Grand canyons on the Moon

Domain: earth_science
Published: 2025-02-04
Editorial outcome: major_revision

Abstract
High energy streams of rock ejected from the Schrödinger impact basin carved two canyons into the lunar crust that are comparable in size to the Grand Canyon of North America. Here we use photogeologic mapping of those canyons and related impact ejecta deposits to show the trajectory of the impacting asteroid or comet, which produced an asymmetrical pattern of crater excavation and transport of ejected debris. The flow directions of that ejected debris and the speed of its subsequent impact with the lunar surface are calculated, as is the energy that carved the canyons in less than ten minutes. The study implies that most of the excavated debris was ejected away from the lunar south pole, minimizing the amount of debris that covers the >4 billion year old units that will be explored by Artemis astronauts.

Introduction
A global lunar landing site analysis1 found that the Schrödinger impact basin2 is the best location for addressing National Research Council science objectives for lunar exploration3, prompting geologic studies of the basin and analyzes of potential landing sites for human and robotic missions4–18. The first robotic mission is scheduled to land in 2026. Schrödinger basin is also the best analog surface expression for Earth’s buried Chicxulub impact crater19, which is linked to the extinction of dinosaurs and most life at the end of the Cretaceous20–23.

The Schrödinger impact basin is remarkable for streams of rocky debris that it ejected, carving two canyons that are comparable to Earth’s Grand Canyon in width and depth. Here we describe those canyons, how they formed, and the implications they have for the trajectory of the impactor.

Results
Geologic context
The Schrödinger impact basin (Fig. 1) is a peak-ring basin, ~320 km in diameter, ~4.5 km deep, centered at 75°S, 132.5°E (ref. 8, Supplementary Fig. 1), with an estimated age of 3.81+0.013/-0.014 Ga (ref. 24), although some models suggest ~0.15 Ga uncertainty on basins with late Imbrian ages25. Schrödinger’s ~150 km diameter peak ring, a circular mountain range that rises 1 to 2.5 km above the basin floor, was produced by the collapse of a central uplift14. Basalt lava flows and a large pyroclastic vent later erupted on the basin floor and ceased erupting 3.70+0.02/-0.03 Ga (ref. 18). Schrödinger formed in the outer margin of the ~2400 km-diameter South Pole-Aitken (SPA) basin26, which is the largest and oldest impact basin on the Moon. The rim of the Schrödinger basin is within 300 km of the south pole and within 125 km of the Artemis exploration zone, which is the first destination of Artemis astronauts.

The Schrödinger basin is surrounded by an asymmetrical ejecta blanket (Fig. 1) that has been mapped27 up to distances of 500 km from the basin rim, although ejecta components not visible from orbit would have been deposited at far greater distances. The basin is also surrounded by several impact ejecta rays (ref. 28; Supplementary Figs. 2 and 3), including Vallis Schrödinger and Vallis Planck (Fig. 1), which are the foci of this work. Such rays are the result of closely-spaced and radially-aligned secondary impact craters that excavated deep canyons into the lunar crust.

Canyon dimensions
Vallis Schrödinger is ~270 km long, ~20 km wide, ~2.7 km deep, and occurs within the limits of the continuous ejecta blanket. The deepest portion of Vallis Planck is ~280 km long, ~27 km wide, and ~3.5 km deep, but a ray of secondary craters extends beyond the continuous ejecta blanket to a length of 860 km. For comparison, the length of the sinuous Grand Canyon is 446 km (277 river miles) between Lees Ferry and Grand Wash Cliffs, which includes the length of Marble Canyon. The deepest point of the Grand Canyon is 1.949 km (6393 ft) below the north rim near Nankoweap Rapids and 1.452 km (4765 ft) below the south rim near Cremation Creek. For illustration purposes (Fig. 2), a transect of Vallis Planck is compared with the Grand Canyon along the Bright Angel Trail, which is a location familiar to most Grand Canyon National Park visitors.

Fifteen secondary craters are discernable along the length of Vallis Schrödinger, with diameters between 10 and 16 km, clearly defined by their eroded but still visible crater rims (Fig. 3, Supplementary Fig. 4). The maximum diameter of secondary craters produced by primary craters ≤260 km in diameter is 4% of the primary crater’s diameter29, which, if extrapolated to the 320 km diameter Schrödinger basin, implies 13 km. Observed 10–16 km dimensions of secondary craters in Vallis Schrödinger approach and slightly exceed that value.

On the Moon, the depth/diameter (d/D) of fresh primary craters <15 km in diameter is 0.196 +0.038/-0.027 (ref. 30) with those 400 m to 10 km in the polar region is 0.21 (ref. 31). The d/D value of lower-velocity secondary craters is less, ∼0.1 for small craters32. While no fresh secondary craters produced by basin-size primary craters exist, we infer that the craters along Vallis Schrödinger likely had a d/D value <0.2 when formed. Measurements after ~3.8 billion years of erosion indicate an average d/D of 0.14 for Vallis Schrödinger and 0.11 for Vallis Planck.

Craters are partly overlapping and superposed by younger craters. Secondary craters are not everywhere preserved along the length of Vallis Planck, which was modified by the collapse of canyon walls as seen in images (Fig. 3) and a topographical transect (Fig. 2). Both canyons suffered erosion, mostly driven by impact gardening of the regolith, seismic slumping, and downslope creep. The upper canyon walls are the steepest portions of the crater rays and have slopes of 27 to 34°, which is the angle of repose for lunar highland material (30°; ref. 33).

Canyon excavation
Velocities of secondary ejecta are 0.95, 0.97, and 1.05 km/s for Vallis Schrödinger assuming ballistic angles of 45, 30, and 20°, respectively (Fig. 4) (see “Methods”). Maximum velocities of ejecta to produce Vallis Planck are larger (1.23 to 1.28 km/s), reflecting longer travel distances. Those velocities are about 50% of the escape velocity from the Moon (2.38 km/s). Flight times of debris producing the 270 km-long main canyon are 4.9 to 15.0 min for Vallis Schrödinger over the entire range of potential ejection angles, with canyon-forming secondary impacts occurring within a 5 min interval. Flight times of debris producing the 280 km-long main canyon are 5.2 to 15.4 min for Vallis Planck over the entire range of potential ejection angles, with canyon-forming secondary impacts occurring within a 5 min interval.

At these ejection speeds, theoretical estimates of mean ejecta fragment sizes range from one-fiftieth to one-twentieth of the primary impactor diameter34, which was ~25 km for the Schrödinger impact14. This implies mean ejecta fragment sizes of 0.5 to 1.25 km diameter at the range of the Schrödinger canyons. This is consistent with our calculated secondary crater projectile diameters for Vallis Planck, which are mostly <2 km (Fig. 4). On the other hand, the implied projectile diameters for the secondary craters that carved Vallis Schrödinger range from 2.3 to 5.2 km, which are considerably larger than theory would suggest. This suggests that the canyon-carving secondary craters for the more proximal Vallis Schrödinger formed by near simultaneous impact of clusters of ejecta fragments, rather than by impact of individual ejecta fragments. Clusters of secondary craters are commonly observed35,36 and produce shallow d/D as observed in the Schrödinger rays. Further, those clusters of fragments spread out with radial distance beyond the continuous ejecta blanket36. Thus, it is logical that clusters closer to the point of impact (i.e., within the continuous ejecta blanket) would have a smaller spread (be more tightly clustered) and produce simple craters like those observed. In the more distal portions of Vallis Planck, where the areal density of ejecta fragments is lower, the secondary craters are more likely to be formed by individual fragments.

