Images from NASA's New Horizons mission indicate Pluto's moon Charon has a 430 mile long canyon named Argo Chasma (the section shown above measures 185 miles). This canyon is also estimated to be about 5.5 miles deep.
Earth's Grand Canyon is only 280 miles long and about 1 mile deep by comparison, representing a much shorter and shallower scar on a much larger world. However, the Valles Marineris is 2,500 miles long and 4.3 miles deep, so Mars still represents a much longer scar. And the longest canyons in the solar system are found in Earth's rift valleys. But none of this takes away from Charon's significant scar given its small size.