Collision Chains among the Terrestrial Planets. III. Formation of the Moon

Abstract

In the canonical model of Moon formation, a Mars-sized protoplanet "Theia" collides with proto-Earth at close to their mutual escape velocity $v_{\mathrm{esc}}$ and a common impact angle 45°. The "graze-and-merge" collision strands a fraction of Theia's mantle into orbit, while Earth accretes most of Theia and its momentum. Simulations show that this produces a hot, high angular momentum, silicate-dominated protolunar system, in substantial agreement with lunar geology, geochemistry, and dynamics. However, a Moon that derives mostly from Theia's mantle, as angular momentum dictates, is challenged by the fact that O, Ti, Cr, radiogenic W, and other elements are indistinguishable in Earth and lunar rocks. Moreover, the model requires an improbably low initial velocity. Here we develop a scenario for Moon formation that begins with a somewhat faster collision, when proto-Theia impacts proto-Earth at ~1.2 $v_{\mathrm{esc}}$, also around 45°. Instead of merging, the bodies come into violent contact for a half-hour and their major components escape, a "hit-and-run collision." N-body evolutions show that the "runner" often returns ~0.1-1 Myr later for a second giant impact, closer to $v_{\mathrm{esc}}$; this produces a postimpact disk of ~2-3 lunar masses in smoothed particle hydrodynamics simulations, with angular momentum comparable to canonical scenarios. The disk ends up substantially inclined, in most cases, because the terminal collision is randomly oriented to the first. Proto-Earth contributions to the silicate disk are enhanced by the compounded mixing and greater energy of a collision chain.

27 pages, 4 figures, 2 tables. Published in PSJ, article available at https://iopscience.iop.org/article/10.3847/PSJ/ac19b2

References in corpus (25)

• Equilibration in the Aftermath of the Lunar-Forming Giant Impact
• Formation of planetary systems by pebble accretion and migration: How the radial pebble flux determines a terrestrial-planet or super-Earth growth mode
• Highly Siderophile Elements in the Earth's Mantle as a Clock for the Moon-forming Impact
• Melting and Mixing States of the Earth's Mantle after the Moon-Forming Impact
• Astrophysical Smooth Particle Hydrodynamics
• Multiple Impact Origin for the Moon
• Collisional Stripping and Disruption of Super-Earths
• SPH calculations of asteroid disruptions: The role of pressure dependent failure models
• A primordial origin for the composition similarity between the Earth and the Moon
• Formation, stratification, and mixing of the cores of Earth and Venus
• The Feeding Zones of Terrestrial Planets and Insights into Moon Formation
• Vapor drainage in the protolunar disk as the cause for the depletion in volatile elements of the Moon
• Inefficient volatile loss from the Moon-forming disk: reconciling the giant impact hypothesis and a wet Moon
• Numerical aspects of Giant Impact simulations
• Realistic On-the-fly Outcomes of Planetary Collisions II: Bringing Machine Learning to N-body Simulations
• Tidal Evolution of the Evection Resonance/Quasi-Resonance and the Angular Momentum of the Earth-Moon System
• Constraints on the pre-impact orbits of Solar System giant impactors
• The energy budget and figure of Earth during recovery from the Moon-forming giant impact
• Dynamical Evolution of the Earth-Moon Progenitors - Whence Theia?
• The Effect of Inefficient Accretion on Planetary Differentiation
• Fate of the runner in hit-and-run collisions
• Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus
• Analytical Model for the Tidal Evolution of the Evection Resonance and the Timing of Resonance Escape
• Examining the Accuracy of Astrophysical Disk Simulations With a Generalized Hydrodynamical Test Problem
• A pebble accretion model for the formation of the terrestrial planets in the Solar System

Cited by in corpus (4)

• Immediate origin of the Moon as a post-impact satellite
• Investigating the Feasibility of an Impact-Induced Martian Dichotomy
• Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus
• Takeout and Delivery: Erasing the Dusty Signature of Late-stage Terrestrial Planet Formation