Brandon Charles Johnson

The Moon's crust exhibits strong magnetic anomalies that likely record ancient magnetic fields produced by a core dynamo. Lunar materials have low magnetic susceptibility, but impactor material with high susceptibility may record the Moon's ancient dynamo. Some magnetic anomalies are antipodal to lunar basins and it is hypothesized that impact ejecta accumulates at the basin antipode and records the ancient lunar magnetic field. Previous impact simulations, which ignored the rotation of the Moon, support this hypothesis. Here, we reevaluate the deposition of antipodal ejecta by considering the rotation of the Moon. Assuming that the rotational period of the Moon is the same as the current day, we find that the rotation of the Moon does not strongly affect the deposition of the antipodal ejecta if impacts occur near the poles. In contrast, rotation has a considerable effect on antipodal deposits for impacts occurring near the equator. An eastward impact at a 45° incidence angle results in almost no antipodal ejecta, however, an eastward impact with a 30° incidence angle accumulates more antipodal ejecta than the non‐rotating case. It is hard to determine the impactor's direction and location, for basins without an ellipsoidal shape and/or an asymmetric ejecta blanket. Nevertheless, our work demonstrates that care must be taken in interpreting the magnetic anomalies antipodal to lunar basins and that the effect of rotation should not be ignored.

Impact‐generated crater rays are well‐documented on the Moon, with most appearing as high‐albedo streaks extending radially from a crater's center. On Mars, however, crater rays are significantly rarer and discernible only through thermal imaging due to their lower thermal inertia compared to surrounding terrain. This study presents the first comparative analysis between the lengths of Martian and lunar crater rays, including lunar albedo rays and cold spots, which are ray‐like thermal anomalies associated with many of the youngest lunar craters. Our findings indicate that both Martian crater rays and lunar cold spots extend significantly farther than lunar albedo rays, with lengths an order of magnitude greater for craters of equivalent diameter. Furthermore, we propose a connection between the formation mechanisms of Martian crater rays and lunar cold spots based on their thermal properties. By integrating thermal rays into existing ejecta models, we refine the understanding of crater‐ray formation and suggest that Martian crater rays and lunar cold spots may share a similar formation mechanism via secondary cratering processes. Advancing knowledge of these features has implications for impact dynamics and surface evolution across planetary bodies.

The primary kill mechanisms responsible for the K‐Pg mass extinction, including the end of the dinosaurs, are much debated. Here, we reexamine the hypothesis that heat and fire were the primary kill mechanisms for terrestrial organisms during the K‐Pg mass extinction. We show that spherules and uncondensed vapor from the impact vapor plume are first ballistically transported globally and then decoupled spatially from one another when they encounter the atmosphere. Expanded K‐Pg boundary sections provide strong evidence that this impact vapor ultimately condenses to abundant fine dust. The flux of thermal energy produced by impact spherules entering the atmosphere globally is strongly enhanced by the presence of this fine dust. This supports the idea that heat and fire were key kill mechanisms for terrestrial organisms.

An impact event excavates a bowl‐shaped transient cavity, which subsequently collapses under gravity. For larger cavities, gravitational forces exceed the strength of the target material beneath the cavity, resulting in central uplift and a reduced final depth‐to‐diameter ratio (1/10). This gravitational collapse—the key to complex crater formation—requires a temporary weakening of the target material. One widely accepted mechanism for this weakening is acoustic fluidization. In this study, we simulate crater formation on a single‐layered, Ganymede‐like icy target using the iSALE‐2D shock physics code, incorporating the Melosh model of acoustic fluidization to explore its behavior in ice. By systematically varying the Melosh model parameters, we examine the influence of quality factor , regeneration coefficient , wavelength of acoustic vibrations , and scattering diffusivity , on the extent of the fluidized region, the persistence of acoustic vibrations, and the overall collapse dynamics. Our results demonstrate that using an impactor‐size‐dependent , a spatially varying , a constant and () effectively reproduces the observed depth‐to‐diameter ratios of Ganymede's craters. Furthermore, craters simulated with the Melosh model show better agreement in observed rim height and produce terraced rims, which are not observed in the Block model results.

Chondrites are composed of formerly partially molten material, known as chondrules, surrounded by fine-grained matrix. They date from the earliest times in Solar System history. However, their role in the formation of the planets is uncertain because, in part, it is not clear how they were produced. Here, we show a robust pathway for forming meteorite-producing asteroids that contain chondrules through embryo-embryo collisions during the late stages of terrestrial planet formation. Melted material from these impacts cool into chondrules and mix with unmelted material in embryo-centric disks that formed from the ejecta. This material accretes into numerous asteroid-sized satellites. These objects are later ejected onto heliocentric orbits because of gravitational encounters with other embryos, thereby becoming the parent bodies of chondrites. This mechanism provides a pathway to form chondrites in Solar System history at times commensurate with measured chondrule ages, while explaining many of their physical properties.

