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Physical and Chemical Evolution of Lunar Mare Regolith Item Type Article; text Authors O'Brien, P.; Byrne, S. Citation O’Brien, P., & Byrne, S. Physical and Chemical Evolution of Lunar Mare Regolith. Journal of Geophysical Research: Planets, e2020JE006634. DOI 10.1029/2020JE006634 Publisher Blackwell Publishing Ltd Journal Journal of Geophysical Research: Planets Rights Copyright © 2020 American Geophysical Union. All Rights Reserved. Download date 26/09/2021 22:26:13 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final published version Link to Item http://hdl.handle.net/10150/660051 RESEARCH ARTICLE Physical and Chemical Evolution of Lunar Mare Regolith 10.1029/2020JE006634 P. O'Brien1 and S. Byrne1 Key Points: 1Lunar and Planetary Laboratory, Tucson, AZ, USA • We present a novel three- dimensional landscape evolution and regolith transport model calibrated to the lunar mare • Gardening by secondary craters is the dominant control on regolith particle surface residence times Abstract The lunar landscape evolves both physically and chemically over time due to impact • Constraints from Apollo samples cratering and energetic processes collectively known as space weathering. Despite returned soil samples and remote sensing indicate space and global remote sensing reflectance measurements, the rate of space weathering in the lunar regolith is weathering maturation timescales of 7–19 Myr of cumulative exposure not well understood. To address this, we developed a novel three-dimensional landscape evolution model to simulate the physical processes that control the burial, excavation, and transport of regolith on airless bodies. Applying this model to the lunar mare, we find that over billions of years of surface evolution, Supporting Information: • Supporting Information S1 material typically spends only a few million years on the surface where it is exposed to the effects of space weathering. The small surface residence times are a result of vigorous mixing by small-scale impacts, predominantly driven by secondary crater formation. We deduce the rate of space weathering Correspondence to P. O'Brien, by comparing our modeled distribution of surface residence times on the lunar mare to measurements [email protected] of space weathering maturity from Apollo soil samples and orbital surface reflectance datasets. These chemical constraints indicate that soil on the lunar mare reaches maturity in 7 Myr of cumulative surface Citation: exposure though due to uncertainties in the rate of small secondary crater production, this timescale could O'Brien, P., & Byrne, S. (2021). Physical be 2–3 times higher. Weathering progresses more rapidly upon initial exposure to space but the surface and chemical evolution of lunar residence time required to achieve maturity is realized over billions of years as regolith is repeatedly mare regolith. Journal of Geophysical Research: Planets, 126, e2020JE006634. buried and exposed by small impacts. https://doi.org/10.1029/2020JE006634 Plain Language Summary On the Moon, large impact craters churn the upper soil layers Received 20 DEC 2019 and micrometeorites, galactic cosmic rays, and the solar wind alter the chemical structure of material Accepted 22 NOV 2020 on the surface in a process called space weathering. The relationship between these processes is crucial to understanding how quickly planetary surfaces are space weathered. To study this problem, we have developed a three-dimensional computer model that simulates lunar-like landscapes that evolve over time from flat surfaces to cratered landscapes. Using this model, we measure how long material spends on the surface where it would be exposed to the space environment. We compared the results of this model to chemical measurements of lunar soil returned by the Apollo astronauts and orbital measurements of the lunar surface. We find that over the last 3.5 billion years most material has been on the surface for only a few million years in total as it is repeatedly buried and excavated by small craters. Space weathering must therefore occur rapidly to produce the observed chemical properties of the lunar soil. 1. Introduction The surfaces of airless bodies like the Moon are directly exposed to the space environment and as a result change both physically and chemically over time due to processes collectively known as space weather- ing. The rate of space weathering on the lunar landscape is poorly understood because these two facets of surface evolution have yet to be fully linked. Macroscopic physical processes like impact cratering disrupt and overturn the upper surface layer comprised of loose, unconsolidated material known as soil or regolith (Shoemaker et al., 1969). As the landscape evolves and material is transported across