Impacts of thrusting, extensional faulting, and glaciation on cratering records of Pluto’s largest moon Charon: Implications for the evolution of Kuiper belt objects

Abstract

A first-order question in the studies of the Solar System is how its outer zone known as the Kuiper belt was created and evolved. Two end-member models, involving coagulation vs. streaming instability, make different predictions—testable by the cratering history of Kuiper Belt Objects (KBOs)—about the cumulative size-frequency distribution (SFD) of the KBOs. Among all of the imaged KBOs, Pluto’s largest icy moon, Charon, appears to preserve the largest size range of seemingly undisturbed craters, their diameters (<italic>D</italic>) on Charon ranging from &lt; 1 km to &gt; 220 km. Current work shows that Charon’s craters with <italic>D</italic> &lt; 10−20 km are fewer than those expected by the coagulation mechanism, but whether this is an artifact of post-cratering modification of smaller craters is unknown. We address this issue by conducting systematic photogeological mapping and performing detailed landform analysis using the highest resolution images obtained by the New Horizons spacecraft, which reveal a range of differentiable terrains on Charon. The most important findings of our work include (1) truncation and obliteration of large craters (diameters &gt; 30−40 km) and their crater rim ridges along the eastern edges of several north-trending, eastward-convex, arcuate ranges in Oz Terra of the northern encountered hemisphere, (2) lobate ridges, lobate knob trains, and lobate aprons resembling glacial moraine landforms on Earth, (3) dendritic channel systems containing hanging valleys, and (4) locally striated surfaces defined by parallel ridges, knob trains, and grooves that are &gt; 40−50 km in length. The above observations and the topographic dichotomy of Charon’s encountered hemisphere can be explained by a landscape-evolution model that involves (i) a giant impact that created the Vulcan Planitia basin and the extensional fault zone along its northern rim, (ii) a transient atmosphere capable of driving N₂-ice glacial erosion of the water-ice bedrock and transporting water-ice debris to sedimentary basins, (iii) regional glacial erosion and transport of earlier emplaced impact ejecta deposits from the highlands of Oz Terra into the lowland basin of Vulcan Planitia, (iv) syn-glaciation north-trending thrusting, interpreted to have been induced by Charon’s despinning, and (v) the development of a water-ice debris cover layer over subsurface N₂ ice below Vulcan Planitia during global deglaciation. The infilling of the Vulcan Planitia could have been accompanied by cryovolcanism. The extensive modification of impact craters means that the size-frequency distributions of Charon’s craters should serve only as a lower bound when used to test formation mechanisms proposed for Kuiper belt objects.