Physical and Chemical Evolution of Lunar Mare Regolith

Abstract

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 weathering. 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 microscopic 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 material is cycled in and out of the uppermost regolith layer. Analysis of lunar soil properties from Apollo program samples and remote sensing surface reflectance measurements have characterized the degree of space weathering, or maturity, of the lunar regolith (Lucey Abstract The lunar landscape evolves both physically and chemically over time due to impact cratering and energetic processes collectively known as space weathering. Despite returned soil samples and global remote sensing reflectance measurements, the rate of space weathering in the lunar regolith is 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, 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 by comparing our modeled distribution of surface residence times on the lunar mare to measurements 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 exposure though due to uncertainties in the rate of small secondary crater production, this timescale could be 2-3 times higher. Weathering progresses more rapidly upon initial exposure to space but the surface residence time required to achieve maturity is realized over billions of years as regolith is repeatedly buried and exposed by small impacts. Plain Language Summary On the Moon, large impact craters churn the upper soil layers and micrometeorites, galactic cosmic rays, and the solar wind alter the chemical structure of material on the surface in a process called space weathering. The relationship between ...

Solar energetic particle tracks in lunar samples: A transmission electron microscope calibration and implications for lunar space weathering

“…Recent modeling work by O’Brien and Byrne (2021) suggests that lunar maria soils reach “maturation” in ˜7–19 Myr of surface exposure that is accumulated in a discontinuous process of burial and excavation by small secondary impacts over the full 3.5 Gyr history of the maria. While we agree with the surface exposure estimate of up 19 Myr, it is unlikely that lunar soil grains have been near‐surface for 3.5 Gyr because exposures of this length, even at depths up to meters, would result in incredibly high solar energetic particle track and GCR effects that are not observed in samples.…”

“…The results from our study show a continuous exposure, at least up to 2 Myr are possible (based on 64455). Grain destruction effects and track densities suggest that the O’Brien and Byrne (2021) approach may be underestimating the overturn rate where grains are either buried deeply and never see the Sun again, or they destroyed by micrometeorite impacts.…”

Intricate Regolith Reworking Processes Revealed by Microstructures on Lunar Impact Glasses

“…Occasional discovery of meteoritic components in regolith samples further suggests multiple material sources involved in regolith formation (Joy et al., 2012). The relative abundance of heterochthonous components in lunar regolith was frequently estimated using parameters‐dependent ballistic sedimentation models (Huang et al., 2017; O’Brien & Byrne, 2021; Petro & Pieters, 2006; Xie et al., 2020). However, there are few constraints on the interaction mechanism between distal ejecta and local regolith particles based on sample analyses.…”

Nanophase Iron Particles Derived From Fayalitic Olivine Decomposition in Chang'E‐5 Lunar Soil: Implications for Thermal Effects During Impacts

“…The thickness of the solar wind rim and the density of the solar flare tracks observed in our study also suggest that the Chang'E‐5 grains are mature. Combined with the relatively younger exposure age of the Chang'E‐5 samples, it is apparent that significant space weathering takes place very quickly on the lunar surface (O'Brien & Byrne, 2021).…”

“…On December 17, 2020, Chang'E‐5 successfully returned the new younger lunar regolith sample from Mons Rümker located in northern Oceanus Procellarum to Earth. The evolution processes (excavation, mixing, burial, transport) of lunar regolith make it likely that younger Chang'E‐5 lunar soils will have a very short surface residence time (O'Brien & Byrne, 2021). The excavation ages of most studied regolith are estimated to be several tens of Ma and linked to the Xu Guangqi (IC‐396) and IC‐265 craters (Qian et al., 2021).…”