A Large Impact Origin for Sputnik Planum and Surrounding Terrains, Pluto?

One of the most prominent features on Pluto discovered by New Horizons is the oval-shaped bright deposit within western Tombaugh Regio (all names used herein are informal). This smooth bright deposit, provisionally identified with frozen nitrogen and methane and informally referred to as Sputnik Planum, is bounded on the northeast by an arcuate scarp (Cousteau Rupes). The smooth bright material there embays what appears to be an eroded plateau 1-2 km high. The arcuate scarp leads to speculation that the deposits formed in an ancient impact basin, but detailed mapping at 2 km pixel scales suggests that this large structure is more complex than any simple impact basin. To the southwest are a series of high peaks and massifs (also embayed by bright material) but these broken massifs have a different morphology from Cousteau Rupes, being both higher and more disrupted. The southern section of this putative 800-km-wide circular structure is completely missing as smooth material extends well to the south of the nominal rim location. A possible analog occurs at the “other End of the Solar System” on Mercury, in Caloris Basin. This 1400-km-wide impact basin is also irregular in shape, with large deviations form circularity, and occasional large massifs along some rim segments. Post-impact smooth plains embay the rim scarp in some areas, though these are likely to be volcanic plains on Mercury. The relief of the rim scarps to the NE and SW and putative evidence for convection within Sputnik Planum suggests that the floor of the deposits lies 1-3 km below the mean surface (pending stereo mapping). This depth is consistent with the filling of an ancient impact basin with ices, deposited either volcanically or atmospherically, although other explanations are also possible. This work was supported by NASA's New Horizons project.