Impact Generated Porosity in Gale Crater and Implications for the Density of Sedimentary Rocks in Lower Aeolis Mons

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Icarus 366 (2021) 114539

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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

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Fig. 2. Free-air gravity (top) Bouguer gravity (mid dle) and porosity (bottom) of our best-fit simulation assuming a pre-impact crustal thickness of 30 km, 1000s after impact. Material shaded by porosity as indicated by the scale bar. The origin marks the point of impact. Thick curves mark crust-mantle and crust- vacuum interfaces. Thin curves are contours of porosity of 0.1, 1, and 5% as indicated. The anomaly is calculated at an altitude of 2.5 km, which is above the crater rim. (Fig. 3 shows the results of the same anomaly calculated at an altitude of 1 km below the pre-impact surface, closer to the location of Curiosity).

include the dilatancy model of Collins (2014), using parameters that Goossens et al. (2017). In both cases, we set h = 3.9 km, which assumes h best reproduce the gravity signature of terrestrial and lunar craters has an inverse dependence on surface gravity. (Maximum dilatancy coefficient βmax = 0.045, Dilatancy pressure limit Gale is a 154-km-diameter basin with a 87-km-diameter peak-ring, a plim = 200 MPa, Critical distension αc = 1.2, Critical friction coefficient ~ 4.5 km rim-floordepth, and a likely central peak covered in sediments μc = 0.4). (Allen, 2015; Baker, 2016; Schwenzer et al., 2012). Apart from a We simulated impacts into targets with crustal thickness of 25, 30, significantly larger crater depth, the dimensions of Gale are similar to and 35 km to reflect uncertainty and azimuthal variation in crustal the well-studied Chicxulub crater on Earth. Previous simulations of the thickness at Gale (Smrekar et al., 2018). The crustal thickness models of Chicxulub impact (e.g, Ivanov, 2005; Collins et al., 2008; Rae et al., Goossens et al. (2017) suggest a pre-impact crustal thickness exceeding 2019), therefore, serve as a valuable reference point and guide for input 40 km. We focus on the lower range of possible crustal thickness, as parameter selection. Our simulation of Gale crater that provides the best mantle uplifts, which affect the post-impact gravity signature, form fit to observations uses a 16-km-diameter impactor and acoustic fluid more readily in thinner crust. Ultimately, we find that the gravity ization parameters γβ = 350 and γη = 0.005. Using these parameters anomaly of Gale is relatively insensitive to assumed crustal thickness. produces an 160 km diameter crater, with an approximately 40 km Observation and modeling demonstrate that pre-impact porosity can radius peak ring, and 4.4 km rim-floor depth, which is in good agree strongly affect the gravity signatures of lunar craters (Soderblom et al., ment with observations (Fig. 1c). In tests, we found that the topographic 2015; Milbury et al., 2015). Thus, in addition to crustal thickness, we expression of the peak ring is sensitive to changes in acoustic fluidization also explore the effect of pre-impact porosity. Besserer et al. (2014) parameters, but the subsurface structure is insensitive to these changes. found that on the Moon crustal porosity as a function of depth is well Thus, the gravity signature of simulated basin in the area of interest z ( ) = h – described by an exponential function, ϕ z ϕ0e , where z is depth, (46 51 km from basin center) is relatively insensitive to our choice of ϕ0 is surface porosity, and h is the porosity e-folding depth (h ≈ 9.0 km acoustic fluidization parameters. on average for the Moon). Here we simulate impacts into two Mars-like We firstdiscuss our best-fitmodel of the formation of Gale and later targets with pre-impact porosity that decays exponentially with depth discuss variations from this simulation. The formation of Gale crater and from a surface porosity ϕ0 of 10 and 23%, respectively, as found by its subsurface density distribution is similar to that seen in simulations of

