Asked by Jon Gray
An impact basin known as Valhalla is Callisto’s largest crater with a diameter of over 300 kilometres (190 miles), while concentric rings that surround it extend out to 3,000 kilometres (1,900 miles) – nearly large enough for the entire USA to fit within its boundaries. Made of rock and ice, this moon of Jupiter is one of the most cratered worlds in the Solar System. In fact, Callisto’s surface is so battered that any new impacts on its surface will likely erase older craters.
Just how Callisto got its battered surface is a tale that’s also true of other bodies in our Solar System, such as our Moon. It’s suggested that Callisto got its distressed surface with the help of a period of heavy bombardment from asteroids and comets, around 4 to 3.8 billion years ago.
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