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Annual Review of Earth and Planetary Sciences

Volume 51, 2023

The Role of Giant Impacts in Planet Formation

Abstract

Planets are expected to conclude their growth through a series of giant impacts: energetic, global events that significantly alter planetary composition and evolution. Computer models and theory have elucidated the diverse outcomes of giant impacts in detail, improving our ability to interpret collision conditions from observations of their remnants. However, many open questions remain, as even the formation of the Moon—a widely suspected giant-impact product for which we have the most information—is still debated. We review giant-impact theory, the diverse nature of giant-impact outcomes, and the governing physical processes. We discuss the importance of computer simulations, informed by experiments, for accurately modeling the impact process. Finally, we outline how the application of probability theory and computational advancements can assist in inferring collision histories from observations, and we identify promising opportunities for advancing giant-impact theory in the future.

  • ▪  Giant impacts exhibit diverse possible outcomes leading to changes in planetary mass, composition, and thermal history depending on the conditions.
  • ▪  Improvements to computer simulation methodologies and new laboratory experiments provide critical insights into the detailed outcomes of giant impacts.
  • ▪  When colliding planets are similar in size, they can merge or escape one another with roughly equal probability, but with different effects on their resulting masses, densities, and orbits.
  • ▪  Different sequences of giant impacts can produce similar planets, encouraging the use of probability theory to evaluate distinct formation hypothesis.

Keyword(s): collisions, giant impacts, planet formation

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The Role of Giant Impacts in Planet Formation, Supplemental Video 5: Two planets undergoing a graze-and-merge giant impact

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The Role of Giant Impacts in Planet Formation, Supplemental Video 4: The disruption of two planets in a giant impact

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The Role of Giant Impacts in Planet Formation, Supplemental Video 3: Erosion of two planets in a giant impact

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The Role of Giant Impacts in Planet Formation, Supplemental Video 2: Two planets undergoing a hit-and-run giant impact

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There are media items related to this article:
The Role of Giant Impacts in Planet Formation, Supplemental Video 1: The merging (accretion) of two planets by giant impact
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The Role of Giant Impacts in Planet Formation
Annual Review of Earth and Planetary Sciences 51, 671 (2023); https://doi.org/10.1146/annurev-earth-031621-055545
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Supplementary Data

  • Supplemental Video 1: The merging (accretion) of two planets by giant impact

    Visualization of two planets undergoing a giant impact that results in a merger (accretion), based on computer simulation output. The larger (target) body is one tenth the mass of the Earth and the smaller (impactor) body is 70% the mass of the target. The planets are colliding at 1.08 times their mutual escape velocity, which equates to 3.63 km/s. The collision angle, defined by the angle between the velocity vector at impact and the line of their centers of mass, is 30°. Off-axis collisions such as these are more probable than on-axis (head-on) collisions.

    Variables: The top-left panel shows mantle and core materials as unique colors for the target and impactor. The top-right panel shows the density of material in kilograms per cubic meter. The bottom-left panel shows temperature in thousands of Kelvins. The bottom right panel shows pressure in Pascals.

    Software: Simulation run by T.S.J. Gabriel ([email protected]) using SPLATCH, a planetary Smooth Particle Hydrodynamics code developed at the University of Bern (Reufer 2011), maintained by A. Emsenhuber (Ludwig Maximillian University of Munich; [email protected]) and H. Ballantyne (University of Bern; [email protected]).

    Citation: Gabriel & Cambioni (2023). The Role of Giant Impacts in Planet Formation, Annual Reviews.

    Supplemental Video 2: Two planets undergoing a hit-and-run giant impact

    Visualization of two planets undergoing a hit-and-run giant impact, based on computer simulation output. This style of collision comprises around half of the giant impacts expected to occur during the latter stages of Solar System formation. The larger (target) body is one tenth the mass of the Earth and the smaller (impactor) body is 70% the mass of the target. When the impactor survives relatively intact after the collision, it is sometimes referred to as the runner. The planets are colliding at 2.5 times their mutual escape velocity, which equates to 8.40 km/s. The collision angle, defined by the angle between the velocity vector at impact and the line of their centers of mass, is 60°.

    Variables: The top-left panel shows mantle and core materials as unique colors for the target and impactor. The top-right panel shows the density of material in kilograms per cubic meter. The bottom-left panel shows temperature in thousands of Kelvins. The bottom right panel shows pressure in Pascals.

