Investigations of a High-Albedo Cliff-forming Impactite Layer in Schrödinger Basin

INVESTIGATIONS OF A HIGH-ALBEDO CLIFF-FORMING IMPACTITE LAYER IN SCHRÖDINGER BASIN. M. Prakash1,2,3, S. P. S. Gulick1,2,3, C. Grima2,3, M. T. K. Jordan1,2,3, G. Y. Kramer4, 1Department of Earth and Planetary Sciences, Jackson School of Geosciences, University Texas at Austin ([email protected]), 2Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, 3Center for Planetary Systems Habitability, University of Texas at Austin, 4Planetary Sciences Institute Introduction: Impact cratering due to asteroid impacts is recognized as the most pervasive planetary process. Craters and basins formed by this process are infilled with a variety of materials, often used as resources on Earth for construction and mineral extraction. We can classify these materials based on location, abundance of melted material, and clast size [1,2]. Analyzing the morphology and petrology of this material provides insight into mechanisms responsible for its deposition and material present before impact. We can look to well-preserved basin floors using geophysical data and remote sensing methods to garner information on both of these aspects. The Moon is an ideal laboratory for such investigations due to its abundance of impact crater materials. Investigating the mechanisms responsible for the formation and deposition of impact cratering materials will provide us insight into surface geology and the diversity of materials produced on the lunar surface, which is the interface humans will largely interact with through lunar missions. Schrödinger Basin is one of the best-preserved impact basins of its size (320 km) on the Moon [3]. Schrödinger Basin allows us to study over 3 billion years of lunar history, perhaps having formed within ring structures of the earlier South Pole-Aitken (SPA) impact overlain by kilometers of SPA ejecta [4]. The impact basin has a peak ring composed of deep crustal material, and Imbrian to Eratosthenian volcanism is preserved as mare and pyroclastic deposits in the crater basin [4,5]. However, the subsurface stratigraphy of the basin is not well constrained. Integrated Measurements and Analysis of Geophysics of Schrödinger (IMAGES) seeks to integrate novel data sources with reflectivity and subsurface radar from the Lunar Radar Sounder (LRS) aboard the Kaguya Lunar Orbiter to update our findings of Schrödinger Basin for future mission use. We report Figure 1. On the right is a Lunar Reconnaissance Orbiter Camera (LROC) image of Schrödinger Basin, with mapped outcrops of this layer depicted in green and white dots. Layer thicknesses calculated with the standard deviation is shown in each region. A) shows a conceptual schematic of the stratigraphy at the pyroclastic vent, Schrödinger G. B) shows the onlapping strata atop the peak ring of the deposit. C) shows the more cohesive rubbly textures seen in less degraded features, while D) shows the powdery textures seen in more degraded graben features. Both green and white dots can coexist in grabens and are not restricted to certain regions in the basin. the identification of outcrops of and boulder falls derived from a cliff-forming, high albedo layer in Schrödinger Basin that we interpret as a suevite deposit. Suevite is a polymict impact-breccia which has high impact melt content as observed on terrestrial analogs (e.g., Ries and Chicxulub Crater) [6,7]. Suevite in the Ries crater is quarried and used as building blocks and concrete [6]. Large deposits of suevite may be useful for extraterrestrial in situ resource utilization (ISRU). Methods: We used high resolution imagery from Lunar Reconnaissance Orbiter (LRO) cameras and topographic data from Lunar Orbiter Laser Altimeter (LOLA). Points denoting the vertical extent of visible outcrops, as well as profiles, were taken across all outcrops and were fitted with a planar surface. Additionally, traces from the pulse-compressed LRS data were averaged across a portion of the north central basin to identify possible impact-related reflectors in the subsurface. Results: High albedo outcrops were mapped in graben cutting across the peak ring, central basin and annular trough, as well as the pyroclastic vent in the basin (Figure 1). Observed outcrops and associated boulder falls are consistent with an impactite layer up to 160 m thick. The apparent thickness is heterogeneous across the basin and additionally increases in local topographic lows in the peak ring. The layer closely follows the surface topography, being found at elevations of ~-4100 m to -4800 m in the basin, consistently buried