As shown in Fig. 4, the transition in fragment size is abrupt and coincides with the edge of the continuous ejecta blanket. The transition is evident both between the rays and within the Vallis Planck data, where one crater lies within the continuous ejecta. This suggests that the difference in fragment size (or crater size) cannot be attributed to target heterogeneities between the two rays. Either side of the transition, fragment sizes are consistent as a function of distance, which is one of the criteria used to discriminate secondary craters from primary craters37.

Trajectories and energy
The canyon-forming rays are asymmetrically distributed around the Schrödinger basin and do not converge at the basin center. Rather, the bearings of Vallis Schrödinger and Vallis Planck converge at 78.21947°S, 143.71996°E (Fig. 1). If that convergence is the point of first impact38, then a line from that point through the basin center suggests a SSE to NNW trajectory (33.5° west of north; Fig. 1) for the impacting asteroid or comet. Thus, rather than a point explosion, like that characterizing small impact events (e.g., Meteor Crater on Earth39), the distance between that point of impact and the center of the basin implies a distributed impact zone. That distributed impact zone is reflected in the bearings of other, smaller rays of ejecta (Supplementary Fig. 3) that were produced by material ejected as the projectile penetrated deeper into the target and it moved towards the basin center40. A distributed impact zone may imply a relatively low primary impact angle38,41,42. Likewise, it may imply relatively low angles for secondary ejecta, ranging from 20 to 45° (ref. 42). Because the secondary craters are circular, the ballistic flight angles are probably in the upper end of that range (i.e., 30 to 45°; ref. 43).

Alternatively, the rays may be lateral sprays of ejecta44 (i.e., part of a butterfly pattern) from an impact with a trajectory of 22.3° east of north, as implied by the largest swath of continuous ejecta (Fig. 1). In that case, ballistic launch angles would have been higher. We derive from published data44 that they would have been 51.3±6.5° and 46.0±6.9° for Vallis Schrödinger and Vallis Planck, respectively.

The minimum kinetic energy necessary to produce measured craters along Vallis Schrödinger is 1.68 × 1020 J. Because most craters are overlapping, the minimum kinetic energy is likely to be overestimated (~15%) in regions with high secondary crater density. However, gaps between identified craters in other regions offset this effect. Assuming a similar distribution of excavating impacts along the entire crater ray implies a total kinetic energy of 3.39 × 1020 J. Values for Vallis Planck are less well constrained, but higher (~1.21 × 1021 J) because of its greater length. The energy to produce the grand canyons on the Moon are 1200–2200 times larger than the nuclear explosion energy once planned to excavate a second Panama Canal on Earth, more than 700 times larger than the total yield of US, USSR, and China’s nuclear explosion tests, and about 130 times larger than the energy in the global inventory of nuclear weapons45–48 (Supplementary Table 1).

The extrapolated convergence of Vallis Schrödinger and Vallis Planck suggest a trajectory 33.5° west of north for the asteroid or comet that produced the Schrödinger basin. In contrast, the continuous ejecta blanket covers the greatest distances in a direction 22.3° east of north. We note that direction is downslope, towards the center of the SPA basin. Thus, ejecta may have flowed farther in the downhill direction, toward the SPA basin center, and ejecta blanket extent may not be a measure of the impactor’s trajectory. Towards the center of SPA, the edge of the continuous ejecta blanket is ~1 km below the rim of the Schrödinger basin, while the topography rises ~4 km in the opposite direction. Nonetheless, an impactor trajectory 22.3° east of north, as implied by the largest swath of continuous ejecta, rather than 33.5° west of north as implied by the orientation of crater rays, remains possible and is consistent with the rays being lateral sprays of ejecta44. In either case, the trajectory of the basin-forming impact is away from the south pole.

Two other asymmetries exist in the Schrödinger basin. A portion of the peak ring is not visible at the surface in one quadrant. That observation was attributed to a preexisting weakness in the crust produced by an older Amundsen-Ganswindt basin-forming impact event2. Also, the peak ring rises higher on one side of the basin than on the other, which has been attributed to a change in crustal thickness from 20 to 40 km in the target rocks hit by the Schrödinger impactor14,19. Those pre-impact target properties should not, however, have produced two rays that converge near the edge of the Schrödinger basin, rather than its center. An oblique impact away from the south pole must still be the root cause for that convergence. We also note that those peak-ring asymmetries are not aligned with the trajectory of the projectile, implying target properties were more important than trajectory in shaping the peak ring.

Implications for Artemis lunar science
The Schrödinger impact trajectory has important implications for the pending Artemis missions to the south polar region. If ejecta had been distributed symmetrically around the basin, scaling relationships for complex crater ejecta49 imply 176+59/-44 m (at the edge of the Artemis exploration zone) to 42+29/-17 m (at the south pole) of ejecta would cover the region. However, a shallow impact trajectory away from the south pole implies highly asymmetric ejecta with a thinner and potentially absent distal ejecta blanket over the Artemis exploration zone. This resolves one of the two great uncertainties in estimating the amount of post-SPA debris in the Artemis exploration zone. Still needed is a similar assessment of the distribution of ejecta around the Amundsen-Ganswindt basin which has the potential to cover SPA ejecta, too.

The asymmetric ejecta distribution implied by Schrödinger’s crater rays suggest there is less Schrödinger impact ejecta covering candidate landing sites and, thus, astronauts and robotic assets will find it easier to sample SPA and underlying primordial crust samples. Impact craters of order 1 km (similar in size to Arizona’s Meteor Crater) can penetrate, excavate, and deposit on the surface material from beneath the Schrödinger impact ejecta blanket. Excavation depth is approximately 1/10 the crater diameter50, which would be 100–200 m for craters 1–2 km diameter. An impact code parameterized for lunar regolith,51 suggests excavation depths down to 300 m are possible for a 1 km diameter crater. Those values are consistent with the observed excavation and transient crater depths of the 1.25 km Meteor Crater, which are ~150 m and ~335 m, respectively52. Of the 5251 impact craters >1 km in diameter within the Artemis exploration zone, 3243 of them have diameters between 1 and 2 km (ref. 53), indicating access to superposed units (Amundsen-Ganswindt and SPA ejecta) may be common.

The Schrödinger impact basin is the second youngest basin on the lunar surface, so samples of impact melt could be used to test its estimated 3.81+0.013/-0.014 Ga age (ref. 24) and, thus, test the lunar impact cataclysm hypothesis, which posits an enhanced period of impact bombardment at that time54; this is the highest priority objective of the National Research Council3. Samples from the underlying SPA impact ejecta blanket will provide samples of SPA impact melt, from which the age of the oldest and largest basin on the Moon can be determined. SPA excavated the entire crustal column and upper mantle55, so a cross-section of the Moon to a depth of ~100 km may be accessible, albeit dismembered. Where craters puncture the SPA ejecta blanket, they can expose primordial crust. Excavated SPA material and any primordial crust can be used to test the lunar magma ocean hypothesis for planetary differentiation and the giant impact hypothesis for the origin of the Earth-Moon system, among many other objectives. Because the Schrödinger impact event dispersed the bulk of its ejecta away from Artemis candidate landing sites, those objectives are more likely to be met.

