The formation and evolution of Pluto's Sputnik basin prior to nitrogen ice fill

Abstract

We simulate Sputnik Planitia (SP), Pluto's largest impact basin, from formation through cooling and relaxation prior to loading of the basin with N2 ice. To assess potential conditions for Pluto's interior that permit the formation of an SP-like basin in terms of depth and diameter, we consider impacts into targets that possess a subsurface ocean with variable ice shell thicknesses and thermal structures. We use a shock physics code to model excavation and transient crater collapse. Then, using the final temperature and density structure from the shock physics code as initial conditions, we simulate the subsequent cooling and viscoelastic relaxation of basin topography using a finite element model (FEM). We show that a thin ice shell (on the order of 100 km) overlying a thick ocean (on the order of 228 km) produces an SP-like basin in terms of diameter and pre-N2 depth. A basin formed in a much thicker ice shell (200 km) overlying a thin ocean (128 km) can reproduce SP if the ice shell is relatively warm and possesses a strong rheology. Our results suggest that SP was not a mascon basin prior to subsequent loading by N2 ice, but rather close to isostatically compensated. We find that SP could have developed into a mascon basin following N2 loading if the ice shell is thin, conductive and possesses a lithosphere capable of supporting the N2 load.