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Fig. 3. Bouguer anomaly of our best-fitsimulation calculated at an altitude 1 km below the pre-impact surface. The simulation has a preimpact crustal thickness of 30 km and the gravity is calculated 1000s after impact. the Chicxulub crater (Collins et al., 2008; Rae et al., 2019). The transient similar upper-crustal subsurface porosity distribution is predicted by crater begins to collapse under gravity and the crater floorstarts to rise simulations of the Chicxulub crater (Rae et al., 2019). Those simulations 150 s after impact (Fig. 1a). Porosity is generated by shearing near the are supported by geophysical data that reveal the peak-ring coincides crater wall and in ejected material. The collapse of the crater then with an annulus of low density and low seismic velocity (Vermeesch produces a central uplift (Fig. 1b). This collapse and uplift deforms et al., 2009; Morgan et al., 2011) and a recent deep drill core through the material in the crater interior and generates porosity through much of Chicxulub peak ring that reveals pervasively fractured granitic rocks the thickness of the martian crust within the crater. Additionally, with measured porosities exceeding 10% (Christeson et al., 2018). The slumping of the crater walls generates more porosity in the near surface predicted subsurface porosity distributions at Gale and Chicxulub differ of the crater’s interior (Fig. 1b). As the central uplift collapses, it pushes in the lower crust, below about 10 km, owing to the three-times greater material outward toward the crater rim (Fig. 1c). The outward moving overburden pressure at the same depth on Earth, which suppresses material, seen as a bump approximately 30 km from the basin center dilatancy below ~10 km on Earth, but ~30 km on Mars. (Fig. 1c), will eventually become a peak ring approximately 40 km from From the final porosity and density structure of our simulated Gale the basin center (Fig. 1.d). crater we can calculate its gravity signature (Collins, 2014). Here we The formation of the peak ring causes significant late-stage defor calculate a synthetic Bouguer anomaly, which represents the Bouguer mation that increases porosity within and around the peak ring, such anomaly that would be measured assuming a reference crustal density of 3 that this region exhibits the highest porosity inside the crater (Fig. 1d). A ρref=2870 kg m , appropriate for non-porous basalt according to our

Fig. 4. Bouguer gravity (top) and Porosity (bottom) of our simulation with a preimpact crustal thickness of 25 km 1000s after impact. Material shaded by porosity as indicated by the scale bar. The origin marks the point of impact. Thick curves mark crust-mantle and crust-vacuum interfaces. Thin curves are contours of porosity of 0.1, 1, and 5% as indicated. The anomaly is calculated at an altitude of 2.5 km, which is above the crater rim.

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Fig. 5. Bouguer gravity (top) and Porosity (bottom) of our simulation with a preimpact crustal thickness of 35 km 1000s after impact. Material shaded by porosity as indicated by the scale bar. The origin marks the point of impact. Thick curves mark crust-mantle and crust-vacuum interfaces. Thin curves are contours of porosity of 0.1, 1, and 5% as indicated. The anomaly is calculated at an altitude of 2.5 km, which is above the crater rim. equation of state. Note that this synthetic Bouguer anomaly is the below the calculation altitude produce a negative anomaly; negative anomaly produced by lateral variation of porosity within the crust and relative densities above the calculation altitude make a positive does not include the gravity signature of the sediments (the corrections contribution to the gravity anomaly. The density anomalies in our for inclusion of sediments are described in the following section). First simulation are relatively long wavelength, however, and we found the we calculate the density anomaly of material, Δρ = ρrefϕ, where ϕ is calculated gravity in the region of interest is relatively insensitive to our the post-impact porosity. This approach ignores density changes caused choice of altitude for the gravity calculation (Figs. 2, 3). The magnitude by impact heating that would disappear after cooling. At any point in the of the change in gravity anomaly over Curiosity’s traverse is 2.0 mGal model, we can calculate the Bouguer anomaly as a sum of the gravita larger when calculated at an altitude 1 km below the pre-impact surface tional acceleration of the annuli, with densities equal to the previously as compared to an altitude 2.5 km above the pre-impact surface. calculated density anomalies, that represent each computational cell In addition to uncertainty in the average crustal thickness of Mars, (Turtle and Melosh, 1997). This approach allows all material below the Gale lies on the dichotomy between potentially thin crust in the northern free surface with a density different from the reference density to lowlands and thicker crust in the southern highlands (Smrekar et al., contribute to the calculated anomaly and is equivalent to performing a 2018). Assuming a crustal thickness of 30 km thick at the InSight landing full terrain correction. We note this also includes any anomalies arising site and a constant crustal density of 2800 kg m 3 the crust to the north from deflection in the crust-mantle interface as well as density differ of Gale is ~25 km thick while to the south the crust is ~50 km thick ences relative to the reference density above and below the calculation (Wieczorek et al., 2020). If the crust of the southern highlands and altitude. The final crater has a Bouguer gravity anomaly that decreases northern lowlands are assumed to have densities of 2550 and 2800 kg from the crater rim to about 30 km from the crater center (Fig. 2). Inside m 3, respectively, the crustal thickness at Gale is ~30 km and less of 30 km from the symmetry axis the Bouguer anomaly is relatively variable with azimuth (Wieczorek et al., 2020). To test how sensitive our constant at 75 mGal. This is a factor of about two larger than the results are to the choice of average crustal thickness or changes in crustal gravity low amplitude of the Chicxulub crater (Hildebrand et al., 1991; thickness from northern to southern portions of the basin we produced Rae et al., 2019), which we attribute to a combination of post-impact simulations with a crustal thickness of 25 and 35 km. Simulations with a sedimentary infill and greater overburden pressure at depth at crustal thickness of 25 or 35 km (Figs. 4, 5) result in similar gravity Chicxulub. signatures to our best-fit simulation with 30 km thick crust (Fig. 2). For our calculation of best-fit density, we focus on the Bouguer Thus, our simulation with a 30-km-thick crust is representative of the anomaly at an altitude 1 km below the pre-impact surface. This altitude expected gravity signature of Gale. We expect further increasing crustal is near the crater surface at Curiosity’s position and minimizes differ thickness would not have a significant effect on our results as there is ences between Curiosity’s measurements and our calculations of the little deformation of the mantle in our simulation with a 35 km pre- gravity signature of the simulated crater. Negative relative densities impact crustal thickness (Fig. 5).

