Lunar Impact Glasses: Probing the Moon's Surface and Constraining its Impact History

Abstract

Lunar impact glasses, formed during impact events when the regolith is quenched during the ejecta's ballistic flight, are small samples whose information can lead to important advances in studies of the Moon. For example, they provide evidence that constrains both the compositional evolution of the lunar crust and the timing of the lunar impact flux starting at ~4,000 million years ago. They are abundant in the lunar regolith and retain geochemical information that tells us where and when they formed. Thus, they provide important details about areas of the Moon both sampled and not sampled by Apollo or Luna missions or lunar meteorites. Additionally, as a result of these glasses possessing a chemical memory of formation location and age, studies of lunar impact glasses provide a foundation on which to conduct studies of impact glasses from other planetary bodies. A summary of past and current lunar impact glass investigations, using glasses from the Apollo 11, 12, 14, 15, 16, and 17 regoliths, along with plans for future work, will be presented.Plain Language Summary Although the location of impact ejection may not be known, the combination of widespread and random sampling makes lunar impact glasses powerful tools for temporal and geochemical explorations of lunar processes. For example, 40 Ar/ 39 Ar ages of lunar impact glasses have revealed evidence for episodes of enhanced impact flux within the first billion years of solar system history that tails off between 3,700 and 1,400 million years ago. Impact glasses, especially spherules, are more abundant in the past <1,000 million years, likely the result of sampling and/or preservation biases. Supporting their value as geochemical probes, impact glasses with compositions that are different from the regolith in which they were collected reveal information about lunar material not directly sampled by the Apollo astronauts or Luna spacecraft. Additionally, glass fragments are more likely to possess these "exotic" compositions compared to glass spherules. In summary, not only do the formation ages of lunar impact glasses constrain the timing of the Moon's impact flux, but their compositions can be used as constraints for lunar lithologies at the Apollo and Luna collection sites and beyond. Moreover, studies of lunar impact glasses can be adapted when it becomes possible to investigate glasses sampled from other planetary bodies.

Discerning Lunar Pyroclastic and Impact Glasses via Raman Spectroscopy

“…Because the PGs were derived from the mantle sources, they would provide valuable geochemical information of the lunar interior and the early differentiation of the Moon (Saal et al., 2008; Shearer & Papike, 1993). During the impact processes (from massive basin‐forming impacts to tiny micrometeorite impacts), impact glasses would be generated by melting and subsequently quenching of lunar surface diverse materials (Papike et al., 1998; Zellner, 2019). These glasses would retain a geochemical memory of their fused target materials and impact chronology (Delano, 1991; Zellner, 2019; Zellner et al., 2009).…”

“…During the impact processes (from massive basin‐forming impacts to tiny micrometeorite impacts), impact glasses would be generated by melting and subsequently quenching of lunar surface diverse materials (Papike et al., 1998; Zellner, 2019). These glasses would retain a geochemical memory of their fused target materials and impact chronology (Delano, 1991; Zellner, 2019; Zellner et al., 2009). In addition, impact processes also can generate nonmelt shocked glasses (e.g., maskelynite) by a solid‐state transition (e.g., Jaret et al., 2015; Kim et al., 2019).…”

Formation of Small Craters in the Lunar Regolith: How Do They Influence the Preservation of Ancient Melt at the Surface?

“…While most impact glass ages record events <3 Ga (Zellner, 2019), our model predicts that the large volumes of melt products that are present in the surface regolith should have formed during the basin-forming period (Figure 4b). As impact glasses only make up about ∼3-5% of the regolith (McKay et al, 1991), it is possible that a large part of these impact melt products can be found as melt rocks in the regolith.…”

“…Previous studies found an apparent excess of ages <0.5 Ga of impact glass spherules (Culler et al., 2000; Levine et al., 2005; Zellner & Delano, 2015). As an example, Figure 6b shows the relative abundance of the 40 Ar‐ 39 Ar ages of 128 lunar impact glass (70 spheres and 58 shards) assessed by Zellner (2019). One explanation for the excess of impact glass in this period is an increase in the impact flux during the late Copernican (Culler, 2000).…”

“…However, the generated impact melt can be altered, redistributed, and even destroyed by subsequent impacts. The long‐term impact mixing makes the distribution of impact melts complicated to interpret (Hartmann, 2003; Liu et al., 2019, 2020; Michael et al., 2018; Norman & Nemchin, 2014; Zellner, 2019).…”