Terrestrial vs lunar canyon carving processes
The well-preserved Schrödinger basin crater rays provided an opportunity to calculate the velocity of secondary impactors (0.95 to 1.28 km/s), the size distribution of those impactors (up to 5.2 km diameter), and kinetic energy that excavated the resulting canyons (3.39 × 1020 J for Vallis Schrödinger and 1.21 × 1021 J for Vallis Planck). The orientations of the rays imply a primary impact bearing of 33.5° west of north and an impact angle <45° if they formed from down-range ejecta or, alternatively, a bearing of 22.3° east of north if they formed from lateral ejecta. Whereas Arizona’s Grand Canyon was carved by water over the last 5 to 6 million years and from integrated paleocanyons that formed over 70 million years56,57, the Moon’s Vallis Schrödinger and Vallis Planck were carved by streams of impacting rock in less than 10 min.

Methods
Spacecraft data analysis
Spacecraft images and elevation data were derived from NASA’s Lunar Reconnaissance Orbiter. Data were accessed through the Data Node for Lunar Reconnaissance Orbiter Camera (LROC) Planetary Data System (PDS) products using its Geographic Information System (GIS) QuickMap tool58, which integrates LROC images with Lunar Orbiter Laser Altimeter (LOLA) elevation data. We used Wide-Angle Camera (WAC) images (100 m/px); narrow-angle camera (NAC) images (0.5 to 2 m/px); NAC region of interest (ROI) mosaics in the Lunar Globe (3D) view; and terrain elevation and slope derived from the LOLA (version SLDEM2015 (+LOLA)) (ref. 59). Secondary craters were identified photogeologically and topographically. QuickMap arc and elevation transect tools were used to measure rim-to-rim diameters of secondary craters in ejecta rays. Three rim-to-rim measurements were taken (i) along the crater ray, (ii) approximately perpendicular to the ray, and (iii) another location where the crater rim was well defined. If the crater rims were ambiguous or the crater seemed heavily modified, a fourth, additional measurement was taken. Possible craters without clearly defined crater rims, too heavily modified craters or craters that are more likely to have been caused by a separate (later) event were disregarded. Some craters in Vallis Planck are marked as “inferred” craters. These likely craters are included in the figures but were not used for any crater scaling or energy calculations. Based on the rim-to-rim diameters, the craters were then (manually) approximated first with a polygon and then with a circle. The final diameter measurement used in subsequent calculations is the diameter of the circle. We compared the average of the three rim-to-rim diameter measurements with the circle diameter. For most craters in Vallis Schrödinger this “error” is less than 5%. It is slightly higher in Vallis Planck due to canyon wall modifications affecting the rim-to-rim diameter measurements. Depth measurements were taken along the profile lines used for the rim-to-rim diameter measurements. Along each of those profile lines, the elevation of the crater rims and the lowest point was determined. The depth was then calculated as (rim_1 + rim_2)/2 – low. For each crater the maximum depth out of those three or four profiles was then selected. The SLDEM has a typical vertical accuracy of ~3 to 4 m which is sufficient for the depth measurements. For each secondary crater, measurements were made of the distance from the Schrödinger basin center, the intersection point with an inferred transient crater rim14 on that trajectory, the intersection point of the two largest crater rays (Fig. 1), and the intersection point with the transient crater rim on this trajectory. The procedure accounts for all possible ejection points and, therefore, can be used to estimate minimum to maximum secondary ejecta velocities. For visualization of the results we used ArcMap (v. 10.5.1) and ArcGIS Pro.

Ballistic flight calculations
The crater rays present a unique opportunity to calculate impact velocity (ν) using measured distances between the secondary craters and their ejection point in a ballistic formula for a curved planetary surface:

ν = [gR tanΦ / (sinθcosθ + cos2θtanΦ)]^1/2 (1)

where Φ = R/2Rp. We considered ejection and secondary impact angles (ϴ) of 20°, 30°, and 45° to account for uncertainty in primary impact angle and ejecta launch position.

Impact scaling calculations
Calculated secondary impact velocities and the diameters of each crater were then used with an analytical scaling relationship for dry sand60,61 assuming anorthositic norite material8 with a density of 2,660 kg/m3 (ref. 62) and a gravity-dominated impact regime, to determine the sizes of impactors and kinetic energy that produced the secondary craters. The secondary craters that produced the canyons are several hundred kilometers away and were formed by impacts at speeds close to 1 km/s. This speed is only just below the speed range of the impact experiments that have been used to develop the crater scaling relationships that we apply in this work60. These scaling laws have been shown to be appropriate for some conditions at even lower impact speeds43,63–66.

