Evolution of Pluto's Impact-Deformed Ice Shell Below Sputnik Planitia Basin

Abstract

Sputnik Planitia basin, the dominant surface feature of the dwarf planet Pluto, is located very close to the far point of Pluto-Charon tidal axis. This position is currently believed to be a result of whole body reorientation driven by the combination of (a) the uplift of a subsurface ocean in response to a basin-forming impact and (b) the nitrogen layer accumulated inside the basin. Since an ice shell made of pure water ice cannot maintain the uplift on timescales of billions of years, the presence of an insulating and highly viscous layer of methane clathrates at the base of the shell has recently been proposed. In this study, we solve the thermo-mechanical evolution of the ice shell in a 2D spherical axisymmetric geometry and evaluate the gravity anomaly associated with the evolving ice shell shape. Taking into account the effect of impact heating and stress-dependent rheology of both ice and clathrates, we show that a thick shell (>= 200 km) loses the impact heat slowly which leads to fast uplift relaxation of the order of hundreds of million years. On the contrary, a thin shell (similar to 100 km) cools down quickly (similar to 10 Myr), becoming rigid and more likely to preserve the ocean/shell interface uplift till the present. These results suggest that a thick ocean may be present beneath Pluto's ice shell.