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Fig. 6. Bouguer gravity (top) and Porosity (bottom) of our simulation with preimpact porosity with ϕ0 = 10% that exponential decreases with an e-folding depth, h = 3.9 km, 1000s after impact. This model has a preimpact crustal thickness of 30 km and. Ma terial shaded by porosity as indicated by the scale bar. The origin marks the point of impact. Thick curves mark crust-mantle and crust-vacuum in terfaces. Thin curves are contours of porosity of 0.1, 1, 5, and 10% as indicated. The anomaly is calculated at an altitude of 2.5 km, which is above the crater rim.

When we include pre-impact porosity, pore space is firstcrushed out in gravity or the gravity anomaly (Lewis et al., 2019). Thus, our models near the point of impact by the expanding shockwave. This results in fit the change in gravity over the rover’s traverse and include a term reduced porosity and higher gravity anomalies at the center of the crater δ0 that accounts for the arbitrary constant offset between the observed (Figs. 6, 7). In our simulations with ϕ0 = 23% and 10%, respectively, the and calculated anomaly. In the case of Curiosity, we do not have an anomaly at the center of the crater is 90.4 and 39.3 mGal higher than the independent reference for leveling so the measured gravity is taken to be anomaly calculated in our simulation with no preimpact porosity vertical in the coordinate system of our impact simulations. Off vertical (Fig. 2). However, regardless of preimpact porosity, the additional pore components of gravity will be small so we expect uncertainty introduced space generated outside of the compaction zone during collapse of the by this necessary assumption will be small. For more information on central uplift and formation of the peak-ring results in a decreasing these corrections see Lewis et al. (2019). In addition to these corrections anomaly, with a similar magnitude, over Curiosity’s traverse (from we also include a correction that accounts for the expected gravity post- approximately 51 to 46 km from basin center). Compared to the simu impact δgimp, which is equal to the Bouguer anomaly at a given distance lation with no pre-impact porosity, the magnitude of the change in from basin center derived from our numerical simulation, as shown in anomaly over the traverse is 1.2 and 2.0 mGal lower for the simulations Fig. 3. This correction adds the anomaly from Fig. 3 as a function of the with ϕ0 = 10% and 23%, respectively. A high pre-impact porosity ϕ0 = rover’s distance from the center of the basin. Adding in the correction for 23% would reduce our best-fitdensity by ~200 kg/m3. Thus pre-impact expected post-impact gravity reduces the expected anomaly below the porosity has a modest effect when estimating the density of sediments free-air calculation (cf., black and blue curves Fig. 8c). Thus, a higher comprising Mount Sharp. If the pre-impact porosity extends deeper into density is needed to reproduce the observed gravity. Including the the Martian subsurface than assumed here (i.e., h > 3.9 km), we would correction for expected post-impact gravity for a target with no pre- expect a further reduction in our best-fit density. impact porosity we find a best-fit density of 2300 ± 130 kg m 3 (1σ) compared to 1680 ± 180 kg m 3 (1σ) assuming no deeper pre-existing 3. Estimating density of Mount Sharp anomalies (Lewis et al., 2019). The reported uncertainty is based on fits to the data, but does not include uncertainty associated with our Following Lewis et al. (2019) we calculate the density of Mt. Sharp impact simulations. that best reproduces the gravity observed by Curiosity. The gravity We note that because Curiosity is generally moving toward the basin anomaly is calculated including corrections for latitude δgλ, free air δgfa, center as it ascends Mt. Sharp, elevation and distance from basin center and complete Bouguer correction δgB. The complete Bouguer correction, exhibit a good negative correlation, meaning as distance from basin not to be confused with the Bouguer anomaly, accounts for the gravi center decreases elevation tends to increase (Fig. 8 a,b). In addition, the tational acceleration of rock between the measurement elevation z and post-impact gravity in our simulation correlates well with distance from the reference z0 = 0; it is given by δgB = ρ(2πGz + δgT), where ρ is density basin center (Fig. 3) and therefore topography. This explains why both of Gale’s mound, G is the gravitational constant, and ρδgT is the terrain models that include the expected post-impact gravity signature and correction out to 75 km. Note that the accelerometers on Curiosity do those that do not can produce good fitsto the observations, albeit with not act as an accurate absolute gravimeter, but are sensitive to changes different best-fit densities. Although both models produce reasonable

