PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Depth of Origin of the Peak (Inner) Ring in Lunar 10.1002/2017GL075207 Impact Basins Key Points: Katarina Miljković1 , Myriam Lemelin2,3 , and Paul G. Lucey2 • Impact simulations were used to determine depth of origin of material 1Department of Applied Geology, Curtin University, Perth, Western Australia, Australia, 2Hawai’i Institute of Geophysics and exposed within the peak ring in lunar basins Planetology, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of 3 • Basin size-dependent trends show Hawai’iatMānoa, Honolulu, HI, USA, Earth and Space Science and Engineering, Department Lassonde School of depth of origin for a range of lunar Engineering, York University, Toronto, Ontario, Canada impact basins, up to the upper mantle • Basin size-dependent relationships help determine lunar composition Abstract Numerical modeling of the peak-ring basin formation showed that the peak-ring forms from the and stratigraphy material that is part of the central uplift outwardly thrust over the inwardly collapsing transient crater rim. Simulations of the lunar basin formation showed that the peak or inner ring in peak ring or multiring basins, Supporting Information: respectively, is composed of the overturned crust and deep-seated material, possibly from the upper • Supporting Information S1 mantle. Numerical impact simulations were used to trace the depth of origin of material exposed within the Correspondence to: peak (or inner) ring. We estimate the scaling trends between basin size and the depth of origin of material K. Miljković, exposed within the ring. We also report on the likely crust, mantle, and projectile abundances exposed within [email protected] the ring. Quantifying the excavation depths during the formation of the peak or inner ring provides a step toward understanding the lunar crust and mantle stratigraphy. Citation: Miljković, K., Lemelin, M., & Lucey, P. G. (2017). Depth of origin of the peak 1. Introduction (inner) ring in lunar impact basins. Geophysical Research Letters, 44, Large impact craters, or impact basins, can be divided in two categories based on their morphology and – 10,140 10,146. https://doi.org/10.1002/ ring formation process: the peak-ring and multiring basins. This crater morphology is present on all rocky 2017GL075207 bodies in the solar system. The transition from the complex crater to peak-ring to multiring basin scales Received 8 AUG 2017 inversely with the parent body gravity (Melosh, 1989). On the Moon, the onset diameters for the formation Accepted 29 SEP 2017 of peak-ring basins are ~150 km and ~300 km for the multiring basins (e.g., Wood & Head, 1976). Accepted article online 9 OCT 2017 Peak-ring basins (e.g., Schrödinger) have a single ring located within their main topographic rim, and mul- Published online 19 OCT 2017 tiring basins (e.g., Orientale) have multiple ring-like features (e.g., Hartmann & Kuiper, 1962; Melosh, 1989; Spudis, 1993). The dynamic theory for the peak-ring formation suggested that the peak ring likely forms through the inter- action of two collapse regimes: (1) the outward collapse of the uplift of the underlying material at the bot- tom of the transient crater and (2) the inward collapse of the transient crater wall (e.g., Murray, 1980). This theory was supported by (a) numerical impact modeling that showed that the peak ring should be com- posed of deeply derived material and that the ring stratigraphy exhibits an overturned structure (e.g., Collins et al., 2002), (b) detailed mineral mapping of lunar impact basins (Baker et al., 2016; Kring et al., 2016), and (c) rock sample analyses obtained by the drilling expedition to the peak ring of the Chicxulub crater (Morgan et al., 2016). Furthermore, recent numerical modeling of the formation of the multiring Orientale basin (Potter, Kring, & Collins, 2013; Johnson et al., 2016) demonstrated that the inner ring in mul- tiring basins forms via the same mechanism as peak rings in peak-ring basins. Previous impact studies have mainly focused on quantifying the maximum depth of excavation during crater formation. According to the Maxwell’s Z-model of the excavation flow (Melosh, 1989) and recent numerical impact modeling work (e.g., Potter et al., 2015), the maximum depth of excavation in lunar basins was estimated to be 0.12 Dtr, where Dtr is the transient crater diameter. Although, Potter, Kring, Collins, and Kiefer et al. (2013) quantified the stratigraphic uplift during basin formation, there is no clear relationship between the depth of origin of the material exposed in the peak or inner ring and impact basin size. In this work, we use the results from numerical impact simulations to determine the scaling trends for the depth of origin of materials exposed within peak and inner rings in lunar peak ring and multiring basins, ©2017. American Geophysical Union. respectively. We also report on the