Why do permanently shadowed craters exist at the Moon’s poles?

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near and far sides of Earth's Moon — Moon (Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.

Permanently shadowed craters exist at the Moon’s poles primarily because the Moon’s spin axis is nearly perpendicular to the plane of the ecliptic—the plane of Earth’s orbit around the Sun—with an inclination of only about 1.5 degrees from the vertical.12

Because of this orientation, sunlight strikes the lunar poles at a nearly horizontal angle. Consequently, the bottoms of deep craters remain in perpetual shadow, creating cold traps where temperatures can drop as low as 40 K (−388 °F, −233 °C). These regions are significant because they have collected volatile substances, such as water ice, over geologic time.12