    Software: Simulation run by T.S.J. Gabriel ([email protected]) using SPLATCH, a planetary Smooth Particle Hydrodynamics code developed at the University of Bern (Reufer 2011), maintained by A. Emsenhuber (Ludwig Maximillian University of Munich; [email protected]) and H. Ballantyne (University of Bern; [email protected]).

    Citation: Gabriel & Cambioni (2023). The Role of Giant Impacts in Planet Formation, Annual Reviews.

    Supplemental Video 3: Erosion of two planets in a giant impact

    Visualization of two planets undergoing a giant impact that results in the erosion of the target and impactor, based on computer simulation output. This giant impact outcome is sometimes referred to as an erosive hit-and-run. The larger (target) body is one tenth the mass of the Earth and the smaller (impactor) body is 70% the mass of the target. The planets are colliding at 3.25 times their mutual escape velocity, which equates to 10.92 km/s. The collision angle, defined by the angle between the velocity vector at impact and the line of their centers of mass, is 30°. At greater multiples of the escape velocity, the runner may be entirely disrupted after the collision.

    Variables: The top-left panel shows mantle and core materials as unique colors for the target and impactor. The top-right panel shows the density of material in kilograms per cubic meter. The bottom-left panel shows temperature in thousands of Kelvins. The bottom right panel shows pressure in Pascals.

    Software: Simulation run by T.S.J. Gabriel ([email protected]) using SPLATCH, a planetary Smooth Particle Hydrodynamics code developed at the University of Bern (Reufer 2011), maintained by A. Emsenhuber (Ludwig Maximillian University of Munich; [email protected]) and H. Ballantyne (University of Bern; [email protected]).

    Citation: Gabriel & Cambioni (2023). The Role of Giant Impacts in Planet Formation, Annual Reviews.

    Supplemental Video 4: The disruption of two planets in a giant impact

    Visualization of two planets undergoing a disruptive giant impact, based on computer simulation output. Disruptive collisions are not expected to be common in Solar System formation and due to numerical effects, the amount of disruption shown here is likely overestimated. The larger (target) body is one tenth the mass of the Earth and the smaller (impactor) body is 70% the mass of the target. The planets are colliding at 3.75 times their mutual escape velocity, which equates to 12.60 km/s. The collision angle, defined by the angle between the velocity vector at impact and the line of their centers of mass, is 5°.

    Variables: The top-left panel shows mantle and core materials as unique colors for the target and impactor. The top-right panel shows the density of material in kilograms per cubic meter. The bottom-left panel shows temperature in thousands of Kelvins. The bottom right panel shows pressure in Pascals.

    Software: Simulation run by T.S.J. Gabriel ([email protected]) using SPLATCH, a planetary Smooth Particle Hydrodynamics code developed at the University of Bern (Reufer 2011), maintained by A. Emsenhuber (Ludwig Maximillian University of Munich; [email protected]) and H. Ballantyne (University of Bern; [email protected]).

    Citation: Gabriel & Cambioni (2023). The Role of Giant Impacts in Planet Formation, Annual Reviews.

    Supplemental Video 5: Two planets undergoing a graze-and-merge giant impact

    Visualization of two planets undergoing a graze-and-merge style giant impact, based on computer simulation output. This style of collision has been widely theorized for the formation of the Moon. The larger (target) body is one tenth the mass of the Earth and the smaller (impactor) body is 70% the mass of the target. The planets are colliding at 1.10 times their mutual escape velocity, which equates to 3.69 km/s. The collision angle, defined by the angle between the velocity vector at impact and the line of their centers of mass, is 45°.

    Variables: The top-left panel shows mantle and core materials as unique colors for the target and impactor. The top-right panel shows the density of material in kilograms per cubic meter. The bottom-left panel shows temperature in thousands of Kelvins. The bottom right panel shows pressure in Pascals.

    Software: Simulation run by T.S.J. Gabriel ([email protected]) using SPLATCH, a planetary Smooth Particle Hydrodynamics code developed at the University of Bern by (Reufer 2011), maintained by A. Emsenhuber (Ludwig Maximillian University of Munich; [email protected]) and H. Ballantyne (University of Bern; [email protected]).

    Citation: Gabriel & Cambioni (2023). The Role of Giant Impacts in Planet Formation, Annual Reviews.

  • Article Type: Review Article

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