Data availability
All data associated with this study are shown in figures and provided in Supplementary Information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Referee report for “Grand Canyons of the Moon” by Kring et al.\nThe manuscript is well written. Schrodinger is a well-studied lunar basin, and as such the manuscript expands on a wide\nbody of literature. The actual new work presented in this manuscript is somewhat limited, but I think it would be an\ninteresting addition given the implications for Artemis astronauts' exploration of the Moon. I suggest publication, pending\nsome minor improvements listed below.\nln 33: The manuscript builds on a large body of work regarding Schrodinger. It would be\ngood to capture uncertainties, when possible. For instance, Schrodinger age is reported following an abstract (ref 24) and\nquoted with two decimal digits. It should be clear that the quoted age uncertainties are not realistic, and furthermore they are\nbased on a chronology model that has been shown to be problematic. For comparison, a close inspection at Figure 4 in\nMorbidelli et al EPSC 2012 shows that for N1=~1e-2 (Schrodinger), the age uncertainty can be up to ~ 300 Myr. A more\nrecent model, Marchi et al Nat Geo 2021, shows a similar age spread in their Figure 1. See also Nesvorny et al ApJ 2022\n(their Figure 2). I will get back to this point later.\nFigure 3, b: One has to wonder if the marked circles are truly impact craters. With low ballistic angles (ln 142), one may\nexpect the craters to be non circular, and the fact that they are so closely packed seems unlikely to me to be individual\nimpacts. In this regard, I wonder if the semi-circular appearance is instead due to interaction of the scouring and slumping of\nthe surface due to high-velocity ejecta streams. This is not a critical point, but the manuscript presents quite a bit of\ndiscussion considering the properties of these presumed craters and their impactors, while it might be these features were\nexcavated in completely different ways.\nln 138: The comment about the distributed impact zone is interesting. The argument is a bit qualitative here, and I wish the\nauthors had included an iSALE simulation (3D) to show this. If such a simulation were available, it would make a great\naddition to the paper.\nln 95-198: See previous comment on Schrodinger basin age. Further, there has been a great deal of work regarding the\nearly lunar impact flux, and quoting NRC 2007 is outdated. Both sample analysis (see refs in Bottke and Norman 2017) and\nmodels (Nesvorny et al 2022; 2023) show that it is very unlikely there was a late impact spike. Still, the earliest lunar\nchronology is a key aspect of the Artemis’ science goals, and the manuscript would benefit\nfrom a more updated discussion.\nIn general, the NRC report is presented as a justification for this work (ln 21-23), but this is a stand-alone research paper that\nshould present interesting science regardless of the NRC goals, so I suggest rephrasing such language.\nSimone Marchi\nSouthwest Research Institute\nBoulder CO, USA", "reviewer_2": "Review of “Grand Canyons on the Moon” by D. A. Kring, D. P. Kallenborn, and G. S. Collins\nI have read and completed my review of “Grand Canyons on the Moon,” a manuscript submitted to Nature Communications\nfor publication. I find the article straightforward and easy to digest, with important implications for the upcoming Artemis\nmissions, as noted in the text. The work describes the secondary craters comprising two impressive canyons associated\nwith Schrodinger Basin, Vallis Schrodinger and Vallis Planck. Using the secondary crater chains which presumably caused\nthese two canyons, the authors calculate the energy required for their excavation. There are several comparisons between\nthese two lunar canyons and the Grand Canyon on Earth, but these are primarily qualitative since the Grand Canyon formed\nvia a very different process. However, the comparisons made in the manuscript are useful to give readers context on what is\ncommonly intangible for most people.\nMy concerns are listed below, categorized into major and minor revisions for the authors.\nMajor Revisions\n• Lines 124-126: The authors state that the larger secondary crater diameters in Vallis Schrodinger imply that they formed\nfrom clusters of fragments rather than larger intact fragments, based on scaling laws. However, this canyon also falls\ncompletely within Schrodinger’s continuous ejecta blanket. Is this conclusion robust? Do the scaling laws hold for impact\neffects that close to the point of contact?\n• Lines 143-144: How is this energy calculated (1.68x1020 J)? Relatedly, what is the difference between this calculation and\nthe one for line 147 (3.39x1020 J). I am guessing the larger number reflects if there was a continuous line of identifiable\nsecondary craters for the length of Vallis Schrodinger? The comparison is not clear.\n• Lines 156-159: The authors mention a local slope in the vicinity of Schrodinger Basin while discussing the asymmetry of\nthe ejecta blanket from the north to the south. This is an interesting idea, were any calculations done to determine if this\nslope contributes to the asymmetry? Could the upward slope to the northwest notably impede ejecta deposition?\nMinor Revisions:\n• Lines 90-92: Change sentence to “Vallis Planck does not show continuous secondary cratering everywhere and appears\nto be modified by the collapse of canyon walls.” For more easy reading (change in bold).\n• Line 102: Please reference the Methods section here, as it is currently unclear how these numbers are calculated.\n• Lines 150-151: Please put references for the different comparisons in the main text\n• Line 165: I believe the reference should go at the end of the sentence.\n• Lines 180-181: Please be more explicit about the “two greatest uncertainties,” I presume that this refers to the presence of\nthe Schrodinger ejecta blanket overlying SPA material, and possibly if any ejecta from Amundsen-Ganswindt Basin will also\ncontaminate a possible SPA collection? I find this very unclear, however.\n• Line 201: Change sentence to “…so a cross-section of the Moon to a depth of ~100 km may be assessable, albeit\ndismembered.” (change in bold)\n• Line 204: Change sentence to “Excavated SPA material and any primordial crust can be used to test the lunar magma\nocean hypothesis for planetary differentiation and the giant impact hypothesis for the origin of the Earth-Moon system,\namong many other objectives.” (change in bold)\n• Supplementary Figures 1 and 2: These would be more informative if there were 2 panels per figure, one with the\nannotations and one without. It is hard to verify the author’s interpretation of the linear features without a “clean” version to\nreview.\n• Supplementary Figures 1-4, Figures 2-3: Please add N/E/S/W annotation to the map views and cross-sections.", "reviewer_3": "Review Summary:\nThis paper provides a focused analysis of the ejecta displacement and radial rays and canyons surrounding the Schrödinger\nimpact basin on the Moon. The “Grand Canyons” described are two deep, radial grooves formed by secondary craters in the\ninferred down-range direction of the impact structure with minor grrooves in the up-range direction. Images and topography\ndata combined with prior mapping of the ejecta are used to conduct measurements of the spatial patterns and dimensions of\nthe radial canyons and the associated secondary craters and those within the ejecta of the basin. This photogeologic\nmapping of Schrödinger’s ejecta is used to infer the trajectory of the impactor and the ejected material. Analytical models of\nballistic trajectories and ejection velocities combined with scaling relationships are used to constrain the velocities and\nenergy required to form the basin sculpture and secondary craters. The results indicate an impact trajectory from the SSE to\nNNW and that only a thin amount of ejecta or no ejecta from Schrödinger may be found in the candidate Artemis exploration\nzone.\nThe paper provides an interesting narrative of the processes and products of ejecta formation for the youngest impact basin\non the Moon. While the paper does not use techniques that are particularly novel (many of the analytical methods and\nscaling relationships have been around for decades), the question of the contribution of ejecta sourced from Schrödinger (or\nother large basin or crater) to the Artemis exploration zones is an important one to answer. The paper’s conclusions that\nthere may be little to no Schrödinger ejecta in the Artemis zones is testable and could mean that material from South-Pole\nAitken basin is more abundant near the surface and accessible to the Artemis missions and sampling strategy. The methods\nof measurements appear sound but could use some clarification, as described in the comments below. It is also a bit odd\nthat so much emphasis of the paper is on the characteristics and process of formation of the deep secondary crater chains,\nVallis Schrödinger and Vallis Planck, when the perhaps the main goal—to determine the direction of impact and ejecta\nemplacement with implications for Artemis exploration zones—could have been mostly completed by simply describing the\nspatial patterns and intersections of ejecta rays with ejecta decay scaling laws. The dramatic descriptions of the topography\nof Vallis Schrödinger and Vallis Planck and the energy required for their formation are quite interesting when compared to\nnatural and atomic process that are more familiar to us on Earth, but are largely superfluous to the end goal of determining\nthe trajectory of impact and pattern and direction of ejecta emplacement.