COMPARATIVE GEOLOGY IN THE KUIPER BELT: PLUTO, CHARON, AND MU69 (Invited Presentation)

In the space of four years, NASA’s New Horizons spacecraft has opened our eyes to very different worlds within the heretofore-unexplored Kuiper Belt. Pluto is a resonant trans-Neptunian object and presents a highly varied and complex surface geology that displays evidence for having been affected by both endogenic and exogenic energy sources, and which remains active through the present day. The crucial factor explaining such activity is Pluto’s retention of a variety of volatile compounds, especially nitrogen and methane ice, which can be readily mobilized by the limited sources of energy, seen most spectacularly in Sputnik Planitia,the vast, convecting sheet of nitrogen ice. Pluto’s extreme obliquity and climate cycles are powerful drivers of the migration of volatiles around the globe within its tenuous atmosphere, and intrusion of volatiles into the subsurface is likely a necessary process to explain the formation of the tentative cryovolcanic edifices named Wright and Piccard Montes. Pluto’s moon Charon is one eighth the mass of Pluto, and while it also displays evidence for geological activity in its early history, with giant fracture belts straddling the encounter hemisphere and possible cryovolcanism in the form of smooth plains at the equator, it has not retained a substantial inventory of volatiles like Pluto has, and so has been geologically inert for the majority of its history.

The planetesimal 2014 MU69 is a cold classical Kuiper Belt object, and unlike Pluto and Charon, has occupied an extremely distant and isolated location within the solar system since its formation. A contact binary with a long dimension of only 35 km, its shape and surface geology indicate that it is the most pristinely preserved planetary body yet seen. MU69’s lightly cratered surface attests to the very low impact rates encountered in the outer solar system, and the bulbous morphology of the larger lobe may represent accretional subunits of smaller planetesimals that formed it prior to the two lobes merging at low velocity. Nevertheless, some evidence of post-formation modification is apparent, including relatively bright material accumulating in depressions; sub-km-sized pits that may originate due to sublimation, surface collapse, or impact; and linear troughs that may indicate extension within the body.