Unusual Iapetus

In Poul Anderson's "The Saturn Game," less than a thousand years ago, a comet swung round Saturn and struck Iapetus, covering one hemisphere with ice, including the "City of Ice" glacier composed of the differently colored ices of diverse materials, water, ammonia, methane, carbon dioxide etc. Landing meteorites collect solar heat and melt their surroundings. The properties of the different ices produce not smooth craters but fantastic concavities. Anderson goes into more detail than I can summarize here.

The upshot is that this Saturnian moon becomes yet another of Poul Anderson's "Unusual Heavenly Bodies." See here.