Differentiation and cryovolcanism on Charon: A view before and after New Horizons

Before the arrival of the New Horizons probe at the Pluto-Charon system, we developed a series of models that predicted that Kuiper Belt Objects, even as small and as cold as Charon, have experienced internal ice-rock differentiation and possibly cryovolcanism. Confronting these predictions is a wide array of spectroscopy, imagery, and other data from New Horizons. In this article we compare the predictions against the new observations, and find that they largely support the expected history of the Pluto system and the evolution of Charon. Following the collision of two partially differentiated impactors with radii ≈1000 km, a disk of material formed around Pluto, from which Charon and Pluto's other moons formed. Because the impactors did not completely differentiate, the disk contained rocky material from their crusts, explaining the moons’ different densities and compositions. Long-lived radionuclides in Charon, assisted by ammonia antifreeze in the ice, melted ice and created a subsurface ocean that eventually refroze ≈1.7−2.5 Gyr ago. The freezing of this ocean would have created extensional stresses that possibly created Serenity Chasma, and could have led to widespread resurfacing, explaining the apparently younger age of Vulcan Planum. Buildup of radiogenic heat then created a second, smaller ocean that refroze 0.5–1.7 Gyr ago. As it froze, cryovolcanism would have been enabled, possibly creating Kubrick Mons. Charon's “moated mountains” such as Kubrick Mons have a natural explanation as cryovolcanoes depressing a thin lithosphere over a cryomagma chamber. We offer further predictions about other aspects of Charon's surface. Our previous predictions that Charon is a world shaped by geological activity have been largely borne out by New Horizons observations.