Seismology on pluto ? ! antipodal terrains produced by sputnik planitia-forming impact

Introduction: Pluto’s interior remains a mystery, including the composition of the core and the existence and extent of a subsurface ocean. Though typical seismological study is not possible on Pluto, the reaction of stress waves released from large impacts to a planet’s interior structure can be used to probe the planet’s interior – seismology by impact. Recent analysis of New Horizons data revealed extensive lineations antipodal to Sputnik Planitia (SP) [1], the 1200 x 2000 km elliptical impact basin on Pluto [2]. These resemble the “hilly and lineated” terrain antipodal to Caloris Basin on Mercury [3]; here, we propose a similar formation mechanism from seismic focusing of impact-generated stress waves. We simulate the formation of SP to determine if these terranes result from stress wave focusing at the basin’s antipode [3-5], and how such focusing may have been influenced by Pluto’s internal structure [3,5]. Our simulations reflect SP’s updated dimensions [2], which require a larger impactor than previously considered [6]. Our numerical simulations consider variations in ocean thickness and core composition, as both are primary uncertainties in Pluto’s internal structure that will exert significant control over wave propagation. We then compare predicted antipodal deformation to the geologic features observed to determine the most likely internal structure of Pluto. Methods: Following [6], we use the iSALE shock physics code [7-9] to simulate a 300, 350, and 400-km diameter impactor striking a Pluto-like target at 2 km/s [10], with the same model setup [11-13]. A 400 km-impactor was found to produce the closest match to observed basin size and used as the fiducial model. We extend 2 km-resolution to all of Pluto to resolve antipodal deformation [i.e., 5]. In our simulations we vary preimpact ocean thickness (0, 50, 100, 150 km) and core composition (dunite [13], serpentine [14]) to determine which, if any, bulk interior structure yields the observed antipodal deformation. The combined ice shell/ocean thickness is held constant at 328 km [6]. Following [5], we infer resulting deformation through measurements of material velocity, total plastic strain, and displacement during and after stress wave arrival at the antipode. Preliminary Results: In our fiducial model (Dimp = 400, tocean = 0, dunite core), the contrast in sound speeds between the ice shell (~3300 m/s) and core (~6500 m/s) causes the impact-induced stress wave to be transmitted quickly through the core and much more slowly through the overlying ice shell (reported sound speeds are calculated by iSALE using the equation of state). The temporal separation of the wave between materials produces separate arrivals at the antipode, as observed in strong peaks (>35 m/s) in material velocity (not to be confused with wave speed) at ~550 and ~800 s after impact. To assess structural deformation at the antipode, we quantify inward/outward displacement experienced by material in the region. Measurements of horizontal displacement after 5000 s (when the antipode is no longer accumulating plastic strain) indicate that greater than 2 km of displacement occurs in a near-surface zone extending 250 -300 km from the antipode, where material is largely carried inwards (Fig. 1a). This near-surface deformation zone is observed in all subsequent runs using a 400-km impactor (Figs. 1-2); decreasing impactor size reduces the width and magnitude of deformation.