\nComments:\nLines 68-72, Lines 89-101, and Figure 3: The outlines of these secondary craters forming the canyons are fairly ambiguous\n(some with only partial arcs of the rim) and come with some level of measurement uncertainty of diameter and also degree of\nconfidence in them being separate craters. Please include some discussion on the uncertainty in diameter measurement\nand how the individual craters were determined. For example, while the secondary craters certainly overlap, some of the\ncrater outlines could be interpreted as a single larger crater instead of two overlapping, which would increase the inferred\ndiameters and decrease the total number of craters identified.\nLine 82: The provided 0.96 ratio value appears to be incorrect; should this be 0.196? It also isn’t clear where in Pike 1974\nwhere this ratio and cited uncertainties are found. Is uncertainty cited in a different paper from Pike or are these new\ncalculations?\nLines 87-88: Please include in the methods section a brief description of how these d/D ratios for the secondary craters were\ndetermined. For example, were depths determined using a single profile (or multiple profiles) and maximum-minimum\nelevations along the profile? What is the spatial resolution of the SLDEM2015 gridded data product, and is this sufficient for\nthe depth measurements?\nLines 125-128: Is there any other evidence for clustered impacts? Is there an analytical\nrelationship for how fragment clusters might evolve with radius from the basin to further\nsupport the inference in transition to more signal fragments. Also, can fragment sizes be\nspatially variable (e.g., target heterogeneity) such that one ray consists of different sized\nfragments than a ray at a different azimuth?\nLines 133-134: The reference cites Schultz and Crawford, 2016, which suggests multiple phases of ejecta emplacement for\nlarge basins and oblique impacts: 1) early phase of ejecta (high speed and low angle), 2) decapitation of the impactor and\nassociated scouring associated with early stages, and then 3) late-stage ejecta that is more radial to the basin center and\ncovers the early stage ejecta and scour. Only the early ejecta patterns (#1, #2) were used to constrain the impact point\nUnder this cited framework, is the formation of Vallis Schrödinger and Vallis Planck being inferred as early stage ejecta that\nis decoupled from the transient cavity or later stages ejecta associated with growth of the transient\ncavity and whose radial trends and intersection, should be more basin-centered. I assume later stage since the canyons are not covered by ejecta and formed by distinct chains of secondaries connected with outer ray patterns. Indeed, Schultz and Crawford (2016) used\nthe *uprange* early-stage ejecta patterns and intersection points to constrain the impact point, with the down-range pattern\nproviding an estimate of the impactor diameter. So, how well can the later stage ejecta, which should be more centric to the\nbasin center be used to infer impact trajectory? Along the same vein, inferring impact trajectory/direction from only two\nprominent late-stage rays seems relying on statistics of small numbers, when Schultz and Crawford show much variability in\nthe intersection points. Indeed, the paper’s Figure S1 shows this variability in the other minor crater rays; why are these not\nconsidered in the trajectory analysis?\nLines 153-163: The ejecta pattern appears to resemble a butterfly pattern bisected in the 33.5 (NNW) direction. This seems to\nfurther support the NNW trajectory.\nLines 164-172: It would be helpful to discuss how the opening of Schrödinger’s peak ring in the inferred up-range direction is\nin contrast to modeling by Schultz and Crawford (2016) showing the peak ring opening in the down-range direction, with\nhigher up-range topography attributed to oblique impacts for other basins (e.g., Moscoviense). This would seem to suggest\nthat, pre-impact topography likely exhibits an even larger influence on peak-ring topographic development, possibly\nbecause the impact that formed Schrödinger wasn’t as oblique as some other basin impacts and due to the influence of the\nlarge gradient in topography at the edge of SPA.\nFigures:\nFigure 3: It would be helpful to include transect lines on the images that show the locations for the profiles in Figure 2b and\nS4.\nFigure 4: The measurement data used for the calculations behind this plot should be provided as supplemental material.\nMethods:\nLines 355-358: Please clarify how many distance measurements are being made. It appears that perhaps there are four\ndistance measurements, but based on the description in the methods it could be interpreted differently: 1) From secondary\ncrater (is this from the center of the crater?) to the basin center, 2) From secondary crater to the transient crater rim (where the\ntrajectory from the basin center and the transient crater rim intersect), and 3) From secondary crater to the point at which the\ntwo largest rays intersect, 4) From secondary crater to the transient crater rim (where the trajectory from the rays’ intersection\npoint and the transient crater rim intersect). Or maybe it’s just two distance measurements from the transient crater rim\nbecause that is what is needed for the velocity calculations? None of the figures show the transient crater rim intersection or\nthese points in one map. Since this is important for the calculation, it would be helpful to include an additional\nsupplement figure showing this. Please also provide values for these distance measurements in a table, including crater dimensions.\nIn addition, should the transient crater be concentric with the final rim-crest-dimeter? Or offset toward the sub-impact point\nsince the impact is inferred to be oblique and the cavity is evolving transiently? This would affect the distance and velocity\ncalculations.\nLines 362: Please give the formula or state that it is a standard ballistic trajectory formula for airless bodies and point to an\nexample reference.\nSupplemental Material:\nFigure S1: Why is one of the “Possible Crater Rays” (dashed orange) in the WNW section almost orthogonal to the basin-\nradial direction and another ray? Also, please label the blue grid on the map – are these latitude and longitudes and at what\nincrements?\nFigure S2: Suggest indicating that mapped rays were extended back through both points on the crater rim outline (in red).\nAlso, are these great-circle paths to account for planetary curvature? If so, please indicate."}, "author_response": "Our replies to the reviewers’ helpful comments, quoted here, are provided in blue font.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nReferee report for “Grand Canyons of the Moon” by Kring et al.\nThe manuscript is well written. Schrodinger is a well-studied lunar basin, and as such the\nmanuscript expands on a wide body of literature. The actual new work presented in this\nmanuscript is somewhat limited, but I think it would be an interesting addition given the\nimplications for Artemis astronauts' exploration of the Moon. I suggest publication, pending\nsome minor improvements listed below.\nThank you.\nln 33: The manuscript builds on a large body of work regarding Schrodinger. It would be\ngood to capture uncertainties, when possible. For instance, Schrodinger age is reported\nfollowing an abstract (ref 24) and quoted with two decimal digits. It should be clear that the\nquoted age uncertainties are not realistic, and furthermore they are based on a chronology\nmodel that has been shown to be problematic. For comparison, a close inspection at\nFigure 4 in Morbidelli et al EPSC 2012 shows that for N1=~1e-2 (Schrodinger), the age\nuncertainty can be up to ~ 300 Myr. A more recent model, Marchi et al Nat Geo 2021, shows\na similar age spread in their Figure 1. See also Nesvorny et al ApJ 2022 (their Figure 2). I will\nget back to this point later.