Unusual Crater Morphologies on Lunar Impact Melt Pools

UNUSUAL CRATER MORPHOLOGIES ON LUNAR IMPACT MELT POOLS. A. Oetting1,2, W. Iqbal2, J. W. Head3, C. H. van der Bogert2, H. Hiesinger2, L. Wueller2, T. Heyer2; 1European Space Agency, European Space Research and Technology Centre (ESA/ESTEC), Directorate of Human and Robotic Exploration, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands ([email protected]); 2Universität Münster, Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany; 3Brown University, Providence, RI 02912 USA. Introduction: Simple craters are generally associated with a bowl-shaped morphology. However, there are cases where craters deviate from the typical bowlshaped morphology when there are different target layers of material with different physical properties (e.g., strength, density). Such craters are e.g., referred to as "bench" or "pan" craters [1-3] and were discovered in the 1960s and 1970s on smooth-appearing material and impact melt pools (e.g., around Aristarchus, King, Tycho, and Theophilus craters). Their formation is suggested to be the result of layering effects of a layer of regolith on top of more competent rock [3]. Further atypical crater morphologies, including central-mound, flat-bottomed, and concentric-shaped craters, have been identified in mare areas [1]. These morphologies are also attributed to the formation in layers of loose, non-cohesive material overlying competent rock [1]. The objective of a detailed investigation of atypical crater morphologies is to assess the presence and/or thickness of different geological layers. This assessment could provide hypotheses and potential constraints on material properties and layering effects. Data: An examination of 20 crater-exterior impact melt pools at Copernican-aged craters was performed to identify craters with atypical morphologies. The image data of the Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) with a pixel scale of ~0.5 m/px [4, 5] were utilized. Supporting data sets, including digital elevation models (DEMs) and spectral Clementine ultravioletvisible color ratio composite maps [6], were insufficient due to inadequate data resolution. The calculation of shadow-length heights could also not be performed due to insufficient heights at the data resolutions. Results: Six different crater classes could be identified (Figure 1) within craters superposed on exterior impact melt pools around Copernican-aged craters, four crater classes identified by [7] and five by [8]. The six identified crater classes have the following characteristics: • Class 1: Bull's eye – smooth surface, no boulders, central peak, inner ring, outer ring; • Class 2: Hummocky floor – hummocky surface, boulders, low relief, steep walls, distinct but undulating rim, no central feature; • Class 3: Flat floor central depression – hummocky surface, boulders, low relief, distinct but undulating rim, central depression; • Class 4: Flat floor central peak – hummocky surface, boulders, low relief, distinct but undulating rim, central peak; • Class 5: No-rim – hummocky surface, boulders, extremely low relief, muted circular rim, which is mostly visible as an albedo difference rather than in the topography data, unclear central feature; • Class 6: Non-circular – hummocky surface, boulders, higher relief than the other crater classes, angular-shaped crater rim, no central feature. Figure 1: Six classes of unusual crater morphologies identified on exterior impact melt pools close to Copernican-aged craters. The illumination direction is from the upper left in all images. The preliminary analysis suggests that the unusual crater morphologies occur at diameters ranging from 10 m to 100 m. Ejecta blankets are either absent or only rudimentarily developed around these craters. Discussion: The formation of these unusual crater morphologies remains to be elucidated. In order to postulate a series of formation hypotheses, we call upon a comparison with laboratory experiments [9-11] and observations on other impact melt deposits [1216]: • Class 1: Bull's eye – impact into viscous or fluid melt? • Class 2: Hummocky floor – impact into possibly partly viscous, partly solid material? • Class 3: Flat floor central depression – impact into partly solidified and partly viscous or fluid material, oscillation of the central peak and solidification in a negative position? • Class 4: Flat floor central peak – impact into partly solidified and partly viscous or fluid material, oscillation of the central peak and solidification in a positive position? • Class 5: No-rim – impact into solidified melt either with extremely high or extremely low impact energy? • Class 6: Non-circular – impact of debris into solidified or semi-solidified melt? In this context, crater classes 2, 3, and 4 could potentially exhibit indications of a mixed and layered target of partly solidified melt. The smooth-appearing class 1 craters may have formed in viscous material [9, 11]. Classes 5 and 6 show no evidence of such viscous material and are therefore presumably formed in a solid target. We also hypothesize that these Copernican-aged impact melt pools may not have had sufficient time since their formation to accumulate a significant amount of regolith. This accumulation would typically result in the formation of an incoherent top layer, which, in turn, would produce crater morphologies such as bench and pan craters. Conclusion: Unusual crater morphologies have been observed to be common features on lunar impact melt deposits, although the total number of craters exhibiting unusual morphologies is significantly lower compared to typical bowl-shaped simple craters. The identified unusual morphologies might have formed in viscous, fluid, or remelted material (i.e., class 1), layered targets of solidified melt and underlying viscous or fluid melt (i.e., classes 2, 3, and 4), and exclusively solidified material (classes 5 and 6). In a subsequent study, we will investigate additional hypotheses for their origins. Contrary to the unusual crater morphologies documented in the literature [1-3], which are attributed to layering effects of regolith on top of more coherent rock, the majority of the six investigated crater classes presented here might have resulted from coherent and solidified impact melt on top of either fluid or viscous melt, or potentially bedrock. A comparison of impact melt cooling and solidification times to this range of morphologies will enable further testing of the aforementioned hypothesis. Acknowledgments: AO was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 263649064 – TRR 170 and is now funded by ESA – ESTEC, Directorate of Human and Robotic Exploration. WI and TH are funded by the Deutsches Zentrum für Luftund Raumfahrt (DLR, German Aerospace Center), Grant 50OO2102. JWH received funding from the NASA Lunar Reconnaissance Orbiter Lunar Laser Altimeter Experiment (80NSSC19K0605). LW, CvdB and HH are funded by the DLR Project 50OW2402. References: [1] Oberbeck, V. R., & Quaide, W. L. (1968) Icarus, 9, 446-465. [2] Schultz, P. H. (1976) UT Press, p. 626. [3] Schultz, P., & Spencer, J. (1979) LPSC, Vol. 10, pp. 1081-1083. [4] Robinson, M. S., et al. (2010) Space Sci. Rev., 150(1), 81–124. [5] Robinson, M. S. (2011) NASA Planetary Data System [Dataset]. [6] Lucey, P. G. et al. (2000) JGR, 105(E8), 20377–20386. [7] Oetting, A. et al. (2024) submitted to JGR: Planets. [8] Oetting, A. (2024) submitted dissertation, Univ. 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