Charon is Pluto’s largest moon — half the diameter of Pluto and about 12% of its mass. The Pluto-Charon system is the only known true binary planetary system where the barycenter lies outside the primary’s surface. Charon’s surface, imaged in detail by New Horizons in 2015, displays a dark reddish north-polar cap (Mordor Macula) likely produced by methane ice migrating from Pluto and reacting under UV radiation to form tholins. A vast equatorial canyon system, Argo Chasma, may be longer than the Grand Canyon.