\nThe latest paper in that series of models is Nesvorný et al. (2023) Early bombardment of the\nMoon: Connecting the lunar crater record to the terrestrial planet formation, Icarus 399\n(2023) 115545. We consulted Alesandro Morbidelli who says the 2023 paper supersedes\nearlier work, including his own. The Nesvorný et al. (2023) paper indicates an uncertainty\nof ~150 Myr for a basin the age of Schrödinger, which we added to the text with a reference\nto Nesvorný et al. (2023). We also note, that in agreement with the spirit of the reviewer’s\ncomment, the text also pointed out that it is important to sample Schrödinger basin\nmaterial to “test its estimated 3.81 +0.013 Ga age (ref. 24)” (line 196 in the original\n-0.014\nmanuscript).\nFigure 3, b: One has to wonder if the marked circles are truly impact craters. With low\nballistic angles (ln 142), one may expect the craters to be non circular, and the fact that\nthey are so closely packed seems unlikely to me to be individual impacts. In this regard, I\nwonder if the semi-circular appearance is instead due to interaction of the scouring and\nslumping of the surface due to high-velocity ejecta streams. This is not a critical point, but\nthe manuscript presents quite a bit of discussion considering the properties of these\npresumed craters and their impactors, while it might be these features were excavated in\ncompletely different ways.\nThe craters have clearly visible crater rims in images, which are also illustrated with two\ntopographic profiles in an additional figure in Supplementary Information. Notionally, the\ninteraction of ejecta curtains could “hide” a more elongate structure caused by low impact\nangles. However, the width of the canyons puts an upper limit on possible crater diameters.\nSchultz and Crawford (2016) suggest ballistic flight angles of 20 to 45 degrees for material\nproducing secondary craters. Gault and Wedekind (1978) found that craters remain\ncircular down to 30 degrees. Thus, as long as the ballistic angle of ejecta making\nsecondary craters is 30 to 45 degrees (which is within the allowable range we consider)\ncircularity is expected. We added a sentence to the line 142 statement indicating the\ncircularity of the crater suggests the angles may be in the upper part of the 20 to 45 degree\nrange.\nWe note that a second hypothesis, that postulates the rays are lateral ejecta that is part of\na butterfly pattern, implies (per Luo et al., 2022) ballistic launch angles of about 50\ndegrees, which would produce circular craters. We added a sentence with those angles to\nthe text, too.\nln 138: The comment about the distributed impact zone is interesting. The argument is a bit\nqualitative here, and I wish the authors had included an iSALE simulation (3D) to show this.\nIf such a simulation were available, it would make a great addition to the paper.\nThis study is based on photogeological and analytical methods; any 3D simulation will\nhave to wait for a subsequent study.\nln 95-198: See previous comment on Schrodinger basin age. Further, there has been a great\ndeal of work regarding the early lunar impact flux, and quoting NRC 2007 is outdated. Both\nsample analysis (see refs in Bottke and Norman 2017) and models (Nesvorny et al 2022;\n2023) show that it is very unlikely there was a late impact spike. Still, the earliest lunar\nchronology is a key aspect of the Artemis’ science goals, and the manuscript would benefit\nfrom a more updated discussion. In general, the NRC report is presented as a justification\nfor this work (ln 21-23), but this is a stand-alone research paper that should present\ninteresting science regardless of the NRC goals, so I suggest rephrasing such language.\nThe objectives identified in the NRC (2007) remain relevant, as it is essential to determine\nthe cadence of impacts during the basin-forming epoch, which we do not believe has been\nresolved by Bottke and Norman (2017) and Nesvorný et al. (2022). Several subsequent\nNASA documents confirm/repeat that objective; good scholarship requires us to cite the\noriginal document, hence NRC (2007). We appreciate the reviewer’s suggestion to\nrephrase the language, but, respectfully, leave it as is. The key to confidently resolving\nuncertainties in the cadence of impacts is the identification of suitable sample sites on the\nlunar surface, collection of those samples, and return of them to Earth for analyses. The\ncurrent paper addresses geologic issues that affect the identification of suitable sample\nsites and that are directly relevant to ongoing site selection discussions for the Artemis III\nand IV missions.\nReviewer #2 (Remarks to the Author):\nReview of “Grand Canyons on the Moon” by D. A. Kring, D. P. Kallenborn, and G. S. Collins\nI have read and completed my review of “Grand Canyons on the Moon,” a manuscript\nsubmitted to Nature Communications for publication. I find the article straightforward and\neasy to digest, with important implications for the upcoming Artemis missions, as noted in\nthe text. The work describes the secondary craters comprising two impressive canyons\nassociated with Schrodinger Basin, Vallis Schrodinger and Vallis Planck. Using the\nsecondary crater chains which presumably caused these two canyons, the authors\ncalculate the energy required for their excavation. There are several comparisons between\nthese two lunar canyons and the Grand Canyon on Earth, but these are primarily qualitative\nsince the Grand Canyon formed via a very different process. However, the comparisons\nmade in the manuscript are useful to give readers context on what is commonly intangible\nfor most people.\nWe thank the reviewer for appreciating our effort to convey the dramatic topography of the\nlunar surface to readers.\nMy concerns are listed below, categorized into major and minor revisions for the authors.\nMajor Revisions\n• Lines 124-126: The authors state that the larger secondary crater diameters in Vallis\nSchrodinger imply that they formed from clusters of fragments rather than larger intact\nfragments, based on scaling laws. However, this canyon also falls completely within\nSchrodinger’s continuous ejecta blanket. Is this conclusion robust? Do the scaling laws\nhold for impact effects that close to the point of contact?\nWhile the canyons are close to the crater in a relative sense, in an absolute sense they are\nseveral hundred kilometers away and were formed by impacts at speeds close to 1 km/s.\nThis speed is only just below the speed range of the impact experiments that have been\nused to develop the crater scaling relationships that we apply in this work (e.g., Schmidt\nand Housen, 1987). Moreover, these scaling laws have been shown to be appropriate for\nsome conditions at even lower impact speeds (e.g., Gault and Wedekind, 1978; Hartmann,\n1985; Holsapple and Schmidt, 1982; Ormö et al., 2015; Yamamoto et al., 2006). We added\nthese details to the methods section.\nClusters of secondary craters are commonly observed (e.g., Lucchitta, 1977; Schultz and\nGault, 1985). Further, those clusters of fragments spread out with radial distance beyond\nthe continuous ejecta blanket (Schultz and Gault, 1985). Thus, it is logical that clusters\ncloser to the point of impact (i.e., within the continuous ejecta blanket) would have a\nsmaller spread (be more tightly clustered) and produce simple craters like those observed.\nThese details and additional references were added to the text.\n• Lines 143-144: How is this energy calculated (1.68x1020 J)? Relatedly, what is the\ndifference between this calculation and the one for line 147 (3.39x1020 J). I am guessing\nthe larger number reflects if there was a continuous line of identifiable secondary craters\nfor the length of Vallis Schrodinger? The comparison is not clear.\nThe energy is calculated using the kinetic energy formula for the determined impactor\nmasses and measured distances they travelled (which provides velocities, as described in\nthe methods section). The first number reflects the sum of the energy necessary to\nproduce the identified craters in Vallis Schrödinger. But, as the paper notes, well-preserved\ncraters do not cover the whole length of Vallis Schrödinger. As stated in the paper,\nassuming a similar distribution of craters along the whole length, a minimum kinetic energy\nof 3.39x1020 J is necessary to produce Vallis Schrödinger. As cratering was the method of\nexcavation, that is straightforward reasoning.\n• Lines 156-159: The authors mention a local slope in the vicinity of Schrodinger Basin\nwhile discussing the asymmetry of the ejecta blanket from the north to the south. This is an\ninteresting idea, were any calculations done to determine if this slope contributes to the\nasymmetry? Could the upward slope to the northwest notably impede ejecta deposition?\nThat is a good thing to check. Upward slopes where ejecta is foreshortened are <20\ndegrees (and often <10 degrees), which is less than the ejecta launch angles. Thus,\nupward slope is unlikely to block ejecta deposition.\nMinor Revisions:\n• Lines 90-92: Change sentence to “Vallis Planck does not show continuous secondary\ncratering everywhere and appears to be modified by the collapse of canyon walls.” For\nmore easy reading (change in bold).