Ephemeral Scars: Electrostatic Dust Redistribution in the Epiregolith Around New Lunar Craters

EPHEMERAL SCARS: ELECTROSTATIC DUST REDISTRIBUTION IN THE EPIREGOLITH AROUND NEW LUNAR CRATERS. E. J. Speyerer, M. S. Robinson, A. K. Boyd, 1Intuitive Machines, 101 E Jackson St, Phoenix, AZ 85004. Introduction: Multi-temporal time series captured by the Lunar Reconnaissance Orbiter Camera provide valuable insights into contemporary surface changes. These images reveal that impact events significantly alter regolith structure by jetting and depositing fragmented rock, melt, and vapor over significant distances (>1000 crater radii). These processes reduce regolith porosity and destroy the highly porous “fairy castle” configuration of the uppermost layer, or epiregolith [1]. Here, we document the first ever observed changes over time in the photometric response of the surface consistent with the rapid reconstruction of this porous structure within the first decade after an impact. We propose that these changes are driven by the redistribution of dust particles through the solar radiation-induced photoelectric effect. This reworking of the epiregolith causes new particle faces to be directly exposed at the surface, affecting the time at which individual facets are exposed to space weathering processes. This continual process and exposure of fresh particle surfaces and porosity change suggest a dynamic equilibrium between impact-driven disruption and regolith recovery. These findings not only enhance our understanding of lunar surface processes but also have broader implications for studying the evolution of regolith on other airless bodies throughout the solar system. Temporal Imaging: Ongoing temporal analysis has revealed hundreds of newly formed impact craters on the surface [2,3]. Initial studies identified distinct ray patterns extending tens to hundreds of crater diameters from these fresh impacts [2]. This study showcases one of these new craters formed in 2012 (Fig. 1). The 70-meter-diameter crater (22.352°S 320.270°E) in Mare Humorum (Fig. 1a) exhibits the most extensive ray system observed to date (Fig. 1b), with rays extending over 1,000 crater diameters. During the cratering process, small amounts of jetted vapor and impact melt scour the surrounding surface, altering the local surface roughness2 (Fig 1c). This scouring can smooth the nearby regolith at microscopic to macroscopic scales, resulting in a less backscattering surface [2,3]. Conversely, the deposition of impact melt and fine ejecta particles can roughen the regolith at similar scales, increasing backscattering. These variations in backscatter are effectively depicted in phase ratio observations (small phase/large phase; Fig. 1c) [3-5], where larger values indicate roughened, more backscattering surfaces and smaller values correspond to smoother areas. When a rough surface is illuminated from small angles above the horizon, many small-scale shadows are cast; when the Sun is high above the horizon, fewer shadows are cast. Following the formation of contemporary impacts, LROC is commanded to target these regions with new WAC observations each month. While the WAC pixel scale is larger (70 to 200 m) than the craters we are observing, the signal-to-noise ratio achieved by combining hundreds of observations allows discrimination of subtle variations in backscattering effects. Ongoing monitoring highlights the ever-changing nature of the lunar surface and offers a way to explore how electrostatic forces influence dust movement. Using an empirical photometric function [6]: where i is the incidence angle, e is the emergence angle, g is the phase angle, and t is time, we can begin tracking time-dependent changes to the surface following each impact event. This smooth function can be solved using a least squares optimization routine. While the first seven parameters (a0 to a6) have not been directly related to the physical characteristics of the regolith, the equation accurately describes the shape of the phase curve, even with a sparse sampling. Additionally, the function accurately models (R-squared > 0.99) the reflectance over a wide range of lighting and viewing geometries, normalizing datasets with low residuals [6]. To track temporal changes, we introduce additional, timedependent components (a7 to a13) to the exponent of the expression. These additional values would be zero in a perfect dataset with no variation. Any shift away from zero suggests that changes have occurred over time. By introducing a single additional timedependent component (a7t), where t denotes the time in years since crater formation, we find that the parameter map for a7 (keeping a8 to a13 pinned to zero; Fig 1d) correlates with the change in backscatter observed across the crater rays (Fig. 1b). This correlation is consistent with significant post-impact modifications along the rays occurring within the decade following crater formation. In the case of the 70 m diameter crater, we found p-values less than 0.05 (2.1E-4 ± 