Young Surface of Pluto’s Sputnik Planitia Caused by Viscous Relaxation

Abstract

One of the most prominent features of Pluto observed by the New Horizon mission is the absence of craters on Sputnik Planitia (SP). Vigorous thermal convection could renew the SP surface with sufficient depth at a timescale of ∼500,000 years. Here we present numerical simulations demonstrating that craters can be removed much more quickly across all of SP by viscous relaxation of nitrogen (N 2 ) ice. The timescale of relaxation is in years if the N 2 layer is 4 km thick and the viscosity is as determined in the lab, and will increase to 10 4 years if the viscosity is 10 4 times larger than the measured value. For such high viscosity, the thermal convection will have a timescale of greater than 10 6 years if it happens at all, so that the relaxation timescale is still more than 2 orders of magnitude shorter. The relaxation timescale decreases with increasing thickness and temperature of the ice layer. The existence of pits on SP can be explained by the surface enhancement of viscosity. Such enhancement does not have significant influence on the relaxation timescale of craters with diameters greater than a few kilometers. Therefore, although convection is required to explain the polygon shapes, it may have a lesser role in the absence of craters on SP. The viscous relaxation mechanism can readily explain the nondetection of both craters and polygon shapes on the southeast SP.