AN IMPACT BASIN ORIGIN FOR SPUTNIK “PLANITIA” AND SURROUNDING TERRAINS, PLUTO

The vast, nitrogen-dominated ice sheet informally known as Sputnik Planum lies within a great, oval-shaped (~1300 km x 900 km) structural depression. Better categorized as a planitia, and the scale and ellipticity of such structures on other bodies are almost always due to basin-forming impacts. Analogues include Hellas on Mars and South Pole-Aitken on the Moon. But what specific geologic features of Sputnik Planitia and its surroundings corroborate this interpretation? New Horizons imagery does not reveal an obvious ejecta blanket, secondary crater chains, or Imbrian sculpture. But Sputnik “basin” is an ancient feature. It lies at the stratigraphic base on Pluto, all old cratered surfaces on Pluto post-date it, and its surroundings been subject to extensive geological (predominantly glacial) modification. Hellas is thus a much better analogue than Orientale, Imbrium, or even SPA on the Moon. As an impact, Sputnik basin has less than a 1% chance of forming in the Kuiper belt over the last ~4 Gyr (Greenstreet et al., 2015), and most likely formed in the ancestral Kuiper Belt (aKB), when Pluto was closer to the Sun. Stereo-derived topography does indicate >1 km excess elevation of the Sputnik basin rim compared with Pluto overall, consistent with an ejecta deposit (and if isostatically compensated by an ocean, the ejecta thickness could be considerable). The Sputnik basin rim is a well-defined scarp to the northeast (informally, Cousteau Rupes) and to the west and southwest are annular arrangements of mountain blocks, all consistent with an impact origin. Extrapolation of crater depth-diameter measurements indicates that the rim-to-floor depth of the structural basin is no greater than 9 km (Schenk et al., LPSC 2016), consistent with estimates of the thickness of the convecting nitrogen ice plain within (McKinnon et al., 2016). The overall structure is consistent with impact of an aKB body > 150 km across, moving from ~N15W to S15E at a moderately oblique angle.