the surface, the micro- scopic chemical structure of material on the surface is altered by micrometeorite impacts, cosmic rays, and the solar wind (Hapke, 2001; Pieters & Noble, 2016). These processes are deeply coupled since the amount of time material spends on the surface exposed to the space environment depends on the rate at which ma- terial is cycled in and out of the uppermost regolith layer. © 2020. American Geophysical Union. Analysis of lunar soil properties from Apollo program samples and remote sensing surface reflectance All Rights Reserved. measurements have characterized the degree of space weathering, or maturity, of the lunar regolith (Lucey O’BRIEN AND BYRNE 1 of 31 Journal of Geophysical Research: Planets 10.1029/2020JE006634 et al., 2000; Morris, 1976). Maturity metrics quantify the amount of weathering products accumulated by a sample, but not the timescale over which those chemical products were accumulated. In the absence of definitive, widespread measurements of cumulative surface exposure ages, the rate at which the surface of the Moon is weathered by the space environment remains unknown. Landscape evolution models, like those applied by Hartmann and Gaskell (1997) and Richardson (2009) to understand cratering equilibrium, can be applied here to investigate the timescales of regolith space weathering exposure on airless bodies. We have developed such a model and calibrated it to reproduce the histories and topographic statistics of surfaces on the lunar maria by incorporating constraints from high-resolution lunar topography (Henriksen et al., 2017; Robinson et al., 2010; Smith et al., 2010). The model is used to track the trajectories of tracer particles as synthetic lunar surfaces evolve over time and to measure how long particles reside in the upper millimeter of regolith; that is, on the surface (Morris, 1978). By combining the distribution of regolith expo- sure ages with measurements of soil maturity (Lemelin et al., 2016; Lucey et al., 2000), we link the physical and chemical evolution of the Moon's surface to estimate the rate of space weathering on the lunar maria. While presently applied only to four Apollo landing sites, this work provides a framework for a broader understanding of how the surface of the Moon and other airless bodies, like Mercury, Ceres, and Vesta are modified over time by space weathering. 2. Background In addition to macroscopic crater-forming impacts, the Moon is continually bombarded by micrometeor- ites, particles from the solar wind, and cosmic rays. These energetic processes, collectively known as space weathering, induce chemical changes in material residing on the surfaces of airless bodies that lack the shielding of an atmosphere or magnetic field (Morris, 1978). Lunar soils that have undergone space weath- ering show lowered albedo, spectral reddening, and subdued mineral absorption bands (Hapke, 2001; Keller & McKay, 1997; Pieters & Noble, 2016; Pieters et al., 1993, 2000; Taylor et al., 2001). From analysis of re- turned lunar samples, it was determined that the primary agent of these chemical changes is the formation of nanophase metallic iron and other optically active opaque (OAOpq) particles as a result of the energetic processes in the space environment operating to reduce FeO in the lunar soil (Britt & Pieters, 1994; Cassidy & Hapke, 1975; Hapke, 2001; Pieters & Noble, 2016). These particles form within agglutinitic glasses and on the rims of grains through irradiation and sputtering processes as well as from condensation of vapor- ized material following micrometeorite impacts (Keller & McKay, 1993, 1997; Noble et al., 2005; Taylor et al., 2001). The degree of space weathering, as characterized by some quantitative measure of the amount of these space weathering products that a surface soil has accumulated, is termed soil maturity (McKay et al., 1974; Morris, 1976). Laboratory techniques can address the rate of space weathering by quantifying the amount of OAOpq particles that have formed in a planetary regolith (e.g., Ramirez & Zega, 2016). Previ- ous studies have placed limits on the space weathering rate using solar flare track densities and amorphized rim widths which accumulate in lunar soil grains and saturate at inferred solar wind exposure ages of a few million years (Keller & Berger, 2017; Keller et al., 2016). Remote sensing reflectance measurements have generated maturity proxies like the optical maturity (OMAT) parameter (Lucey et al., 2000). Though sensitive only to the uppermost few regolith grains, orbital measurements provide information about how maturity varies across the entire lunar surface and relates to age and composition. Space weathering is ubiquitous on airless bodies and much work has been done to characterize
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