Icarus 366 (2021) 114539 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Research Paper Impact generated porosity in Gale crater and implications for the density of sedimentary rocks in lower Aeolis Mons B.C. Johnson a,b,*, R.E. Milliken c, K.W. Lewis d, G.S. Collins e a Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA b Department of Physics and Astronomy, Purdue University, West Lafayette, IN, USA c Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA d Earth and Planetary Sciences Department, Johns Hopkins University, Baltimore, MD, USA e Impacts and Astromaterials Research Centre, Dept. Earth Science and Engineering, Imperial College London, London, UK ARTICLE INFO ABSTRACT Keywords: Sedimentary rocks in Gale crater record important information about the climatic history and evolution of Mars. Impact cratering Recent gravity measurements and modeling indicate strata encountered by the Curiosity rover have a very low Mars density (1680 ± 180 kg m 3) and thus unusually high porosity. Missing in these models, however, is the role of Gravity deeper crustal porosity on the observed gravity signatures. Here we simulate the impact formation of Gale crater Gale crater and find that impact generated porosity results in a negative gravity anomaly that decreases in magnitude with distance from the basin center. Incorporating this expected post-impact gravity signature into models for the bulk density of strata in lower Mt. Sharp, we find a best-fit density of 2300 ± 130 kg m 3 for an impact into a target with no pre-impact porosity. Models incorporating pre-impact porosity result in densities that are up to 200 kg/ m3 lower. These revised densities increase the maximum potential burial depth of rocks along the rover traverse, allowing for the possibility Gale crater may once have been filled with sediment. 1. Introduction Vaniman et al., 2014; Rampe et al., 2017; Smith et al., 2018), possibly occurring as a cement, the rocks are clearly lithified and a number of Gale crater is a ~ 150-km-diameter impact crater situated along the them have estimated unconfined compressive strengths similar to north-south hemispheric topographic dichotomy boundary on Mars. terrestrial mudstones (e.g., Peters et al., 2018). These characteristics are Estimated to have formed near the end of the Noachian period indicated by visual rock textures, erosional weathering patterns, (~3.5–3.7 Ga) (e.g., Thomson et al., 2011), the crater hosts a ~ 5 km tall mineralized fractures, and rate of penetration and energy from drilling mountain known as Aeolis Mons and referred to informally as Mt. Sharp activities that can be qualitatively linked to rock hardness (e.g., Grot- (Grotzinger et al., 2014). The NASA Curiosity rover has been traversing zinger et al., 2014; Peters et al., 2018; Abbey et al., 2019). Many of the the northwestern crater floor and lower slopes of Mt. Sharp since its rocks encountered by Curiosity do exhibit evidence of early and/or late landing in August 2012, during which time it has identified and char- diagenetic alteration (e.g., Siebach et al., 2014; Stack et al., 2014; acterized strata interpreted as part of an ancient alluvial-fluvial- Nachon et al., 2016; Kronyak et al., 2019; Kah et al., 2018; Sun et al., lacustrine system (Grotzinger et al., 2014, 2015a, 2015b; Hurowitz 2019), but there are no clear visual or other indicators of unusually high et al., 2017). The most common lithology observed thus far on Mount primary or secondary porosity. Rather, the presence of hydraulic frac- Sharp has been phyllosilicate-bearing mudstones, although thin basaltic tures in the martian mudstones is consistent with rocks in lower Mt. sandstone units and fluvialconglomerates have also been observed (e.g., Sharp having mechanical properties similar to mudstones on Earth Edgar et al., 2017; Grotzinger et al., 2014, 2015a; Williams et al., 2013; (Schieber et al., 2017; Caswell and Milliken, 2017), and geochemical Rampe et al., 2017; Banham et al., 2018). data for mudstones drilled early in the mission are consistent with low All rocks drilled and analyzed to date with the CheMin X-ray permeability at the time of fracturing (Grotzinger et al., 2014). Cumu- diffraction instrument indicate the presence of a significant portion of latively, these observations are consistent with rocks in Mt. Sharp having poorly crystalline (X-ray amorphous) material (Dehouck et al., 2014; porosity values akin to sedimentary rocks on Earth. * Corresponding author at: Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA. E-mail address: [email protected] (B.C. Johnson). https://doi.org/10.1016/j.icarus.2021.114539 Received 29 January 2021; Received in revised form 13 May 2021; Accepted 13 May 2021 Available online 15 May 2021 0019-1035/© 2021 