Lunar impact ejecta flux on the Earth (2504.15502v1)

Abstract: The transfer of material between planetary bodies due to impact events is important for understanding planetary evolution, meteoroid impact fluxes, the formation of near-Earth objects (NEOs), and even the provenance of volatile and organic materials at Earth. This study investigates the dynamics and fate of lunar ejecta reaching Earth. We employ the high-accuracy IAS15 integrator withing the REBOUND package to track for 100,000 years the trajectories of 6,000 test particles launched from various lunar latitudes and longitudes. Our model incorporates a realistic velocity distribution for ejecta fragments (tens of meters in size), derived form large lunar cratering events. Our results show that 22.6% of lunar ejecta collide with Earth, following a power-law $C(t) \propto t^{0.315}$ with half of the impacts occurring within ~10,000 years. We also confirm that impact events on the Moon's trailing hemisphere serve as a dominant source of Earth-bound ejecta, consistent with previous studies. Additionally, a small fraction of ejecta remains transiently in near-Earth space, providing evidence that lunar ejecta may contribute to the NEO population. This aligns with recent discoveries of Earth co-orbitals such as Kamo'oalewa (469219, 2016 HO3) and 2024 PT5, both exhibiting spectral properties consistent with lunar material. These findings enhance our understanding of the lunar ejecta flux to Earth, providing insights into the spatial and temporal patterns of this flux and its broader influence on the near-Earth environment.

Lunar Impact Ejecta Flux on Earth

The paper entitled "Lunar impact ejecta flux on the Earth" by Castro-Cisneros and colleagues represents a comprehensive paper into the dynamics of lunar ejecta and their potential contribution to the Earth's vicinity and the near-Earth object (NEO) population. Utilizing advanced numerical simulations, the authors investigate the trajectories of lunar ejecta originating from sizable lunar cratering events, with a focus on how these particles interact with Earth and their broader implications within the solar system.

Methodology

The research employs high-fidelity numerical simulations using the IAS15 integrator within the REBOUND package, tracking 6,000 particles over 100,000 years. This approach surpasses earlier models by incorporating all planets and the Moon throughout the entire simulation, offering a more accurate depiction of the dynamical evolution of lunar ejecta. The initial velocity distribution for ejecta fragments is derived from hydrodynamic-impact simulations, reflecting realistic conditions and enhancing the prior assumptions.

Main Findings

A significant outcome of the paper is the determination that 22.6% of lunar ejecta end up colliding with Earth, with half of these impacts occurring within approximately 10,000 years. This collision rate aligns with earlier estimates and follows a power-law distribution indicative of scaling behavior over time. The research highlights that ejecta from the Moon's trailing hemisphere are more likely to impact Earth, providing consistent support with previous findings.

Additionally, the paper explores the velocities and spatial distributions of impacts on Earth. Lunar ejecta impact velocities range from 11.0 to 13.1 km/s, with a notable concentration of collisions occurring at equatorial latitudes. This geographical distribution contrasts with that of the general NEO population, suggesting characteristic orbital dynamics unique to lunar-originated objects.

Implications

The authors discuss the broader implications of their findings, particularly the role of lunar ejecta in contributing to the NEO population. Recent observations of objects like Kamo'oalewa and 2024 PT5, exhibiting spectra consistent with lunar materials, support the hypothesis that some NEOs may derive from lunar ejecta. This insight adds a new dimension to the understanding of NEO origins beyond the traditional asteroid belt sources.

From a planetary impact perspective, the paper offers a quantitative assessment of the potential effects of lunar ejecta on Earth's biological and geological history. The impact velocities and collision frequencies suggest that under current conditions, lunar ejecta do not pose a significant threat comparable to larger NEO impacts, such as the Chelyabinsk event.

Future Directions

The authors outline avenues for future research, emphasizing the incorporation of oblique impact models to better simulate realistic launching conditions. There is also interest in exploring the dynamics of lunar ejecta during ancient periods, when the Moon was significantly closer to Earth, which could yield insights into the early Earth-Moon system and the potential exchange of materials during formative planetary epochs.

In conclusion, the paper by Castro-Cisneros and colleagues delivers a detailed analysis of lunar ejecta dynamics, shedding light on its implications for Earth's impact history and the composition of near-Earth space. Their work serves as a foundation for future investigations into lunar ejecta trajectories and their contribution to the NEO population, advancing our understanding of planetary material exchange and the evolution of the Earth-Moon system.