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Fig. 7. Bouguer gravity (top) and Porosity (bottom) of our simulation with preimpact porosity with ϕ0 = 23% that exponential decreases with an e-folding depth, h = 3.9 km, 1000s after impact. This model has a preimpact crustal thickness of 30 km and. Ma terial shaded by porosity as indicated by the scale bar. The origin marks the point of impact. Thick curves mark crust-mantle and crust-vacuum in terfaces. Thin curves are contours of porosity of 0.1, 1, 5, 10, and 15% as indicated. The anomaly is calculated at an altitude of 2.5 km, which is above the crater rim.

fits,the model including the expected post-impact gravity signature has worth noting that our analysis is only sensitive to the average density of a slightly better fit (± 130 kg m 3 at 1σ level) than the model that ig sediments, we expect sediment near the surface contain more porosity nores the possibility of a post-impact signature (± 180 kg m 3 at 1σ and are less dense than deeper material. Through the terrain correction level). Perhaps as Curiosity continues toward the basin center there will our calculation also assumes the density of any surrounding topography be a deviation between the two models and the data will more strongly has the same density as the sediments. Curiosity is relatively far from the favor one interpretation over the other. This is most likely to happen if crater rim, the nearest topographic feature expected to have a density the rover continues toward the basin center without ascending much in substantially higher than the sediments (Fig. 9). The terrain correction elevation or if it begins to ascend Mt. Sharp more steeply than it did for results in 2.6 mGal of variation over the rover traverse and scales line the data used in the present study. The latter is relatively likely given arly with assumed density. Thus, our results are relatively insensitive to that the regional surface slopes of Mt. Sharp are generally steeper the assumed density in the terrain correction. The final assumption is upsection. In the meantime, we note that a density value of 2300 ± 130 that the sediments of Gale lay atop a relatively flat basin floor. An kg m 3 is fully consistent with the textural, sedimentological, and dril alternative assumption, which predicts the same gravity, is that the ling data observations discussed above. basement rocks have appreciable topography along Curiosity’s traverse but exhibit no density contrast across the post-impact surface and sed 4. Discussion iments of Mt. Sharp. The models of Lewis et al. (2019) contain these same assumptions. Unfortunately, we do not have estimates of the basin Although most impacts are oblique, to reduce computational floor topography underneath Mt. Sharp. Our current simulations only expense, our impact simulations are axisymmetric and assume impacts resolve basin topography to at best ±400 m, the size of two high- of vertical incidence. In addition to possible North-South asymmetries in resolution cells. It is not clear that impact simulations should be trus crustal thickness at Gale, oblique impacts are known to cause asym ted to predict topography at scales less than 100 m for a basin of this metries in crater collapse (Elbeshausen et al., 2009). These asymmetries size, especially considering the possibility of erosion and degradation of would likely cause some uprange-downrange asymmetries in the post-impact topography. porosity produced during collapse of the transient crater and formation We put forward an example of basin floor topography and density of the peak ring. However, we expect that the variation in gravity with contrast to understand the magnitude of the effect they would have on azimuth to be substantially less than the variation with radius. At modeled gravity. If the relief at the interface between sedimentary de Chicxulub, for example, which was likely formed by a moderately posits and the basement changes by 100 m over Curiosity’s traverse and oblique impact (Collins et al., 2020), the azimuthal variation in gravity the density contrast between the sediments and basement is 250 kg m 3, anomaly over the peak ring is <10 mGal, which is less than one third of this would provide an added 1 mGal change in the gravity anomaly the anomaly magnitude (Rae et al., 2019). along the traverse. This would roughly correspond to a change in the Our model of the measured gravity has three major assumptions. One best-fit density of sediments by 100 kg m 3. A proportionally larger is that the density of the sediments that infilled Gale crater, including signal is expected for a larger density contrast or change in relief along those that form Mt. Sharp, is constant along Curiosity’s traverse. It is also the traverse.

