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We investigate the implications of prolonging the equilibrium crystallization (EQX) stage of lunar magma ocean (LMO) solidification beyond the oft-modeled 50% volume solids, to 60%. Most models of two-stage LMO crystallization halt the EQX phase once 50% of a molten Moon (post-core formation) solidifies, after which the remaining 50% of the LMO solidifies via fractional crystallization (FRX). We quantitatively show through a simple scaling analysis that compares crystal settling velocity to vertical convective velocity that the early EQX regime can operate up to (and possible even slightly beyond) 60% volume solids. Phases that stabilize during the EQX and FRX regimes are then computed using Perple_X (thermodynamic calculator) along with the hp633ver database and associated activity-composition relations for solid solutions, and consider an adiabat that remains between the liquidus and solidus. Early results show two key findings: 1) only low volumes (~2%) of ilmenite form over ~50-km thick upper mantle layers for both 50% and 60% EQX regimes, suggesting that a mantle overturn may have been sluggish and/or limited in depth (dense ilmenite is thought to have been a critical driver of late-stage mantle mixing); and 2) contrary to most published two-stage LMO models, a refractory-enriched (i.e. high Al2O3) bulk silicate Moon is not required to produce garnet in the lunar mantle, assuming an Earth-like bulk silicate Moon composition with an alumina content of ~4 wt.%. To complement and test these numerical phase equilibria model results, a series of piston-cylinder experiments is underway that simulate the pressures and temperatures experienced by an FeO+TiO2-rich residual LMO in order to assess the volume and distribution of ilmenite produced during LMO solidification. These results are compared to those of the numerical phase equilibria models. Despite the model-dependent nature of these results, they provide a unique insight into potential LMO crystallization that has not been previously considered in the literature. 

Abstract It is generally accepted that the Moon accreted from the disk formed by an impact between the proto-Earth and impactor, but its details are highly debated. Some models suggest that a Mars-sized impactor formed a silicate melt-rich (vapor-poor) disk around Earth, whereas other models suggest that a highly energetic impact produced a silicate vapor-rich disk. Such a vapor-rich disk, however, may not be suitable for the Moon formation, because moonlets, building blocks of the Moon, of 100 m–100 km in radius may experience strong gas drag and fall onto Earth on a short timescale, failing to grow further. This problem may be avoided if large moonlets (≫100 km) form very quickly by streaming instability, which is a process to concentrate particles enough to cause gravitational collapse and rapid formation of planetesimals or moonlets. Here, we investigate the effect of the streaming instability in the Moon-forming disk for the first time and find that this instability can quickly form ∼100 km-sized moonlets. However, these moonlets are not large enough to avoid strong drag, and they still fall onto Earth quickly. This suggests that the vapor-rich disks may not form the large Moon, and therefore the models that produce vapor-poor disks are supported. This result is applicable to general impact-induced moon-forming disks, supporting the previous suggestion that small planets (<1.6R_⊕) are good candidates to host large moons because their impact-induced disks would likely be vapor-poor. We find a limited role of streaming instability in satellite formation in an impact-induced disk, whereas it plays a key role during planet formation. 

Abstract The amount of vapor in the impact-generated protolunar disk carries implications for the dynamics, devolatilization, and moderately volatile element isotope fractionation during lunar formation. The equation of state (EoS) used in simulations of the giant impact is required to calculate the vapor mass fraction (VMF) of the modeled protolunar disk. Recently, a new version of M-ANEOS (Stewart M-ANEOS) was released with an improved treatment of heat capacity and expanded experimental Hugoniot. Here, we compare this new M-ANEOS version with a previous version (N-SPH M-ANEOS) and assess the resulting differences in smoothed particle hydrodynamics (SPH) simulations. We find that Stewart M-ANEOS results in cooler disks with smaller values of VMF and in differences in disk mass that are dependent on the initial impact angle. We also assess the implications of the minimum “cutoff” density (ρ_c), similar to a maximum smoothing length, that is set as a fast-computing alternative to an iteratively calculated smoothing length. We find that the low particle resolution of the disk typically results in >40% of disk particles falling toρ_c, influencing the dynamical evolution and VMF of the disk. Our results show that the choice of EoS,ρ_c, and particle resolution can cause the VMF and disk mass to vary by tens of percent. Moreover, small values ofρ_cproduce disks that are prone to numerical instability and artificial shocks. We recommend that future giant impact SPH studies review smoothing methods and ensure the thermodynamic stability of the disk over simulated time. 

Abstract One of the unique aspects of Earth is that it has a fractionally large Moon, which is thought to have formed from a Moon-forming disk generated by a giant impact. The Moon stabilizes the Earth’s spin axis at least by several degrees and contributes to Earth’s stable climate. Given that impacts are common during planet formation, exomoons, which are moons around planets in extrasolar systems, should be common as well, but no exomoon has been confirmed. Here we propose that an initially vapor-rich moon-forming disk is not capable of forming a moon that is large with respect to the size of the planet because growing moonlets, which are building blocks of a moon, experience strong gas drag and quickly fall toward the planet. Our impact simulations show that terrestrial and icy planets that are larger than ~1.3−1.6 R ⊕ produce entirely vapor disks, which fail to form a fractionally large moon. This indicates that (1) our model supports the Moon-formation models that produce vapor-poor disks and (2) rocky and icy exoplanets whose radii are smaller than ~1.6 R ⊕ are ideal candidates for hosting fractionally large exomoons. 

Determining the presence or absence of a past long-lived lunar magnetic field is crucial for understanding how the Moon’s interior and surface evolved. Here, we show that Apollo impact glass associated with a young 2 million–year–old crater records a strong Earth-like magnetization, providing evidence that impacts can impart intense signals to samples recovered from the Moon and other planetary bodies. Moreover, we show that silicate crystals bearing magnetic inclusions from Apollo samples formed at ∼3.9, 3.6, 3.3, and 3.2 billion years ago are capable of recording strong core dynamo–like fields but do not. Together, these data indicate that the Moon did not have a long-lived core dynamo. As a result, the Moon was not sheltered by a sustained paleomagnetosphere, and the lunar regolith should hold buried 3 He, water, and other volatile resources acquired from solar winds and Earth’s magnetosphere over some 4 billion years. 

Abstract The Vredefort impact structure, located in South Africa and formed 2.02 Ga, is the largest confirmed remnant impact crater on Earth. The widely accepted impactor diameter and velocity to form this crater are 15 km and 15 km/s, respectively, which produce a crater diameter of 172 km. This is much smaller than the most commonly cited estimates (250–280 km), and while previous results were able to match the geologic evidence known at that time, these impact parameters are not consistent with more recent geological constraints. Here, we conduct impact simulations to model the Vredefort crater formation with the shock physics code impact Simplified Arbitrary Lagrangian Eulerian (iSALE). Our numerical simulations show that combinations of the impactor diameter and impact velocity of 25 km and 15 km/s or 20 km and 25 km/s are able to recreate the larger crater size of ∼250 km. Moreover, these configurations can reproduce shock‐metamorphic features present in the impact structure today, including the distributions of breccia, shatter cones, planar deformation features in quartz and zircon, and melt. Our model also predicts that Vredefort and Karelia, Russia, where an ejecta layer from the impact was found, were approximately 2,000–2,500 km apart based on the layer thickness. Additionally, we use this model to predict the potential global effects of such a large impact by estimating the amount of climatically important gases released to the atmosphere at the time. Our work demonstrates the need to revisit previously estimated impactor parameters for large terrestrial craters in order to better characterize impact events on Earth and elsewhere.