under 10-20 m of material. The layer was additionally found atop the peak ring, where we observed a series of dipping planes that thin upslope, forming an onlapping geometry (Figure 1, B). Two textural types were identified within the mapped outcrops (Figure 1, C and D). One was more cohesive and mostly found in less degraded grabens, with boulder falls clearly originating from the outcrop. The second type is powdery in appearance, with less extensive boulder falls. Both largely appear heterogeneous and rubbly. The slopes of the texturally distinct, high albedo layer reach up to 53 degrees, and consistently were steeper than those of units below or above it, whether they were pyroclastic or regolith. This demonstrates this layer is a cliff-former relative to overlying or underlying material. Discussion: Our results suggest that the grabens formed post-impact expose a buried layer of material consistent with a melt-bearing impact breccia (suevite) that formed during the Schrödinger impact 3.8 Ga, buried under a layer of regolith 10-20 m thick in most parts of the basin. The magnitude of thickness of the layer is similar to those in terrestrial analog craters. Spectroscopic work has identified much of the regolith and the impactite layer in the annular trough and central basin to be melt rich, as seen in broader, shallower peaks in the spectrum compared to nonglassy counterparts of the same composition [5]. Photogeologic units mapped in the central basins were also identified in graben structures cutting through the peak ring where we identified these outcrops [5]. Hydrocode modeling of the Schrödinger impact implies that rocks which experience shock-pressures necessary to produce impact melt (>60 GPa) would be concentrated in the central basin forming a melt sheet, whereas the mapped melt-rich layer deposit is found capping the peak ring crystalline rocks, within the annular trough, and in the central basin and varying elevations. This pattern of deposition indicates some movement needed for emplacement [8]. The onlapping nature of deposits in the peak ring region allude to a lateral flow mechanism of emplacement consistent with a ground-hugging flow. Such a mechanism could explain the deposition of this material throughout the basin allowing for the unit’s presence inside and outside the topographic peak ring. Similar lateral transport evidence was observed in the Ries crater suevite in seismic data, which suggested a ground-hugging flow as an emplacement mechanism [9]. The observation of a somewhat slightly rubbly texture and presence of boulder falls beneath the cliff forming layer is consistent with a heterogenous material, such as a breccia, as opposed to columnar jointing expected in volcanic flows or melt sheets [10]. These lines of evidence are consistent with the cliff forming high albedo layer representing a massive suevite deposit. Such a deposit would be imperative in determining the stratigraphy of the Schrödinger impact basin and the relative timing of peak-ring excavation and deposition of melt-breccias in large impact basins on the Moon. Constraining thicknesses of this deposit will provide important context future geophysical investigations by the Farside Seismic Suite. Additionally, as possible sample collection from traverses across the basin are planned for the Endurance mission concept, identification of these outcrops highlights possible sampling areas for material that will teach us about the target lithology and basin formation kinematics. Finally, as suevite is used as a construction material on Earth, large deposits of this material could be useful for discussions of ISRU. Future investigations to understand the thickness variation and stratigraphic relationships of this suevite layer can help constrain models of the generation of impactite stratigraphy on the Moon, and as a planetary analog, and thus how shallow impact processes form crater floors. References: [1] French, B. M. & Koeberl, C. (2010) Earth-Science Reviews, 1-2, 123-170. [2] Stöffler, D. & Grieve, R. A. F. (2007), Impactites, 82-92, 111-125, 126-242. [3] Wilhelms D. (1987) Professional Paper 1348. [4] Shoemaker E. M. et al. (1994) Science, 266, 1851-1854. [5] Kramer G. Y. et al. (2013) Icarus, 223, 131-148. [6] Shoemaker E. M. et al. (1961) Journal of Geophysical Research, 66 (10), 3371-3378. [7] Christeson G. L. et al. (2021) Journal of Geophysical Research: Planets, 126, e2021JE006938. [8] Kring D. A. et al. (2016) Nature Communications, 7, 13161 [9] McCall N. et al. (2024) Geology, 52 (1), 39-44. [10] Masaitis (1999), Meteoritics and Planetary Science, 34, 691-711.