The craters Alphonsus and Arzachel, part of the trio that includes Ptolemaeus to the north, are located near the center of the Moon's visible disk. Alphonsus, with a width of 118 kilometers, is estimated to be between 3.8 to 3.9 billion years old and has an unusually smooth floor with a small central peak. This suggests that it was partially buried by debris from the Moon's Late Heavy Bombardment era. In contrast, Arzachel, with a width of 98 kilometers, is estimated to be a billion years younger and was smoothed by volcanic flows. It also displays a strangely offset central peak.
Both Alphonsus and Arzachel are known as floor-fractured craters, meaning they have huge cracks on their smooth floors, called Rima Alphonsus and Rima Arzachel, created by ancient volcanism that pushed up from below each crater. There is further evidence of volcanism inside the eastern rim of Alphonsus, where four dark halos surround small craters, either on or near Rima Alphonsus. A fifth dark halo lies along the western wall, which are dark volcanic ash deposits that erupted onto the floor of Alphonsus over a billion years ago.
Arzachel holds a special place in the heart of many due to its connection with a science fiction book, which inspired the imagination of many young readers. The book, which has long been forgotten, features American and Russian astronauts who landed on the Moon and encountered difficulties that jeopardized their return to Earth. The crew of both nationalities learned that they had to cooperate to survive the harsh lunar environment and return safely to Earth. The action in the story took place within the Arzachel crater.
