Take what you know about the solar system and applt it to the formation of the galilean moons of jupiter.?

Jupiter doesnt produce heat like star, so comparison is not so easy. Fact of how those moons are constructed position them on orbit. Their proportion of overall m with size and speed on orbit settle them to the position of planet. Its seems you noticed that Io is the most dense and Castillo is the least. Ice is even a less dense form of water. Unfortunatelly measuring m of planets and other giants nowadays is taken more by guess then real estimation. Scientists cant drill planets cores so they depend more on what they know about Earth and substances. Here is reason why gravitation is not yet fully discovered. I believe. You ask good question about traces of its secrets.