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a 51

Distance from basin center (km) 46 0 200 400 600 800 1000 1200 1400 1600 1800 Sol b -4200

-4300

-4400 Elevation (m) -4500

0 200 400 600 800 1000 1200 1400 1600 1800 Sol c 30

-10

-20 Anomaly (mGal) -30 Observed -3 -40 Best fit =2300 260 kg m Free air -50 Free air + iSALE

-60 0 200 400 600 800 1000 1200 1400 1600 1800 Sol

Fig. 8. Modeled effect of rovers distance from basin center (a) and elevation (b) on the gravity anomaly measured by Curiosity (c). Gravity anomaly models in (c) are “Free-air” = δgλ + δgfa + δ0, “Free-air + iSALE” = δgλ + δgfa + δgimp + δ0, and “Best fit” = δgλ + δgfa + δgimp + δgB + δ0. The best fitis plotted along with the 2σ range in density. This is calculated for the impact model with no pre-impact porosity.

Another implicit assumption is that the observed gravity produced an impact into a target with no pre-impact porosity, implies the strata in by Lewis et al. (2019) is accurate. One source of uncertainty in this lower Mt. Sharp have a porosity of 18 ± 6%. Considering a pre-impact assumption is gravimeter drift. Usually gravimeter drift is accounted for porosity with ϕ0 = 23% reduces our estimate of bulk density by by taking multiple readings at the same location at different times in the ~200 kg/m3 implying a porosity of ~25%. According to the pore space gravity survey and assuming a linear drift in time. Making multiple closure calculations of Lewis et al. (2019) a porosity of 18 ± 6% cor measurements at the same location at different times is not feasible in responds to a burial depth of up to ~4–5 km for fine-grainedsediments. this case. Lewis et al. (2019) found that including a correction for linear This maximum burial depth could be larger if sediments were buried drift did not reduce residuals between observed and modeled gravity. It under dry conditions. Maintaining this porosity during the construction is likely that some of the short wavelength variation in the observed of Mt. Sharp allows for, but does not require, the entirety of Gale crater gravity with an amplitude ~10 mGal are caused by gravimeter drift. to have once been filled with sediment (e.g., Malin and Edgett, 2000). Regardless of the veracity of the observed gravity produced by Lewis Although the specific cementing agents and processes responsible for et al. (2019), our simulations show that the impact produced porosity lithificationof strata on Mars remain largely unknown, the observation will tend to increase the inferred density of the sediments of Mt. Sharp. of various diagenetic features indicative of late stage fluidcirculation (e. With a grain density of the Murray formation of 2810 ± 133 kg m 3 g., Stack et al., 2014; Kronyak et al., 2019; Sun et al., 2019), coupled (Lewis et al., 2019), our best-fitbulk density of 2300 ± 130 kg m 3, for with potential burial by several kilometers of material, indicate

