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The Sizes and Nature of Basin Impactors on Mercury and the Moon

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The Sizes and Nature of Basin Impactors on Mercury and the Moon Lunar and Planetary Science XLVIII (2017) 2704.pdf THE SIZES AND NATURE OF BASIN IMPACTORS ON MERCURY AND THE MOON. P. H. Schultz, Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Book Street, Box 1846, Providence, RI 02912; [email protected]. Introduction: Oblique impacts map out different exhibit strong contrasts. An estimate for the oblique stages of crater formation across the surface and can impact crater Dantu (with an elongate central pit) on reveal the controlling independent variables. Ceres was also included. Laboratory experiments allow isolating these different Results: Mapped sculpture patterns on Mercury stages and variables, even though they cannot directly yielded the following sizes of the impacting bodies: simulate a large-scale event. One critical set of Caloris (525 km diameter), Rembrandt (115 km), variables includes impact angle and impactor size. On Beethoven (246 km), Tolstoj (197 km), Vivaldi (43 the planets, crater morphology and ejecta distribution km), Dürer (86 km), Wang Meng (36 km), and Valmki can be used to constrain the size of the impacting body, (23 km). Such sizes may seem large but are generally independent of scaling relations [1] and provide clues consistent with extrapolated scaling relations (Fig. 1a). to the origin of the inner rings of large basins [2]. Scaling relations refer to impact diameters referenced Background: At large scales, early stages of crater to the pre-impact surface, i.e., the “apparent” diameter, formation become more evident due to the reduced rather than the final post-collapse diameter. Here, the cratering efficiency [3]. For an oblique impact, rim-rim diameters of each basin was reduced by 25% surface expressions of an evolving flow field include a to account for crater enlargement by slumping and migrating source region of ejecta [4], non-circular another 25% in order to adjust to the pre-impact elements in crater shape [5,6], and downrange scours surface. Figure 1a reveals that lunar and Mercurian by the impactor [1,7]. The Imbrium Sculpture basins (and Dantu on Ceres) fall on the extrapolated illustrates each of these signatures [1]. For sufficiently scaling relation for non-porous gravity-controlled oblique angles, fragments of the impacting body begin targets after making small (but reasonable) adjustments to fail soon after first contact such that the tops and for impact speed and angle. sides of the impactor scour the surface beyond the final While the inner rings of two-ring basins are easy to crater rim. Back-extrapolated trends of the resulting recognize, the correct identification becomes grooves either converge beyond the uprange rim or problematic for multi-ring structures. For this study, it form sub-parallel sets converging far uprange. Based is assumed that the inner, oblong wrinkle ridge system on experiments and 3D hydrocode models, the width of Imbrium and the inner oblong shelf in Orientale of these grooves and scours near the uprange rim can represent the inner ring. Even then, the multi-ring be used to constrain the diameter of the impactor. This basins (Rembrandt, Orientale, and Imbrium) require a strategy yielded a diameter of the Imbrium impactor as much higher speed, much lower angle, or a different at least 250 km, along with estimates of 110 km, 100 transient diameter in order to be consistent with the km, and 45 km for Orientale, Moscoviense, and peak-ring basins in Fig.1a. If the apparent transient Schrödinger, respectively [1]. Here we extend this crater diameter for Imbrium were placed between the approach to basins on other bodies, compare the Montes Alpes and oblong massif ring delineated by derived impactor diameters with the measured interior Montes Recti-Pico-Pito, the scaling relation would peak ring diameters, and assess predictions based on hold. For Orientale, the a working diameter would scaling relations. The onset transition in interior correspond to a position between Inner and Outer morphologies (such as central peaks and rings) not Rooke Mountains. only depends on gravity but also impact speed [8,9,10]. The derived impactor diameter also can be Consequently, such features should be expressed in the compared with the inner ring diameter (Figure 1b). An scaling relations as well. average ring diameter is not assumed; rather, the Selected Basins: By definition, such a strategy can physically relevant diameter is orthogonal to the include only those craters formed by oblique trajectory, as from inferred from the morphologic trajectories with clearly expressed impactor signatures. signatures (ejecta and crater structure). In general, the Nevertheless, this subset establishes a benchmark for ring-impactor diameter ratio (DIR/a) is about 2, but