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IMPACT ORIGIN of SPUTNIK PLANITIA BASIN, PLUTO. William B Lunar and Planetary Science XLVIII (2017) 2854.pdf IMPACT ORIGIN OF SPUTNIK PLANITIA BASIN, PLUTO. William B. McKinnon1, P.M. Schenk2, X. Mao1, J.M. Moore3, J.R. Spencer4, F. Nimmo5, L.A. Young4, C.B. Olkin4, K. Ennico3, H.A. Weaver6, S.A. Stern4, and the New Horizons Geology, Geophysics & Imaging Theme Team; 1Dept. Earth and Planet. Sci. & McDonnell Center for the Space Sci., Washington Univ. in St. Louis, Saint Louis, MO 63130 ([email protected]), 2LPI, Houston, TX 77058, 3NASA Ames Research Center, Moffett Field, CA 94035, 4SwRI, Boulder, CO 80302, 5Dept. Earth and Planetary Sci., UC Santa Cruz, Santa Cruz CA, 95064, 6JHUAPL, Laurel, MD 20723. Introduction: The vast, nitrogen-dominated ice sheet informally known as Sputnik Planitia lies within a great, oval-shaped (~1300 km x 900 km) structural depression ([1], Fig. 1). The scale and ellipticity of such structures on other bodies are almost always due to basin-forming impacts [2,3]. Analogues include Hellas on Mars and South Pole-Aitken on the Moon. New Horizons imagery does not reveal obvious large secondary craters, secondary crater chains, or Imbrium sculpture, but Sputnik basin is an ancient feature. It lies at the stratigraphic base on Pluto, all old cratered sur- faces on Pluto post-date it, and its surroundings have been subject to extensive geological (predominantly glacial) modification. Hellas on Mars 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 [4], and most likely formed in the ancestral Figure 1. Orthographic projection of New Horizons (NH) Kuiper Belt (aKB), when Pluto was closer to the Sun. LORRI-MVIC stereo-derived DEM centered on Sputnik Stereo-derived topography indicates 1 km excess ele- Planitia. Outer ellipse is 1300 x 900 km. vation of the Sputnik basin rim compared with Pluto (e.g., the Noachian-age Hellespontus Montes to the overall, consistent with an ejecta blanket (Fig. 2). The west [8]). Hellas’ identity as an impact basin relies on Sputnik basin rim is a well-defined scarp to the north- 1) the likelihood of impact basins from the LHB on the east (Cousteau Rupes) and to the west and southwest cratered southern highlands on Mars, 2) its structural are annular arrangements of mountain blocks (the al- analogy to the slightly younger but much less degraded Idrisi, Baré, and Hillary Montes; all nomenclature Argyre basin, and 3) gravity results that clearly show herein being informal), all consistent with an impact, if thickened highlands crust surrounding a greatly not a multiring basin origin (Fig. 1). Extrapolation of thinned (<20 km) crust underlying the basin floor [9]. crater depth-diameter measurements indicates that the rim-to-floor depth of the structural basin is no greater than 9 km, consistent with estimates of the thickness of the convecting nitrogen ice plain within [5]. The over- all structure is consistent with impact of an aKB body >150 km across, moving from ~N15°W to S15°E at a moderately oblique angle (≳45°) [6]. Characterizing Impact Basins: Impact basins are expected to exhibit the following: 1) characteristic ejecta facies (a massive ejecta “blanket,” large second- ary craters and crater chains, Imbrium sculpture), 2) structural rim uplift, 3) multiple mountain rings, and 4) gravity signature of mass redistribution. These charac- teristics are exemplified by impact basins on the Moon [7], but can be highly modified on worlds with active geology and climates, such as Mars and Pluto. Hellas is a striking example. Lying at the base of Mars’ stra- Figure 2. Cylindrical projection of DEM of the New Ho- tigraphy, it is highly degraded and eroded. There are rizons Pluto encounter hemisphere, with topographic no identified secondary craters, crater chains, or sculp- profile. Sputnik Planitia is flat-lying at an elevation 2.5 km ture caused by the ballistic emplacement of ejecta. below the best mean datum for Pluto. Elevated rim topogra- Only the barest remnants of basin rings are identifiable phy surrounds the basin. Lunar and Planetary Science XLVIII (2017) 2854.pdf Table 1. Extreme Elliptical Impact Basins ly or mascon, one substantial enough to drive true po- Figure 3. Estimate of the size of the Sputnik basin im- lar wander [3,12]. A thinned water-ice shell above a pactor. A 3 km/s impact (appropriate to the ancestral Kuiper quasi-permanent cold ocean uplift has been proposed belt) at 45° is assumed, with equal impactor and Pluto mantle in particular [3] as the primary mascon source. We densities. The 2 curves are for two proposed transient-to- note uplift of a