Europa: September 7, 1996. Europa is the smallest of Jupiter’s moons. The bright crater on the bottom right is named Pwyll, after Pwyll, Prince of Dyfed, a mythical Welsh king whose tale is told in the Mabinogion.

One day, while out hunting in the forest, Pwyll comes across a pack of hounds feasting on a slain stag. He drives them away and allows his own hounds to feed, earning the anger of the hounds’ owner, Arawn, lord of the mystical kingdom of Annwn—a paradisiacal realm that, with the influence of Christianity, is identified with the afterlife. To make amends, Pwyll assents to trading places with Arawn for a year and a day, taking on his likeness so no one will know the difference. At the end of the year, Pwyll (still in the guise of Arawn) fights Hafgan, Arawn’s rival, mortally wounding him and thus earning Arawn possession of all Annwn. During the year, Pwyll has also slept chastely with Arawn’s wife, so when the two meet again to trade back appearances and places, they become lasting friends.