While humans are still a long way away from going canyoneering on the moons of Pluto, we can at least start scoping out the terrain. In images shot last summer by NASA's New Horizons spacecraft, we can see one immense and interesting feature of Charon: a deep canyon dubbed the Argo Chasma that slices through one hemisphere of Pluto's largest satellite.
Thanks to the viewing angle New Horizons got on its closest approach to Charon last July, scientists were able to estimate the depth of the Argo Chasma. In sections, the canyon is believed to be about 5.5 miles (or 9 kilometers) deep with sheer cliff faces several miles tall. Without a complete picture of the canyon, scientists believe Argo is about 430 miles (700 km) long. For reference: Earth's Grand Canyon is about 280 miles (450 km) long and a mile deep. Argo's massive scale means it also beats the three-mile-high cliffs at Verona Rupes on Uranus' moon Miranda for the tallest (known) cliff face in the solar system.
Until extra-planetary rock climbing becomes a reality, however, Earthbound humans hoping to explore Argo will have to be content to don our cardboard VR headsets and gaze up at Charon from Pluto's virtual surface.