Final answer:
Callisto is not fully differentiated; it lacks a dense core and has remained geologically inactive, with much of its interior being icy and a surface covered in impact craters.
Step-by-step explanation:
When astronomers refer to Callisto as a differentiated body, they are speaking to its internal structural division into layers of different density materials. Specifically, a differentiated body would have a heavy core surrounded by a lighter, icy mantle and crust. However, in the case of Callisto, it has not fully differentiated, meaning that it hasn't separated into distinct layers with a dense core.
Measurements of Callisto's gravitational field, such as those by the Galileo spacecraft, indicate that it lacks a dense core, suggesting an absence of full differentiation. This was unexpected, as it is generally thought to be easier for an icy body to differentiate than a rocky one due to the low melting temperature of ice, where even minimal heating can start the process of allowing heavier materials like rock and metal to sink and lighter icy materials to rise to the surface. Despite this, Callisto seems to have frozen solid before this process could be completed, leading to limited geological activity and maintaining its icy composition throughout much of its interior.
Callisto's surface is densely covered with impact craters and shows no signs of geological changes driven by internal forces, unlike its neighbor Ganymede, which is a fully differentiated world with a magnetic field and evidence of geological activity. This suggests that Callisto has remained geologically 'dead' for over 4 billion years. Its low density compared to the rocky and metallic materials found in the inner planets reinforces the idea that it is an icy body through and through.