interpreting higher angle impacts. Stereographic impactors larger than about 100 km are consistently projections centered on selected basins on Mercury above the extrapolated linear trend for smaller provided the base maps for determination of impactors. downrange groove trends. The selected basins Discussion: The correlation between the inner ring included: Caloris (1250 km diameter), Rembrandt (720 diameter and observationally determined impactor km), Beethoven (640 km), Tolstoj (490 km), Vivaldi diameter (Fig. 1b) is consistent with the “footprint” (210 km), Dürer (190 km), Wang Meng (160 km), and model of inner ring formation [2,11]. In this model, Valmki (97 km). The morphologies of these basins the inner ring is related to dynamic uplift of the Lunar and Planetary Science XLVIII (2017) 2704.pdf displaced-downward crust. For an oblique impact at Fassett, C. I. et al. (2012), JGR 117, E00L08, doi: 30°, the downward-directed peak pressure is reduced 10.1029/2012JE004154; [13] Ernst, C. M. et al. by a factor of four. This reduced pressure increases the (2016), LPSC 47, #1374; [14] Barnouin, O. et al. role of target strength at depth, even at speeds less than (2015), LPSC 46 #2672; [15] Le Feuvre, M. and 15 km/s. At higher speeds (>20 km/s), energy plays a Wieczorek, M. A. (2011), Icarus 214, 1-20; [16] greater role and results in loss of expression of the Barlow, N. G., and T. L. Bradley (1990), Icarus 87, downward-displaced zone and the formation of central 156–179; [17] Robbins, S. J. and Hynek, B.M. (2012), peaks. In laboratory experiments, the ratio of the edge JGR 117, E06001, doi:10.1029/2011JE003967. of the downward-displaced pit is also nearly constant as a function of (v sinθ), even though the diameter and 2.0# 100# depth of the crater changes. But it also depends on the MercUry# Moon# a#–#ReMbrandt##(80#kM/s,#30°)# 1#–#Schrödinger#(8#kM/s#@#15°)*# impedance ratio between the impactor and target b#–Vivaldi#(30#kM/s,#30°)# 2#–#Moscoviense#(15#km/s#@#30°)*# 50# c#–#Dürer#(15#km/s,#30°)# 3#–#Orientale#(25#kM/s#@#30°)*# (Ip/It). Hence, the upward offsets for the inner rings of d#–#ValMke#(15#km/s,#30°)# 4#–#ImbriUM#(25#kM/s#@#30°)*# Schrödinger, Orientale, Rembrandt, and Imbrium in e#–#Wang#Meng#(15km/s,#30°)# f#–#Tolstoj#(15#km/s,#30°)**# 20# /#a)# D/2r# Fig. 1b could represent an effect of differentiated g#–#Beethoven#(15#kM/s,#30°)## A# asteroids with a higher (Ip/It). Conversely, values of 1.0# h#–#Caloris#15#km/s,#30°)## 10# low (Ip/It), such as a cometary impact, should result in Log#(D 1# a# 5# smaller and poorly expressed central rings. z# b# 3# c#e# 4#MRB# Implications: First, lower speed impactors may be 2# Ceres# d# f# g# 2# responsible for the apparent excess of the 200-350 km z#–#Dantu#(5#km/s,#30°)## h# diameter basins on Mercury, consistent with the observed excess of basins per unit area in this size !6# !5# !4# !3# !2# !1# range relative to the Moon [12]. Second, most craters 2 post-dating the Late Heavy Bombardment on Mercury Log#[π2#=##gr/(vsinθ) ]# (99%) and the Moon (90%) were formed at speeds > Fig. 1a: Apparent crater diameter (DA) scaled by impactor 15 km/s, consistent with the greater abundance of diameter (a) as a function of the pi-2 scaling relation [3]. central peaks. But certain craters with small, broken Impactor diameters were determined by the convergence of inner rings (e.g., Eminescu) may represent comets downrange grooves/scours following [1]. Assumed values impacting at the lower end of expected impact speeds. are shown in the legend. The crater diameter was adjusted The unusual Hokusai crater [13, 14] could represent an for the target/projectile density ratio. It is assumed that low- unusually slow cometary impact (<10 km/s). Third, speed undifferentiated asteroids formed the two-ring basins central pit craters represent an extension of the on Mercury, in contrast with larger multi-ring basins (except Tolstoj, which is consistent with a comet). footprint model for smaller bodies where impactor momentum, penetration depths, and dynamic response 3.00$ were insufficient to produce an uplifted ring but did 4$ produce downward displacement. This is consistent 2$ with oblong central pits in oblique impacts and double a$ pits in simultaneous impacts [8]. Fourth, the bi-modal 1$ 3$ distribution of impact speeds on Mars [15] would 2.00$ b$ c$ produce a population of craters of the same size with e$ central peaks, central pits, and central rings in the same d$ terrains [16, 17]. References: [1] Schultz, P.H. and Crawford, D.A. (2016), Nature 535, 391-394; [2] Schultz, P.H. (2016), 1.00$ LPSC 47, #2931; [3] Holsapple, K.A. and Schmidt, R. Log$(Inner$Ring$Diameter)$ M. (1987), JGR 92, 6350-6376; [4] Anderson, J. L. B. (2004), MAPS 39,303-320; [5] Gault, D. E., and 1.00$ 2.00$ 3.0$ Wedekind, J. A. (1978), Proc. Lunar Planet. Sci. Log$(Impactor$Diameter)$ Conf., 9th, 3843–3875; [6] Schultz, P.
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