crust, mantle, and projectile abundances exposed within the ring, as a All Rights Reserved. function of impact condition and target properties. MILJKOVIĆ ET AL. DEPTH OF ORIGIN OF BASIN INNER RING 10,140 Geophysical Research Letters 10.1002/2017GL075207 Figure 1. (left) Lunar impact basin made by a 30 km projectile impacting at 17 km/s shown in vertical cross section. The target temperature profile is “intermediate.” The crust (shown by green tracer particles) is 45 km thick, overlaying the mantle (shown in blue tracer particles). Tracers shown in red represent residues of the projectile. The gray box is centered at the radius of the peak ring, which also corresponds to the radius of the crustal thinning (Dthin/2); (right) relative mass distribution of tracer particles and their respective depth of origin, extracted from the sampling box shown on the left. The crustal component shown in green and mantle component in blue. 2. Method To simulate the lunar impact basin formation, we used the iSALE-2D hydrocode, a multimaterial, multirheol- ogy finite difference shock-physics code used for simulating impact processes in geologic media (Amsden et al., 1980; Collins et al., 2004; Wünnemann et al., 2006). This code has been extensively used for modeling impact basin formation on the Moon (Johnson et al., 2016; Melosh et al., 2013; Miljković et al., 2013, 2015, 2016; Potter et al., 2012; Potter, Kring, & Collins, 2013; Potter, Kring, Collins, Kiefer et al., 2013; Potter et al., 2015; Zhu et al., 2015) and was benchmarked against other hydrocodes (Pierazzo et al., 2008). We investigated impacts made by 15, 30, 45, and 60 km diameter impactors hitting the Moon at an average speed of 17 km/s (Le Feuvre & Wieczorek, 2011), as used and justified in Miljković et al. (2013). The average lunar crustal thickness on the nearside hemisphere is 34 km, and 43 km on the farside hemisphere (Wieczorek et al., 2013). Therefore, the Moon was assumed to be a flat target made of a 30 or 45 km thick crust overlaying mantle. To represent the thermal state of the Moon at the time lunar basins had formed, we applied “cold,” “intermediate,” and “hot” target temperature profiles, as described in Miljković et al. (2013, 2015, 2016), typi- cal for the lunar farside, nearside (except the PKT), and the PKT region, respectively. The crust (modeled using basalt ANEOS), mantle, and projectile (both modeled using dunite ANEOS) used material models from Miljković et al. (2013, 2015, 2016). These sets of impactor and target properties cover the entire size range of lunar basins, except the South Pole-Aitken (SPA) basin and possibly Imbrium. The reason for excluding the largest lunar basins was because of difficulty in modeling the formation of the inner ring in such large basins due to the very large amount of melt forming in the center of the basin. The depth of origin of the material exposed in the peak or inner ring was tracked using Lagrangian tracer par- ticles placed in every cell of the Eulerian numerical mesh. Numerical cell size was set to 1.5 by 1.5 km in all simulations, which provided sufficient resolution to observe the peak-ring formation process. Given the resolution, we can properly resolve the combination of the collapse of the transient crater, stratigraphic uplift of the transient crater floor, and collapse of the overturned crustal material that contribute to the peak-ring formation. The trajectory of each tracer was recorded during the basin formation process, so that the initial location of the material can be determined at the end of a simulation. We identified a region of 10 km radial distance (at final surface level) by 10 km depth centered at the radius of the peak ring (gray box, Figure 1), and we analyzed all tracers located in this region. This method allowed for sampling a statistically significant num- ber of tracers that will provide a good representation of the possible range of the material’s depth of origin, for all materials exposed within the ring. We also found that the tracer sample box was moderately insensitive to its size. If the sample box increases by 100% in both radial and vertical directions, it still produced less than 10% variation in the mean depth of origin of the materials as well as the crust-mantle proportions. Considering all numerical simulations were made in 2-D, it was important to estimate the mass of each cell that tracers represented in 3-D. All numerical models were cylindrically symmetric; therefore, the volume of each cell was calculated as a volume of a torus around the vertical axis. Thickness of the torus MILJKOVIĆ ET AL. DEPTH OF ORIGIN OF BASIN INNER RING 10,141 Geophysical Research Letters 10.1002/2017GL075207 Figure 2. (a) (left) The mean depth of origin of material exposed within the peak or inner ring for a range of basin sizes, represented via the coupling parameter (LV0.58, where L is the projectile diameter in km and V speed in km/s).