When the New Horizons probe flew past Pluto’s largest moon last year, it gave scientists the best view of Charon they’ll have for decades. Rather than the sedentary world researchers had imagined, Charon has a rich geologic history including mountains, landslides and canyons — one stretches four times longer and up to twice as deep as the Grand Canyon. A dark region near its north pole, dubbed Mordor, might be the result of gases from Pluto’s atmosphere condensing on the moon’s surface.
Charon spans about half of Pluto’s diameter and has an eighth of the dwarf planet’s mass, resulting in a lopsided double orbit; Charon goes around Pluto, but the two also orbit a point in space known as a barycenter. Because of this, some astronomers consider both bodies dwarf planets. But the International Astronomical Union, which rules in such matters (it officially demoted Pluto to dwarf planet in 2006) considers Charon a moon. Pluto’s other four moons orbit the same barycenter.
Officially, the moon is named after the mythological ferryman, but it also honors discoverer James Christy’s wife, Charlene. This has led to some disagreement over its pronunciation, but many American scientists favor Christy’s choice: “SHAHR-on.”
[This article originally appeared in print as "Moons of Our Solar System."]