dense(r) carbonaceous or organic-rich complex crater scalings. layer as an alternative [13], and the “ejecta” ridge-ring (considered in [3]) and any (rock-rich) impactor rem- The Case for Impact: In terms of a raised rim, nants (see below) will also contribute to any positive Sputnik Planitia (SP) is enclosed by an eroded and mass anomaly. modified broad raised ridge 250-300 km wide, which Finally, we remark that a proposed alternative rises up to 1000 meters or so above the exterior plains origin for the SP basin, by surface loading by condens- and towers above the surface of SP proper (Fig. 2). ing N2-ice alone [14], is speculative, lacks structural The exact dimensions of this outer ridge-ring depend analogues elsewhere in the Solar System, and would be on how it is defined. Taking its definition as the high- (height) limited by the weakness of N2-ice as a geolog- est elevation across this broad topographic swell, we ical material. obtain a diameter of 1800 by 1100 km. Assuming a An Extreme Elliptical Impact Basin: Based on transient crater diameter of ~500 km (half the mean gravity regime impact scaling, and a simple-to-com- diameter of SP proper), 1 km falls short of the ejecta plex crater transition diameter near 4.5 km, we esti- thickness expected at the distance of the ridge by a mate the SP impactor to be somewhere in the 150 to factor of ~3 [cf. 10]. This suggests a combination of 300 km diameter range (Fig. 3). This is consistent with isostatic adjustment, subcrustal flow, and/or surface the SP impact simulations of [17], who modeled 200- erosion has occurred. The presence of substantial km diameter vertical impacts at 2 km/s. Low speed bounding high topography is, however, consistent with impacts of this scale on Pluto imply low cratering effi- a impact origin for the basin. ciencies, on the order of 2-3, and between this [18] and The prominent mountain chains and arrays of Pluto’s sphericity an elliptical impact basin (with a mountain blocks on SP’s western side are possible downrange extension due the decapitated impactor further evidence of an impact basin origin, as their [18]) is more than likely. Sputnik Planitia thus joins distribution fits within the classic √2 spacing of lunar other extreme elliptical (oblique) impact basins in the Solar System (Table 1), with implications for excava- basin rings [10] (and large-scale impact would be an tion depth, ejecta distribution, impactor fate, and more. efficient way to create such crustal blocks in the first References: [1] Stern S.A. et al. (2015) Science 350, place) (Fig. 2). That such structures can form on Pluto 10.1126/science.aad1815. [2] Moore J.M. et al. (2016) Sci- is further supported by the next smallest impact feature ence 351, 1284–1293. [3] Nimmo F. et al. (2016) Nature identified by New Horizons, Venetia Burney. Burney is 540, 94–96. [4] Greenstreet S. et al. (2015) Icarus 258, 267– characterized by a depression ~180 km wide and 1.8- 288. [5] McKinnon W.B. et al. (2016) Nature 534, 82–85. 2.0 km deep, in turn surrounded by 2-4 concentric sets [6] McKinnon, W.B. et al. (2016) GSA Annual Mtg., abs. 48- of discontinuous crenulated ridges 0.5-to-1.0 km high, 6. [7] Zuber M.T. et al. (2013) Science 339, 668–671. [8] Leonard G.J. and Tanaka K.L. (2001) USGS Misc. Inv. Se- bringing its overall diameter to ~350 km. The terrain ries I-2694. [9] Genova A. et al. (2016) Icarus 272, 228–245. between the ridges is also depressed a few hundred [10] Melosh H.J. (1989) Impact Cratering, OUP. meters with respect to surrounding plains. This is rem- [11] McKinnon W.B. and Melosh H.J. (1980) Icarus 44, iniscent of Orientale on the Moon or Gilgamesh on 454-471. [12] Keane J.T. et al. (2016) Nature 540, 90–93. Ganymede, and suggests that Pluto’s interior thermal [13] McKinnon W.B. et al. (1997) in Pluto and Charon, structure permitted Gilgamesh-style (as opposed to Univ. Ariz. Press, 295-343. [14] Hamilton D.P. et al. (2016) Valhalla-style) multiring basin formation [11]. Nature 540, 97–99. [15] Schenk P.M. and McKinnon W.B. (1985) LPSC XVI, 544–545. [16] McKinnon W.B. and As to a gravity signature for SP, New Horizons Schenk P.M. (1995) GRL 32, 1829–1832. [17] Johnson B.C. could not make direct measurements at its flyby dis- et al. (2016) GRL 43, 10,068–10,077. [18] Elbeshausen D. et tance and speed. Nevertheless, the position of SP near al. (2013) JGR 118, 2295–2309. [19] Andrews-Hanna J.C. et the tidal axis (sub-Charon point) has prompted the hy- al. (2008) Nature 453, 1212–1215. [20] Garrick-Bethel I. and pothesis that SP coincides with a positive mass anoma- Zuber M.T.
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