Compensation State and Geophysical Evolution of Sputnik Basin on Pluto

Abstract

Sputnik basin is an ∼2,000 × 1,000 km elongated impact basin located in the equatorial region of Pluto. It contains a low-viscosity N2-rich ice deposit that has prompted comparisons between the Sputnik basin and mascon basins in the inner Solar System—some of which were initially isostatically compensated by the uplift of high-density mantle materials in the subsurface and evolved to a mass excess from the lithospheric support of infilling material. In the absence of gravitational data, novel approaches must be considered to examine the structure of the Sputnik basin. Here, we assume that the surface of the low-viscosity infill conforms to Pluto's geoid, and we use this constraint to evaluate the local gravity field over the basin considering a range of ice shell and N2-deposit thicknesses. Our results show that an isostatically compensated pre-fill Sputnik basin has a strongly negative free-air gravity anomaly due to the attenuation of the gravity signature through Pluto's relatively thick ice shell compared to the mean radius. The best fit models to the Sputnik basin reflect a present-day mass deficit and a largely uncompensated basin, which is at odds with previous work suggesting a past mass excess and overcompensated basin. To reconcile these findings, we propose that the substantially thinned post-impact ice shell beneath the basin was out of thermal equilibrium, leading to the refreezing of the uplifted ocean and to the basin transitioning to a present-day mass deficit. This new evolutionary pathway has important implications for Pluto's interior structure and the evolution, and longevity, of a subsurface ocean.