Here's why:
* Density: Io has a density of 3.53 g/cm³.
* Comparison: The densest moon in the solar system is Europa, another moon of Jupiter, with a density of 3.01 g/cm³. While Io's density is not significantly higher, it's still higher than other major moons like Ganymede and Callisto.
Important Note: Density is not the same as mass. While Io is denser than Europa, Europa is actually much more massive.
