What geological characteristic makes Callisto's surface arguably the oldest in the entire Solar System?

Answer

It shows no evidence of endogenic processes like plate tectonics or volcanism.

Callisto lacks evidence of endogenic processes such as plate tectonics or active volcanism, meaning its evolution has been almost entirely dominated by impacts, resulting in a surface close to saturation cratering.

What geological characteristic makes Callisto's surface arguably the oldest in the entire Solar System?
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