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). B.C. Johnson et al. Icarus 366 (2021) 114539 Estimates of the bulk density of rocks in lower Mt. Sharp were recently discussed by Lewis et al. (2019), who used a novel approach to extract gravimetric data from the Curiosity rover along its traverse path. Their model that best reproduces the observed gravity indicates the strata under the rover have a bulk density of 1680 ± 180 kg m 3. Based on observed mineral abundances, Lewis et al. (2019) estimate the rocks to have a grain density of 2810 ± 133 kg m 3, which implies the sedi- mentary rocks encountered in Gale have a high porosity of ϕ = 40 ± 6% and have experienced a maximum burial depth under aqueous condi- +1000 tions of 1600 800 m (Lewis et al., 2019). Together, these observations present a somewhat contradictory story. Drill penetration rates, rock textures, evidence of hydraulic frac- turing, and other visual evidence are consistent with strata in lower Mt. Sharp having physical properties similar to lithified sedimentary rocks on Earth. However, none of these observations provide a direct mea- surement of bulk density, and in some cases they are highly qualitative and poorly constrained (e.g., visual estimates of rock porosity). In contrast, the gravity data have been interpreted to indicate low bulk density and thus unusually high porosity values for martian mudstones and sandstones relative to their terrestrial counterparts, particularly for terrestrial rocks that may have been deeply buried. Density values estimated from the gravity data, however, are highly model-dependent. Lewis et al. (2019) assume that the observed gravity signature is produced only by the wedge of sediments with thickness equal to the change in elevation of the rover over its traverse. Left un- explored is the effect on gravity of deep pre-existing subsurface porosity associated with the original formation of the Gale impact crater. Inclu- sion of lateral variation in porosity deeper in the crust in models of observed gravity will change the estimates of the density of the sedi- mentary rocks encountered in Gale. In an attempt to reconcile the multitude of geological and geophysical observations, this study ex- plores the role of impact-induced porosity to better constrain gravity- derived density and implied porosity values for rocks in Gale crater. 2. Impact simulations To provide estimates of Gale’s post-impact gravity signature we simulate its formation using the iSALE-2D shock physics code (Collins et al., 2011; Wünnemann et al., 2006). We simulate a dunite impactor striking a target consisting of a basaltic crust overlying a dunite mantle, with a similar set-up to recent simulations of large lunar craters (Johnson et al., 2018). The material model used to represent the impactor and mantle comprises a tabular equation of state (Benz et al., 1989) and strength model (Davison et al., 2010) for dunite. The near surface of the martian crust is well described by finegrained basalt. The mechanical properties of the average Martian crust, however, are better represented by coarser grained gabbro of similar chemical composition (e.g. Taylor and McLennan, 2009). We therefore use a basalt equation of state for the crust (Pierazzo et al., 2005) combined with a temperature-, pressure- and damage-dependent strength model for gabbro (Potter et al., 2012). For convenience, we assume that the initial state of the crust is undamaged; however, as the rocks surrounding the crater are pervasively fractured by the expanding shock wave, the inclusion of preimpact damage in the target is not necessary. To facilitate late-stage collapse of the crater we employ the block model of acoustic fluidization (Melosh and Ivanov, 1999), which has been used previously to repro- duce the morphometry and subsurface structure of lunar (e.g., Wünne- Fig. 1. Generation of porosity during the formation of Gale crater assuming a mann and Ivanov, 2003; Milbury et al., 2015; Baker et al., 2016) and pre-impact crustal thickness of 30 km. Material shaded by porosity as indicated terrestrial (e.g., Ivanov, 2005; Collins et al., 2008; Collins, 2014; Rae by the scale bar at various times during crater formation as noted in the legend. et al., 2017, 2019) craters. The origin marks the point of impact. Thick curves mark crust-mantle and crust- The typical impact velocity at Mars is 13.1 km/s (Minton and Mal- vacuum interfaces. ◦ hotra, 2010) and the most likely impact angle is 45 . Thus, we simulate vertical impacts at a velocity of 9.3 km/s, the vertical component of a ◦ 13.1 km/s impact at 45 . Our simulations have a resolution of 200 m corresponding to 40 cells per projectile radius for our best-fitmodel. To account for the creation of pore space as geologic material deforms we 2 B.C.