Final answer:
Callisto is not locked in orbital resonance and has not undergone full differentiation, unlike other Galilean moons such as Ganymede. It lacks a dense core and has not separated into layers of different density materials, remaining a primitive and ancient moon in the Jovian system.
Step-by-step explanation:
Callisto is the only Galilean moon that is not locked in orbital resonance and has never been . Callisto's distance from Jupiter is about 2 million kilometers, and it orbits the planet in 17 days. Unlike its neighboring moons like Ganymede, which has a differentiated internal structure with a core, mantle, and crust, and Europa and Io, which are predominantly rocky, Callisto remains an undifferentiated body with mixed material throughout. The Galileo spacecraft observed that Callisto has not separated into layers of different density materials and lacks a dense core.
This was unexpected because icy bodies are thought to differentiate more easily due to the lower melting temperature of ice. While the nearby Ganymede shows signs of geological activity and a magnetic field indicative of a partially molten interior, Callisto has frozen solid before any significant internal structuring took place, remaining a primitive and ancient world in the Jovian system.