\nWe replaced the sentence as suggested, but with a caveat. The verb “show” might be\nmisinterpreted to mean that secondary cratering did not produce some sections of the\nVallis. Instead of the word “show,” we modified the sentence with the verb “preserved.”\n• Line 102: Please reference the Methods section here, as it is currently unclear how these\nnumbers are calculated.\nDone.\n• Lines 150-151: Please put references for the different comparisons in the main text\nDone.\n• Line 165: I believe the reference should go at the end of the sentence.\nDone.\n• Lines 180-181: Please be more explicit about the “two greatest uncertainties,” I presume\nthat this refers to the presence of the Schrodinger ejecta blanket overlying SPA material,\nand possibly if any ejecta from Amundsen-Ganswindt Basin will also contaminate a\npossible SPA collection? I find this very unclear, however.\nThe text was amended for clarity and now reads “…needed is a similar assessment of the\ndistribution of ejecta around the Amundsen-Ganswindt basin which has the potential to\ncover SPA ejecta, too.\n• Line 201: Change sentence to “…so a cross-section of the Moon to a depth of ~100 km\nmay be assessable, albeit dismembered.” (change in bold\nDone.\n• Line 204: Change sentence to “Excavated SPA material and any primordial crust can be used to test the lunar magma ocean hypothesis for planetary differentiation and the giant\nimpact hypothesis for the origin of the Earth-Moon system, among many other objectives.”\n(change in bold)\nDone.\n• Supplementary Figures 1 and 2: These would be more informative if there were 2 panels\nper figure, one with the annotations and one without. It is hard to verify the author’s\ninterpretation of the linear features without a “clean” version to review.\nA side-by-side view replaces the first of those Supplementary Figures (originally labeled 1,\nbut now labeled 2, because of the addition of a new Supplementary Figure 1). We did not\nrepeat that for the next figure, because it would have duplicated the view that a reader can\nsee in the new Supplementary Figure 2.\n• Supplementary Figures 1-4, Figures 2-3: Please add N/E/S/W annotation to the map views\nand cross-sections.\nWhen working in a southern circumpolar region, north radiates in all directions from the\npole. Thus, a single north-indicating arrow on a map is not appropriate. To help readers\nunderstand cardinal directions in the region, we added to the Figure 1 caption that “North\nradiates in all directions from the south pole.” We added similar notes to the relevant\nSupplementary Figures. For clarity, we also inserted a new Supplementary Figure (#1) that\nshows clearly the location of the Schrödinger basin relative to the south pole and the\nnorthward-directed meridians that radiate from the south pole. We might add that ‘south\npole’ is not a formal (IAU) geographic name on the Moon, so it is not capitalized like it is on\nEarth.\nReviewer #3 (Remarks to the Author):\nReview Summary:\nThis paper provides a focused analysis of the ejecta displacement and radial rays and\ncanyons surrounding the Schrödinger impact basin on the Moon. The “Grand Canyons”\ndescribed are two deep, radial grooves formed by secondary craters in the inferred down-\nrange direction of the impact structure with minor grrooves in the up-range direction.\nImages and topography data combined with prior mapping of the ejecta are used to\nconduct measurements of the spatial patterns and dimensions of the radial canyons and\nthe associated secondary craters and those within the ejecta of the basin. This\nphotogeologic mapping of Schrödinger’s ejecta is used to infer the trajectory of the\nimpactor and the ejected material. Analytical models of ballistic trajectories and ejection\nvelocities combined with scaling relationships are used to constrain the velocities and\nenergy required to form the basin sculpture and secondary craters. The results indicate an\nimpact trajectory from the SSE to NNW and that only a thin amount of ejecta or no ejecta\nfrom Schrödinger may be found in the candidate Artemis exploration zone.\nThe paper provides an interesting narrative of the processes and products of ejecta\nformation for the youngest impact basin on the Moon. While the paper does not use\ntechniques that are particularly novel (many of the analytical methods and scaling\nrelationships have been around for decades), the question of the contribution of ejecta\nsourced from Schrödinger (or other large basin or crater) to the Artemis exploration zones is\nan important one to answer. The paper’s conclusions that there may be little to no\nSchrödinger ejecta in the Artemis zones is testable and could mean that material from\nSouth-Pole Aitken basin is more abundant near the surface and accessible to the Artemis\nmissions and sampling strategy. The methods of measurements appear sound but could\nuse some clarification, as described in the comments below. It is also a bit odd that so\nmuch emphasis of the paper is on the characteristics and process of formation of the deep\nsecondary crater chains, Vallis Schrödinger and Vallis Planck, when the perhaps the main\ngoal—to determine the direction of impact and ejecta emplacement with implications for\nArtemis exploration zones—could have been mostly completed by simply describing the\nspatial patterns and intersections of ejecta rays with ejecta decay scaling laws. The\ndramatic descriptions of the topography of Vallis Schrödinger and Vallis Planck and the\nenergy required for their formation are quite interesting when compared to natural and\natomic process that are more familiar to us on Earth, but are largely superfluous to the end\ngoal of determining the trajectory of impact and pattern and direction of ejecta\nemplacement.\nComments:\nLines 68-72, Lines 89-101, and Figure 3: The outlines of these secondary craters forming\nthe canyons are fairly ambiguous (some with only partial arcs of the rim) and come with\nsome level of measurement uncertainty of diameter and also degree of confidence in them\nbeing separate craters. Please include some discussion on the uncertainty in diameter\nmeasurement and how the individual craters were determined. For example, while the\nsecondary craters certainly overlap, some of the crater outlines could be interpreted as a\nsingle larger crater instead of two overlapping, which would increase the inferred diameters\nand decrease the total number of craters identified.\nAs suggested, we expanded measurement details in the methods section. In summary,\nmultiple measurements of secondary crater diameters were made with a variation that was\ngenerally <5%.\nLine 82: The provided 0.96 ratio value appears to be incorrect; should this be 0.196? It also\nisn’t clear where in Pike 1974 where this ratio and cited uncertainties are found. Is\nuncertainty cited in a different paper from Pike or are these new calculations?\nThank you for catching the typographical error, which we fixed (changing 0.96 to 0.196).\nThe source of that number and uncertainties are data in Pike (1974), as cited in the\nsubmitted manuscript. We acknowledge, however, that it was Pike (1980) that presented\nthe data in the format we use. Thus, we changed the reference from Pike (1974) to Pike\n(1980), the latter of which will then refer readers to Pike (1974) for the data.\nLines 87-88: Please include in the methods section a brief description of how these d/D\nratios for the secondary craters were determined. For example, were depths determined\nusing a single profile (or multiple profiles) and maximum-minimum elevations along the\nprofile? What is the spatial resolution of the SLDEM2015 gridded data product, and is this\nsufficient for the depth measurements?\nThe requested details were added to the methods section. The SLDEM has a typical vertical\naccuracy of ~3 to 4 m which is sufficient for the depth measurements.\nLines 125-128: Is there any other evidence for clustered impacts? Is there an analytical\nrelationship for how fragment clusters might evolve with radius from the basin to further\nsupport the inference in transition to more signal fragments. Also, can fragment sizes be\nspatially variable (e.g., target heterogeneity) such that one ray consists of different sized\nfragments than a ray at a different azimuth?\nThe issue of clustered impacts as a function of radial distance was noted by Reviewer #1\nand addressed as described above.\nRegarding target heterogeneity: While notionally target lithologies may control fragment\nsizes (e.g., as at Meteor Crater), that is not evident in our own data. As shown in Figure 4,\nthe transition in fragment size is not only evident between the rays, but is also evident\nwithin the Vallis Planck data (see the triangular symbols). Moreover, no evidence of that\nhas been detected in studies of lunar crater rays. Indeed, consistency in fragment size as a\nfunction of distance is one of the criteria used to discriminate secondary craters from\nprimary craters (e.g., Singer et al., 2020). We added a paragraph saying as much.