Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto

To explain the position of the Sputnik Planitia basin on Pluto, the feature would need to have formed via impact and Pluto would need to have a subsurface ocean. The present location of Sputnik Planitia—the prominent deep icy basin on Pluto—is close to one of the longitudes of the dwarf planet's tidal axis. By analogy with other large basins in the Solar System, it is thought to be an impact feature. Although reorientation arising from tidal and rotational torques can explain the present-day location of the basin, it requires the feature to be a positive gravity anomaly, despite its negative topography. Francis Nimmo et al. argue that if Sputnik Planitia formed via impact and if Pluto possesses a subsurface ocean, a positive gravity anomaly would naturally result because of shell thinning and ocean uplift, followed by later modest nitrogen deposition. This is one of four papers on the geology of Sputnik Planitia in this issue of Nature. In News & Views, Amy Barr puts these latest contributions into context. The deep nitrogen-covered basin on Pluto, informally named Sputnik Planitia, is located very close to the longitude of Pluto’s tidal axis1 and may be an impact feature2, by analogy with other large basins in the Solar System3,4. Reorientation5,6,7 of Sputnik Planitia arising from tidal and rotational torques can explain the basin’s present-day location, but requires the feature to be a positive gravity anomaly7, despite its negative topography. Here we argue that if Sputnik Planitia did indeed form as a result of an impact and if Pluto possesses a subsurface ocean, the required positive gravity anomaly would naturally result because of shell thinning and ocean uplift, followed by later modest nitrogen deposition. Without a subsurface ocean, a positive gravity anomaly requires an implausibly thick nitrogen layer (exceeding 40 kilometres). To prolong the lifetime of such a subsurface ocean to the present day8 and to maintain ocean uplift, a rigid, conductive water-ice shell is required. Because nitrogen deposition is latitude-dependent9, nitrogen loading and reorientation may have exhibited complex feedbacks7.