Late-Stage Accretion and Giant Impacts

The chaotic final phase of terrestrial-planet building, when a few dozen Moon-to-Mars-sized embryos collide to make the rocky planets.

Definition

Late-stage accretion is the final phase of terrestrial-planet formation in which large planetary embryos collide and merge through giant impacts, completing the assembly of the rocky planets.

Scope

This topic covers the last stage of rocky-planet formation, in which gravitational interactions among planetary embryos drive crossing orbits and a sequence of giant collisions over tens of millions of years. It includes N-body models of terrestrial-planet assembly, the energetics and aftermath of giant impacts such as magma oceans and core merging, the giant-impact origin of the Moon, and the role of late accretion in delivering volatiles and the highly siderophile elements of planetary mantles.

Core questions

• How do gravitational interactions among embryos lead to the final number and spacing of terrestrial planets?
• What were the conditions of the impact that formed the Moon, and why is the Moon depleted in iron?
• How did giant impacts set the spin states, obliquities, and bulk compositions of the rocky planets?
• How much of Earth's water and volatiles arrived during and after late accretion?

Key theories

Giant-impact origin of the Moon

A collision between the proto-Earth and a Mars-sized body ejected a disk of mostly mantle material from which the Moon accreted, accounting for the Moon's small iron core and the high angular momentum of the Earth-Moon system.

Chaotic terrestrial-planet assembly

N-body simulations show that a population of planetary embryos evolves through crossing orbits and stochastic giant collisions into a small number of terrestrial planets, naturally producing variety in their masses and spins.

Mechanisms

After the gas disk disperses, gravitational perturbations excite the orbits of planetary embryos until they cross and collide. Giant impacts deposit enormous energy, melting planets into magma oceans, merging metallic cores, and ejecting debris that can re-accrete or form a satellite. Continued bombardment after core formation, called late accretion, adds a veneer of material to the mantle.

Clinical relevance

Giant impacts explain key features of the rocky planets and the Moon, and they help constrain the delivery of water and life-essential volatiles to early Earth.

History

The giant-impact hypothesis for the Moon emerged in the mid-1970s from the work of Hartmann and Davis and, independently, Cameron and Ward, and gained quantitative support from hydrodynamic simulations such as those of Canup and Asphaug in 2001. N-body studies from the 1990s onward established the chaotic, collision-dominated picture of terrestrial-planet formation.

Debates

Isotopic similarity of Earth and Moon

The near-identical isotopic compositions of Earth and the Moon are hard to reconcile with a Moon formed mostly from the impactor, motivating a range of alternative high-energy impact scenarios.

Key figures

• William Hartmann
• Robin Canup
• Erik Asphaug
• John Chambers

Seminal works

• hartmanndavis1975
• canup2001
• chambers2001

Frequently asked questions

How did the Moon form?

The leading view is that a Mars-sized body struck the young Earth, throwing molten and vaporized rock into orbit, from which the Moon quickly accreted; this explains the Moon's small iron core.

How long did the final assembly of Earth take?

Models and isotopic dating suggest Earth completed most of its growth within roughly the first hundred million years of Solar System history, ending with the Moon-forming impact.