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 (mckinnon@wustl.edu), 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 Kuiper Belt (aKB), when Pluto was closer to the Sun. Stereo-derived topography indicates 1 km excess ele-vation of the Sputnik basin rim compared with Pluto overall, consistent with an ejecta blanket (Fig. 2). The Sputnik basin rim is a well-defined scarp to the north-east (Cousteau Rupes) and to the west and southwest are annular arrangements of mountain blocks (the al-Idrisi, Baré, and Hillary Montes; all nomenclature herein being informal), all consistent with an impact, if not a multiring basin origin (Fig. 1). Extrapolation of 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-tigraphy, it is highly degraded and eroded. There are no identified secondary craters, crater chains, or sculp-ture caused by the ballistic emplacement of ejecta. Only the barest remnants of basin rings are identifiable

Figure 1. Orthographic projection of New Horizons (NH) LORRI-MVIC stereo-derived DEM centered on Sputnik Planitia. Outer ellipse is 1300 x 900 km. (e.g., the Noachian-age Hellespontus Montes to the west [8]). Hellas’ identity as an impact basin relies on 1) the likelihood of impact basins from the LHB on the cratered southern highlands on Mars, 2) its structural analogy to the slightly younger but much less degraded Argyre basin, and 3) gravity results that clearly show thickened highlands crust surrounding a greatly thinned (<20 km) crust underlying the basin floor [9].

Figure 2. Cylindrical projection of DEM of the New Ho-rizons Pluto encounter hemisphere, with topographic profile. Sputnik Planitia is flat-lying at an elevation 2.5 km below the best mean datum for Pluto. Elevated rim topogra-phy surrounds the basin.

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Figure 3. Estimate of the size of the Sputnik basin im-pactor. A 3 km/s impact (appropriate to the ancestral Kuiper belt) at 45° is assumed, with equal impactor and Pluto mantle densities. The 2 curves are for two proposed transient-to-complex crater scalings.

The Case for Impact: In terms of a raised rim,

Sputnik Planitia (SP) is enclosed by an eroded and modified broad raised ridge 250-300 km wide, which rises up to 1000 meters or so above the exterior plains and towers above the surface of SP proper (Fig. 2). The exact dimensions of this outer ridge-ring depend on how it is defined. Taking its definition as the high-est elevation across this broad topographic swell, we obtain a diameter of 1800 by 1100 km. Assuming a transient crater diameter of ~500 km (half the mean diameter of SP proper), 1 km falls short of the ejecta thickness expected at the distance of the ridge by a factor of ~3 [cf. 10]. This suggests a combination of isostatic adjustment, subcrustal flow, and/or surface erosion has occurred. The presence of substantial bounding high topography is, however, consistent with a impact origin for the basin.

The prominent mountain chains and arrays of mountain blocks on SP’s western side are possible further evidence of an impact basin origin, as their distribution fits within the classic √2 spacing of lunar basin rings [10] (and large-scale impact would be an efficient way to create such crustal blocks in the first place) (Fig. 2). That such structures can form on Pluto is further supported by the next smallest impact feature identified by New Horizons, Venetia Burney. Burney is characterized by a depression ~180 km wide and 1.8-2.0 km deep, in turn surrounded by 2-4 concentric sets of discontinuous crenulated ridges 0.5-to-1.0 km high, bringing its overall diameter to ~350 km. The terrain between the ridges is also depressed a few hundred meters with respect to surrounding plains. This is rem-iniscent of Orientale on the Moon or Gilgamesh on Ganymede, and suggests that Pluto’s interior thermal structure permitted Gilgamesh-style (as opposed to Valhalla-style) multiring basin formation [11].

As to a gravity signature for SP, New Horizons could not make direct measurements at its flyby dis-tance and speed. Nevertheless, the position of SP near the tidal axis (sub-Charon point) has prompted the hy-pothesis that SP coincides with a positive mass anoma-

Table 1. Extreme Elliptical Impact Basins

ly or mascon, one substantial enough to drive true po-lar wander [3,12]. A thinned water-ice shell above a quasi-permanent cold ocean uplift has been proposed in particular [3] as the primary mascon source. We note uplift of a dense(r) carbonaceous or organic-rich layer as an alternative [13], and the “ejecta” ridge-ring (considered in [3]) and any (rock-rich) impactor rem-nants (see below) will also contribute to any positive mass anomaly.

Finally, we remark that a proposed alternative origin for the SP basin, by surface loading by condens-ing N2-ice alone [14], is speculative, lacks structural analogues elsewhere in the Solar System, and would be (height) limited by the weakness of N2-ice as a geolog-ical material.

An Extreme Elliptical Impact Basin: Based on gravity regime impact scaling, and a simple-to-com-plex crater transition diameter near 4.5 km, we esti-mate the SP impactor to be somewhere in the 150 to 300 km diameter range (Fig. 3). This is consistent with the SP impact simulations of [17], who modeled 200-km diameter vertical impacts at 2 km/s. Low speed impacts of this scale on Pluto imply low cratering effi-ciencies, on the order of 2-3, and between this [18] and Pluto’s sphericity an elliptical impact basin (with a downrange extension due the decapitated impactor [18]) is more than likely. Sputnik Planitia thus joins other extreme elliptical (oblique) impact basins in the Solar System (Table 1), with implications for excava-tion depth, ejecta distribution, impactor fate, and more.

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