparent size in the sky would be larger.

While Ganymede is the largest moon in the solar system, and Europa is very reflective (due to its icy surface), their distances from Jupiter make them appear dimmer than Io. Callisto, the outermost of the four, is the least reflective and also the farthest, making it the dimmest of the bunch from Jupiter's surface.

Let me know if you'd like to explore more about the moons of Jupiter!