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Fig. 9. Topography of Gale crater with Curiosity traverse (white). Mars orbiter laser altimeter (MOLA) topography (Smith et al., 2001) on High-Resolution Stereo Camera imagery (Jaumann et al., 2007). circulation of deep basinal brines may have been an important process in during the Mars science Laboratory’s prime Mission. Icarus 319, 1–13. https://doi. the cementation (e.g., precipitation of solutes) and evolution of Mt. org/10.1016/j.icarus.2018.09.004. Allen, C., 2015. A massive central peak and low peak ring in Gale crater – Important Sharp. Our updated estimates of rock density and porosity based on in influences on the formation of Mt. Sharp. In: 46th Lunar and Planetary Science situ gravity data are in broad agreement with values typical of many Conference #2787. sedimentary rocks on Earth. The lower lithostatic pressures on Mars is a Baker, D., 2016. Updated catalogs of peak-ring and protobasins on Mars. In: 47th Lunar and Planetary Science Conference #3046. key difference affecting lithification of sediments on Mars compared to Baker, D.M.H., Head, J.W., Collins, G.S., Potter, R.W.K., 2016. The formation of peak- Earth, but ultimately the processes associated with lithification and ring basins: working hypotheses and path forward in using observations to constrain burial of sediments on Mars are likely just as diverse and complex as on models of impact-basin formation. Icarus 273, 146–163. https://doi.org/10.1016/j. icarus.2015.11.033. Earth. Banham, S.G., Gupta, S., Rubin, D.M., Watkins, J.A., Sumner, D.Y., Edgett, K.S., Grotzinger, J.P., Lewis, K.W., Edgar, L.A., Stack-Morgan, K.M., Barnes, R., Bell, J.F., Declaration of Competing Interest Day, M.D., Ewing, R.C., Lapotre, M.G.A., Stein, N.T., Rivera-Hernandez, F., Vasavada, A.R., 2018. Ancient Martian aeolian processes and palaeomorphology reconstructed from the Stimson formation on the lower slope of Aeolis Mons, Gale None. crater, Mars. Sedimentology 65, 993–1042. https://doi.org/10.1111/sed.12469. Benz, W., Cameron, A., Melosh, H.J., 1989. The origin of the moon and the single-impact hypothesis III. Icarus 81, 113–131. https://doi.org/10.1016/0019-1035(89)90129- Acknowledgements 2. Besserer, J., Nimmo, F., Wieczorek, M.A., 2014. GRAIL gravity constraints on the vertical – We thank two anonymous reviewers for their helpful comments. We and lateral density structure of the lunar crust. Geophys. Res. Lett. 41, 5771 5777. https://doi.org/10.1002/2014gl060240. gratefully acknowledge the developers of iSALE-2D, including Kai Caswell, T.E., Milliken, R.E., 2017. Evidence for hydraulic fracturing at Gale crater, Mars: Wünnemann, Dirk Elbeshausen, Tom Davison, Boris Ivanov and Jay implications for burial depth of the Yellowknife Bay formation. Earth Planet. Sci. Melosh. Some plots in this work were created with the pySALEPlot tool Lett. 468, 72–84. https://doi.org/10.1016/j.epsl.2017.03.033. Christeson, G.L., Gulick, S., Planetary, J.M.E.A., 2018. Extraordinary rocks from the peak written by Tom Davison. GSC acknowledges funding from the Science ring of the Chicxulub impact crater: P-wave velocity, density, and porosity and Technology Facilities Council, Grant ST/S000615/1. All data asso measurements from IODP/ICDP Expedition 364. Icarus 495, 1–11. https://doi.org/ ciated with figures including iSALE simulation inputs and results are 10.1016/j.epsl.2018.05.013. Collins, G.S., 2014. Numerical simulations of impact crater formation with dilatancy. available on Harvard Dataverse https://doi.org/10.7910/DVN/62D J. Geophys. Res. Planets 119 (12), 2600–2619. https://doi.org/10.1002/ LBN. 2014JE004708. Collins, G.S., Morgan, J., Barton, P., Christeson, G.L., Gulick, S., Urrutia, J., Warner, M., Wünnemann, K., 2008. Dynamic modeling suggests terrace zone asymmetry in the References Chicxulub crater is caused by target heterogeneity. Earth Planet. Sci. Lett. 270, 221–230. https://doi.org/10.1016/j.epsl.2008.03.032. Abbey, W., Anderson, R., Beegle, L., Hurowitz, J., Williford, K., Peters, G., Morookian, J. Collins, G.S., Melosh, H.J., Wünnemann, K., 2011. Improvements to the ε-α porous M., Collins, C., Feldman, J., Kinnett, R., Jandura, L., Limonadi, D., Logan, C., compaction model for simulating impacts into high-porosity solar system objects. McCloskey, S., Melko, J., Okon, A., Robinson, M., Roumeliotis, C., Seybold, C., Int. J. Impact Eng. 38, 434–439. https://doi.org/10.1016/j.ijimpeng.2010.10.013. Singer, J., Warner, N., 2019. A look back: the drilling campaign of the curiosity rover

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