4.7E4) in the crater ejecta. Furthermore, the surrounding area, unaffected by the impact event, had p-values > 0.05 (0.37 ± 0.30), indicating that time-varying effects do not alter the background as fast as the freshly formed crater rays. The complete model described by the equation illustrates how the photometric response changes over time. The change to the surface is most significant in the decades after the impact event, and as time passes, the surface change gradually dissipates. The photometric response of the surface is closer to that of the original surface before the impact occurred. By plotting the resulting phase function in various periods, we can see how long it takes for the distal rays of the crater to disappear. We find that after only a decade, the distal rays are already fading in the case of the 70 m diameter crater. We can extrapolate that they may disappear within 50 years, although this process may slow as the reflectance of the ejecta returns closer to pre-impact values. Discussion: Our study reveals that the rays of newly formed impact craters undergo modification within a decade of their formation, as evidenced by changes in backscattering properties observed in the LROC WAC observations, contradicting the prevailing view of the surface as largely static. These observations provide direct evidence that shorttimescale dynamic processes are crucial in continuously reshaping the regolith. The primary mechanism we propose for this rapid modification is electrostatic levitation induced by the photoelectric effect from solar ultraviolet radiation. Differential charging occurs as the surface transitions from day to night across the terminator: the sunlit side is positively charged, and the shadowed areas are negatively charged. This charge differential can mobilize fine dust particles, lifting them several centimeters above the surface [7]. Once lofted, these grains resettle since the induced velocities cannot overcome the Moon’s gravitational pull. As the individual grains resettle, residual electrostatic charges and Van der Waals forces form the “fairy castle” structure—a delicate, porous arrangement of particles that comprise the epiregolith (highly porous (~90%) upper portion of the regolith, ~250 microns thick [1]) characterized by high surface roughness and high porosity in the upper few microns of regolith. When the distal rays initially form, the delicate fairy castle structure is destroyed, similar to blast zones observed at crewed and robotic landing sites. This change to the physical properties of the regolith by smoothing the surface on a microscopic scale. The force of the jetted particles and vapor contributes to macroscopic alterations on the surface, significantly impacting the backscattering properties observable at the resolution of NAC and WAC pixels. Then, as particles are levitated with each terminator crossing and softly settling back down to the surface, they allow the regolith to reform the fairy castle structure. This reformation increases the surface’s extreme backscattering properties, effectively diminishing the visibility of the crater rays over time. Consequently, the rays become less discernible, and the surface appears to “heal” from the impact event within decadal-length timescales. Reworking the top few microns of regolith will affect space weathering rates by exposing unaltered grain sides to cosmic and solar ray irradiation, solar wind implantation, and sputtering. Each pass of the terminator allows the top grains to be redistributed and churned, making this epiregolith homogenous compared to the material at depth, in which exposure is only influenced by the emplacement of a new impact crater or other resurfacing processes. Since we witness these changes fading the distal reflectance zones of newly formed craters, we can also confirm that this process is acting on the top few microns much faster than that of micrometeorites, minor impacts, and distal secondary impacts. The rapid crater ray reflectance changes documented here are consistent with the hypothesis that electrostatic levitation and the subsequent resettlement of dust particles are critical factors in modifying the surface post-impact. These findings highlight the ever-changing nature of the regolith’s top layer and confirm that electrostatic processes are key in shaping the physical properties of the epiregolith. References: [1] Mendell & Noble (2010) LPSC #1348 [2] Speyerer, et al. (2016). Nature 538, 215218 [3] Robinson, et al. (2015) Icarus 252, 229-235 [4] Kaydash, et al. (2011) Icarus 211, 89-96 [5] Shkuratov, et al. (2011) Planet and Space Sci 59, 1326-1371 [6] Boyd & Robinson (2019) LPSC, #1992. [7] Wang, et al. (2016) GRL 43, 6103-6110. Fig 1. Newly formed 70 m crater. (a) after image 200 m across; (b) temporal ratio (after/before) images made with LROC WAC observations before and after the impact event; (c) WAC phase ratio images: (i=g=30° e=0°)/(i=g=60° e=0°); (d) Parameter map for coefficient a7 (a8 to a13=0); (b) to (d) 200 km across.