Rethinking Lunar Mare Basalt Regolith Formation: New Concepts of Lava Flow Protolith and Evolution of Regolith Thickness and Internal Structure

Abstract

Lunar mare regolith is traditionally thought to have formed by impact bombardment of newly emplaced coherent solidified basaltic lava. We use new models for initial emplacement of basalt magma to predict and map out thicknesses, surface topographies and internal structures of the fresh lava flows, and pyroclastic deposits that form the lunar mare regolith parent rock, or protolith. The range of basaltic eruption types produce widely varying initial conditions for regolith protolith, including (1) autoregolith, a fragmental meter-thick surface deposit that forms upon eruption and mimics impact-generated regolith in physical properties, (2) lava flows with significant near-surface vesicularity and macroporosity, (3) magmatic foams, and (4) dense, vesicle-poor flows. Each protolith has important implications for the subsequent growth, maturation, and regional variability of regolith deposits, suggesting wide spatial variations in the properties and thickness of regolith of similar age. Regolith may thus provide key insights into mare basalt protolith and its mode of emplacement. Plain Language Summary Following recent studies of how lava eruptions are emplaced on the lunar surface, we show that solid basalt is only one of a wide range of starting conditions in the process of forming lunar soil (regolith). Gas present in the lavas during eruption also produced bubbles, foams, and explosive products, disrupting the lava and forming other starting conditions for mare soil parent material.

The Lunar Mare Ring‐Moat Dome Structure (RMDS) Age Conundrum: Contemporaneous With Imbrian‐Aged Host Lava Flows or Emplaced in the Copernican?

“…Regolith is created by impact gardening of the lunar surface (Shoemaker et al., 1969) and its mean thickness in Maria is estimated to be about 4–5 m (e.g., Bart et al., 2011; Basilevsky, 1974; McKay et al., 1991; Shkuratov & Bondarenko, 2001). The process of regolith formation encompasses two interrelated issues: (a) craters are excavated at the impact point by penetration through the regolith into underlying basaltic regolith protolith (e.g., Head & Wilson, 2020), resulting in an increase in the thickness of the regolith in any given region; (b) ejecta from these craters form regolith‐like material, which is thicker closer to the crater and progressively thinner with distance. Although a minimal part of the ejecta is ballistically transported to great distances (kilometers and tens of kilometers), a significant part of the ejecta is deposited near the point of impact from a few tens to a few hundred meters depending on the size and velocity of the impactor.…”

“…Finally, the uppermost part of the foam layer reaches the hard vacuum causing the bubbles to burst, releasing gas, and generating a layer of glass fragments. This disintegration process extends down into the spreading foam and this produces a fine‐grained fragmental layer with essentially no cohesive strength—a kind of instant regolith (autoregolith; see Head & Wilson, 2020)—at the top of the foam layer (Wilson & Head, 2017a; Wilson et al., 2019). It seems unlikely that this fragmental layer will have a surface appearance that reflects the cracks in the underlying original crust of the flow or even cracks in the foam itself as it cools.…”

“…Such linear patterns should be investigated in further analysis of individual RMDS clusters. Immediate Post‐Emplacement Deformation of Domes: The second boiling process that leads to RMDS formation occurs in a lava flow that has come to a halt and is cooling and solidifying, and thus RMDSs are predicted not to show any deformation by shearing due to any lateral flow movement. Composition/Mineralogy of Domes and Host Lava Flows: The basic composition and mineralogy of the domes and the host lava flows should be very similar, but the domes themselves should contain a higher proportion of glass shards due to the fragmentation of the magma emerging from the surface crack at the start of dome growth. Nature of Dome Material: RMDS dome material is predicted to be a basaltic lava foam characterized by a vesicularity of ∼50%–60%. This range is dictated by the requirement that to explain the dome morphology the foam must have a vesicularity that is, large enough (>30–40%) to ensure a significant yield strength and viscosity, but is small enough (<∼75%) to ensure that the foam does not disintegrate under shearing forces. Initial Upper Layer of Dome Material: Dome material should be overlain by an “autoregolith,” a layer of shattered foam caused by eruption into a vacuum, composed of a mixture of loosely packed glass shards and chilled magma droplets up to a few meters‐thick and with ∼30% void space, the amount expected for any unwelded accumulation of irregular brittle fragments. Dome Stratigraphy and Regolith Development: What is the “regolith protolith” (Head & Wilson, 2020) of the domes, the stratigraphy of the substrate immediately following their emplacement? The Wilson et al.…”

“…Dome Stratigraphy and Regolith Development: What is the “regolith protolith” (Head & Wilson, 2020) of the domes, the stratigraphy of the substrate immediately following their emplacement? The Wilson et al.…”

The Cauchy 5 Small, Low‐Volume Lunar Shield Volcano: Evidence for Volatile Exsolution‐Eruption Patterns and Type 1/Type 2 Hybrid Irregular Mare Patch Formation

“…First, plates of the partly solidified lava lake floor will be rafted out onto the small shield rim and flanks. Second, the upper surfaces of the extremely vesicular flows will undergo a mild explosive activity into the overlying vacuum to form a meters‐thick layer of “auto‐regolith” (Head & Wilson, ), a carpet of explosively ruptured bubble wall fragments and glass shards that protects the underlying flow from further explosive disruption (Wilson et al, ). As the very bubble‐rich/vesicular lava flows on the flanks of the shield continue to cool below the auto‐regolith layer, second boiling causes the exsolved bubbles and foams to continue to form, grow, and to migrate laterally and rise vertically; shear from final flow emplacement and cooling can locally break down bubbles and form voids beneath the cooling and thickening auto‐regolith and solidified flow surface.…”