Multiple lines of evidence suggest that a methane‐clathrate crustal layer exists in Titan. However, we have not directly confirmed the existence or thickness of this hypothesized layer, a potentially important methane reservoir. Here, we model impact crater formation and subsequent viscoelastic evolution in Titan's ice shell with methane‐clathrate crusts 0–15 km thick. We show that Titan's peculiarly shallow impact craters are most consistent with the presence of a methane‐clathrate crust, which influences the initial crater shape and rate of topographic relaxation over time. Craters that form in a pure‐water‐ice shell are kilometers deeper than those observed on Titan, even after relaxation. The methane‐clathrate crust thickness influences the thermal and yield strength profiles, which affect the initial crater shape and topographic evolution. Impacts into methane‐clathrate crusts form shallower craters and they topographically relax to depths closer to what is currently observed. We find that the simulations of impacts into a 5 km thick clathrate crust best reproduce the observed crater topography. Such a crust would store at least ∼250× the current mass of atmospheric methane for potential replenishment. The clathrate crust drastically insulates Titan's interior, and implies that Titan is more active than previously assumed.

The early Earth (i.e., Archean and Hadean Eons, 2.5–4.0 and 4.0–4.5 Ga, respectively) experienced frequent cosmic bombardment. Impacts have been shown to stimulate crustal alteration, for instance via hydrothermal systems active for up to millions of years post‐impact. Using a shock physics code that includes the effects of tensile fragmentation and porosity generation, we conduct the first comprehensive study to quantify impact‐generated permeable volumes into the upper crust of the early Earth for a suite of environmental (e.g., geothermal gradient, crustal thickness, presence of an ocean) and impact conditions (e.g., impactor velocity and size). Inferences from our suite of impact simulations suggest that the volume of impact‐induced permeable regions is strongly dependent on impact energy (i.e., impactor size and velocity). The range of generated permeability (10 ⁻²⁰ to 10 ⁻¹² m ² ) within those regions, however, depends on the geothermal gradient and crustal composition. Moreover, we use a bombardment history model to infer the cumulative effects of recursive impacts onto the early Earth. We estimate that the upper 8 km shell of the Earth's crust may have been made highly permeable by impacts prior to 4.3 Ga, and that a significant portion of this volume would have been permeable until 3.5 Ga. These results show that impacts were instrumental in driving hydrothermal alteration of the early Earth's crust, with important consequences for the geochemical evolution of near‐surface environments.

The South Pole–Aitken (SPA) basin-forming impact was a critical event in the Moon’s history. Despite being the oldest and largest acknowledged basin, critical details including the impactor’s size, nature, direction, and fate of the ejecta remain uncertain. Here, we simulate SPA’s formation and resultant crustal structure. We find that SPA’s observed shape of an ellipse tapered toward the south is best reproduced by a 260-km-diameter differentiated impactor striking with a north-to-south trajectory. In this scenario, the impact disperses lunar mantle materials in the cross-range and the downrange directions away from the lunar farside. Much of the mantle ejecta collapses into the basin interior, consistent with the distribution inferred from gravity. These results suggest that the Artemis landing sites near the lunar south pole should contain abundant SPA ejecta including mantle materials. Samples returned from these regions should therefore reveal the age of SPA and the composition of the lunar mantle.

The South Pole‐Aitken basin (SPA) is the largest and oldest impact basin on the Moon and was excavated through the crust and into the mantle early in lunar history. Here, we use gravity data to identify heterogeneities in crustal density that could indicate deeply sourced material in the crust despite a lack of surface spectral signatures. We show the presence of a ∼400‐km‐wide annulus of large‐amplitude, small‐scale gravity anomalies associated with the basin's rim region, which we interpret as the SPA ejecta blanket. We perform a pair of gravity inversions, varying either the thickness of a dense layer or the concentration of dense material in a mixed layer to match the gravity observations, and determine that the SPA ejecta blanket likely contains ≳5 million km ³ of mantle‐derived material. Three craters excavate the sources of these anomalies, revealing varied compositions that imply that the lunar mantle may have had orthopyroxene‐ and clinopyroxene‐rich components at similar depths at the time of the SPA impact, with important implications for the timing of mantle overturn. Gravity anomalies also reveal a previously unnoticed 1,440‐km‐diameter peak‐ring within SPA, delineating the transition between the collapsed ejecta and the central melt sheet. The annulus of strong anomalies associated with the ejecta encompasses the south polar region, suggesting a strongly heterogeneous subsurface near several future missions' landing targets, including the upcoming Artemis missions. These results place an important constraint on basin formation, reveal the composition of the mantle, and will guide future studies in targeting this material.