\nLines 133-134: The reference cites Schultz and Crawford, 2016, which suggests multiple\nphases of ejecta emplacement for large basins and oblique impacts: 1) early phase of\nejecta (high speed and low angle), 2) decapitation of the impactor and associated scouring\nassociated with early stages, and then 3) late-stage ejecta that is more radial to the basin\ncenter and covers the early stage ejecta and scour. Only the early ejecta patterns (#1, #2) were used to constrain the impact point\nUnder this cited framework, is the formation of Vallis Schrödinger and Vallis Planck being inferred as early stage ejecta that\nis decoupled from the transient cavity or later stages ejecta associated with growth of the transient cavity and whose radial\ntrends and intersection, should be more basin-centered. I assume later stage since the canyons are not covered by ejecta\nand formed by distinct chains of secondaries connected with outer ray patterns. Indeed, Schultz and Crawford (2016) used\nthe *uprange* early-stage ejecta patterns and intersection points to constrain the impact point, with the down-range pattern\nproviding an estimate of the impactor diameter. So, how well can the later stage ejecta, which should be more centric to the\nbasin center be used to infer impact trajectory? Along the same vein, inferring impact trajectory/direction from only two\nprominent late-stage rays seems relying on statistics of small numbers, when Schultz and Crawford show much variability in\nthe intersection points. Indeed, the paper’s Figure S1 shows this variability in the other minor crater rays; why are these not\nconsidered in the trajectory analysis?\nAs described in the original manuscript, the two prominent rays are produced early in the\nexcavation process (lines 132-133 in the original manuscript) and several smaller rays\nproduced later in the process (lines 137-138 in the original manuscript); all rays, including\nthose in Figure S1 (lines 138-140 in the original manuscript), were used in the analysis.\nLate ejecta may have filled in part of the rays; i.e., the rays may have once extended closer\nto the basin rim. Standard ejecta thickness calculations suggest total ejecta thickness at\nthe beginning of the still-visible Vallis Schrödinger would be about 300 to 400 m in the\nnearly 2-km-deep chasm (and, thus, partially responsible for low d/D values), but only a\nfew tens of meters thick at the outer edge of Vallis Schrödinger and, thus, would not\nsignificantly bury that portion the ray.\nLines 153-163: The ejecta pattern appears to resemble a butterfly pattern bisected in the\n33.5 (NNW) direction. This seems to further support the NNW trajectory.\nWe amended the text to say so.\nLines 164-172: It would be helpful to discuss how the opening of Schrödinger’s peak ring in\nthe inferred up-range direction is in contrast to modeling by Schultz and Crawford (2016)\nshowing the peak ring opening in the down-range direction, with higher up-range\ntopography attributed to oblique impacts for other basins (e.g., Moscoviense). This would\nseem to suggest that, pre-impact topography likely exhibits an even larger influence on\npeak-ring topographic development, possibly because the impact that formed Schrödinger\nwasn’t as oblique as some other basin impacts and due to the influence of the large\ngradient in topography at the edge of SPA.\nDone.\nFigures:\nFigure 3: It would be helpful to include transect lines on the images that show the locations\nfor the profiles in Figure 2b and S4.\nDone.\nFigure 4: The measurement data used for the calculations behind this plot should be\nprovided as supplemental material.\nDone. Data appear as Supplementary Table 1.\nMethods:\nLines 355-358: Please clarify how many distance measurements are being made. It\nappears that perhaps there are four distance measurements, but based on the description\nin the methods it could be interpreted differently: 1) From secondary crater (is this from the\ncenter of the crater?) to the basin center, 2) From secondary crater to the transient crater\nrim (where the trajectory from the basin center and the transient crater rim intersect), and\n3) From secondary crater to the point at which the two largest rays intersect, 4) From\nsecondary crater to the transient crater rim (where the trajectory from the rays’ intersection\npoint and the transient crater rim intersect). Or maybe it’s just two distance measurements\nfrom the transient crater rim because that is what is needed for the velocity calculations?\nNone of the figures show the transient crater rim intersection or these points in one map.\nSince this is important for the calculation, it would be helpful to include an additional\nsupplement figure showing this. Please also provide values for these distance\nmeasurements in a table, including crater dimensions.\nThe methods section was amended as suggested for clarification.\nIn addition, should the transient crater be concentric with the final rim-crest-dimeter? Or\noffset toward the sub-impact point since the impact is inferred to be oblique and the cavity\nis evolving transiently? This would affect the distance and velocity calculations.\nAs described in the paper (methods, lines 355-359 in the original manuscript), four\ndifferent distances were used in the calculations and encompass this particular\nuncertainty.\nLines 362: Please give the formula or state that it is a standard ballistic trajectory formula\nfor airless bodies and point to an example reference.\nThe formula was added to the methods.\nSupplemental Material:\nFigure S1: Why is one of the “Possible Crater Rays” (dashed orange) in the WNW section\nalmost orthogonal to the basin-radial direction and another ray? Also, please label the blue\ngrid on the map – are these latitude and longitudes and at what increments?\nGood catch; the dashed orange line was a processing artifact and deleted.\nThe blue gridded lines represent distance segments in radii from the basin center (not\nlatitude or longitude) were used to determine if there were systematic geographic patterns\namong secondary features. Text was added for clarifcation.\nFigure S2: Suggest indicating that mapped rays were extended back through both points on\nthe crater rim outline (in red). Also, are these great-circle paths to account for planetary\ncurvature? If so, please indicate.\nThe lines trace the rays as they appear on a Lambert Azimuthal Equal Area Projection, as\nindicated at the top of the map."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have done a reasonable job at addressing my comments.", "reviewer_2": "I have read the revisions for the manuscript entitled “Grand Canyons on the Moon” by Kring, Kallenborn, and Collins. The\nrevisions to the article have made the work much more understandable and clear. My recommendation is that the article is\nready for publication, but I have a small number of minor comments.\nLine 34: Should “latest” be “late”?\nFigure 4: Explicitly state what the black dashed line connecting the red points represents.\nLine 220: Should “assessable” be “accessible”?\nLine 232: Please add a quick statement or sentence about what conditions would lead to the added (red text) bearing\npossibility.", "reviewer_3": "The authors have sufficiently addressed my comments and concerns. The additional methods descriptions and\nsupplemental material are greatly appreciated, which improve clarity and better support the presented\nobservations and interpretations."}, "author_response": "POINT-BY-POINT REPLY TO REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author): The authors have done a reasonable job at\naddressing my comments.\nThank you.\nReviewer #2 (Remarks to the Author): I have read the revisions for the manuscript\nentitled “Grand Canyons on the Moon” by Kring, Kallenborn, and Collins. The revisions\nto the article have made the work much more understandable and clear. My\nrecommendation is that the article is ready for publication, but I have a small number of\nminor comments.\nLine 34: Should “latest” be “late”?\nThank you; spelling corrected.\nFigure 4: Explicitly state what the black dashed line connecting the red points\nrepresents.\nDone.\nLine 220: Should “assessable” be “accessible”?\nThank you; spelling corrected.\nLine 232: Please add a quick statement or sentence about what conditions would lead\nto the added (red text) bearing possibility.\nDone.\nReviewer #3 (Remarks to the Author): The authors have sufficiently addressed my\ncomments and concerns. The additional methods descriptions and supplemental\nmaterial are greatly appreciated, which improve clarity and better support the presented\nobservations and interpretations.\nThank you."}]

Decision letter