Immediate origin of the Moon as a post-impact satellite (2210.01814v1)

Abstract: The Moon is traditionally thought to have coalesced from the debris ejected by a giant impact onto the early Earth. However, such models struggle to explain the similar isotopic compositions of Earth and lunar rocks at the same time as the system's angular momentum, and the details of potential impact scenarios are hotly debated. Above a high resolution threshold for simulations, we find that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside the Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped then torqued onto wider, stable orbits. Furthermore, the outer layers of these directly formed satellites are molten over cooler interiors and are composed of around 60% proto-Earth material. This could alleviate the tension between the Moon's Earth-like isotopic composition and the different signature expected for the impactor. Immediate formation opens up new options for the Moon's early orbit and evolution, including the possibility of a highly tilted orbit to explain the lunar inclination, and offers a simpler, single-stage scenario for the origin of the Moon.

• The paper demonstrates that high-resolution SPH simulations reveal a giant impact can immediately form a Moon-like satellite with around 60% proto-Earth material.
• The paper shows that even satellites formed near Earth’s Roche limit can survive by migrating into stable, wider orbits.
• The paper finds that the nascent satellite’s molten outer layer and cooler interior help explain the Moon’s observed thermal and compositional structure.

Immediate Origin of the Moon as a Post-Impact Satellite: An Expert Summary

The lunar formation mechanism has long remained a challenging enigma in planetary science. "Immediate Origin of the Moon as a Post-impact Satellite," authored by J.A. Kegerreis and colleagues, explores the possible formation of the Moon as an immediate post-impact satellite, using high-resolution hydrodynamical simulations. This research draws significant attention due to its novel perspective and computational rigor, offering new insights into the Earth-Moon system's early evolution.

High-Resolution Simulations of Giant Impacts

The traditional giant impact hypothesis suggests that the Moon formed from debris ejected following a collision between the early Earth and a Mars-sized body, often referred to as Theia. Such scenarios face difficulties explaining the isotopic similarities between terrestrial and lunar rocks, alongside the angular momentum of the Earth-Moon system. Kegerreis et al. address these challenges through a series of high-resolution smoothed particle hydrodynamics (SPH) simulations that utilize up to $10^8$ particles, surpassing previous studies in numerical fidelity and detail.

Key Findings and Results

1. Immediate Satellite Formation:
   • The simulations reveal that giant impacts can result in the immediate formation of a satellite with mass and iron content akin to the modern Moon. These satellites can be placed into orbit significantly outside Earth's Roche limit.
   • The results suggest that immediately formed satellites consist of approximately 60% proto-Earth material, matching Earth-like isotopic characteristics, providing a potential resolution to the isotopic conundrum associated with the canonical model.
2. Survivability and Stability:
   • The simulations also show that satellites initially on trajectories passing within the Roche limit can survive through tidal disruption. These bodies are torqued into stable, wider orbits, expanding the range of plausible initial conditions that lead to stable lunar formation.
   • Crucially, these outcomes are consistently reproduced at resolutions above $10^{6.5}$ particles, affirming the robustness of these results against numerical artifacts.
3. Compositional and Thermal Structure:
   • The directly formed satellites are characterized by molten outer layers over relatively cooler interiors, potentially improving the isotopic affinity between Earth and Moon. The thermal gradient aligns with existing geophysical observations of the Moon’s crust and mantle structure.
4. Implications for Orbital Dynamics:
   • The study hypothesizes new initial conditions for lunar orbital evolution, including potentially highly inclined orbits. This could offer explanations for the present inclination of the Moon relative to Earth's orbit.

Implications and Future Developments

The study by Kegerreis et al. holds significant implications for understanding early solar system dynamics and planetary formation theories. Direct satellite formation after impacts might present a simpler alternative to multi-stage lunar accretion models, while potentially reconciling isotopic differences without invoking improbable initial conditions.

Practical implications extend to understanding satellite formation processes in other planetary systems, where direct satellite formation from debris might be more prevalent than currently believed.

For the future, the exploration could benefit from extended analyses on long-term orbital stability and thermal evolution, considering the tidal interactions of the nascent satellite with the proto-Earth and any residual disk material. Improved equations of state and additional high-resolution simulations across varied parameter spaces would further constrain the conditions viable for such direct satellite productions. Additionally, evolving isotopic and observational metrics from lunar samples can refine these models, offering deeper insights into our Moon's complex ancestry.