Giant Impact of Protoplanets

In the region where the terrestrial planets formed in our Solar System, about ten to 20 Mars-sized protoplanets formed first through the accretion of planetsimals. Our terrestrial planets are thought to have been formed through collisions among these protoplanets. These collisions are called “giant impacts,” and Earth and Venus likely experienced many giant impacts. In particular, the final giant impact on Earth created our Moon. This video shows a typical giant impact between two Mars-sized protoplanets, which likely happened early in the giant impact stage.

Two Mars-sized protoplanets are about to collide with each other. The impact velocity is about 5 km/s, which is about ten times faster than the speed of a pistol bullet.
Due to the high-velocity impact, the solid bodies made of rock change their shapes fluidly. Since intense energy is released by a giant impact, the vicinity of the impact point heats up to 5000 K; almost all rocks are melted, and even partially vaporized.
Under these impact conditions, the protoplanets do not simply merge after the first contact, but they eventually merge after the second contact because the relative velocities of these two protoplanets are greatly reduced by the first contact. A lot of material is also ejected around the resulting protoplanet.
Since the collision is not a head-on one, but an oblique one, the created protoplanet rotates very quickly. Due to the fast spin, a lot of molten rock material starts to stretch outward in the form of two arm-structures, and spreads around the equatorial plane of the protoplanet.
Since the rotational speed of these arms depends on the distance from the protoplanet, they stretch further and the inner parts of them are rolled up by the protoplanet.
Tidal stretching is relatively weak in the outer part, so that small clumps start to form due to their self-gravity. Once these clumps come close to the protoplanet, they often stretch due to the strong tidal force.
Under these impact conditions, a wide massive disk is ultimately formed around the equatorial plane of the protoplanet.
After the disk cools down, a huge satellite like our Moon may form from this disk.

Details of Numerical Simulation

PurposeUnderstanding the process of giant impacts in terrestrial planet formation
Calculation ModelFluid Clculation: Standard SPH method
Gravity Calculation: Tree Method
Equation of State: Silicate mantle and iron core (Tillotson EOS)
Number of Particles: 1 million particles
ComputerCommercially available computer (ELSI VT-Cluster)
Time Scale1 day
Spatial ScaleA protoplanet of a diameter of 5000km
ResearchersHidenori Genda (Earth-Life Science Institute, Tokyo Institute of Technology)
ReferenceGenda, Kokubo & Ida, 2012, ApJ, 744, 137 (8pp)
Genda, Kobayashi & Kokubo 2015, ApJ, 810, 136 (8pp).

Details of Visualization

The colors in the video reflect the internal energy of the particles. Yellow indicates higher energy than red.
Since a 360-degree stereoscopic version of this movie has been created, it is also possible to view it with a head-mounted display (HMD).

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  • Simulation: Hidenori Genda
  • Visualization: Hirotaka Nakayama
  • Four-Dimensional Digital Universe Project, NAOJ

360p (.mov, zip file : 78.8 MB) (.wmv, zip file : 53.7 MB)

720p (.mov, zip file : 197.3 MB) (.wmv, zip file : 204.8 MB)

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