The role of Pluto's ocean's salinity in supporting nitrogen ice loads within the Sputnik Planitia basin

Abstract

The existence of a subsurface water ocean on Pluto has been inferred on numerous grounds. The characteristics of such an ocean are vital inputs for various types of models that are carried out to illuminate the interior structure and evolution of Pluto, especially in light of the complementary relationship between the dimensions of the ocean and an outer water ice shell. In particular, models of loading of the Sputnik Planitia (SP) impact basin by nitrogen ice infill require knowledge of ocean properties such as density, and previous studies did not consider the potential densification of Pluto's ocean from salinity. Here we carry out analytic and Finite Element Method (FEM) modeling of loading deformations and stresses in an ice shell lithosphere, to determine the conditions, including ocean density, that best account for the observed distributions of tectonic features around the SP basin. Our results, while generally consistent with those of previous studies, provide more complete constraints on the initial basin depth (no deeper than 6 km, with most likely values in the 3–4 km range). Further, we find that increasing density (salinity) generally increases disagreement between model tectonic predictions and observations, and therefore we infer modest salinity-based densification of Pluto's ocean (ρocean < 1100 kg/m3) relative to values seen or inferred on other worlds.