Reprint

Title: Hit-and-run planetary collisions

Authors: Erik Asphaug, Craig B. Agnor, & Quentin Williams

Status: Nature 439, pp. 155-160.

Abstract: Terrestrial planet formation is believed to have concluded in our Solar System with about 10 million to 100 million years of giant impacts, where hundreds of Moon- to Mars-sized planetary embryos acquired random velocities through gravitational encounters and resonances with one another and with Jupiter. This led to planet-crossing orbits and collisions that produced the four terrestrial planets, the Moon and asteroids. But here we show that colliding planets do not simply merge, as is commonly assumed. In many cases, the smaller planet escapes from the collision highly deformed, spun up, depressurized from equilibrium, stripped of its outer layers, and sometimes pulled apart into a chain of diverse objects. Remnants of these hit-and-run collisions are predicted to be common among remnant planet-forming populations, and thus to be relevant to asteroid formation and meteorite petrogenesis.



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Collision Animations

Here we show two animations of the collision in Figure 3b of the paper. Namely, a roughly lunar-mass (0.01Mearth) impactor hitting a Mars-mass (0.1Mearth) target. The collision occurs with an impact velocity twice the initial escape velocity and at a 30 degree impact angle. The first animation is the view in the target center-of-mass frame and the second is in the impactor center-of-mass frame. In both simulations the mantle particles are shown in blue with a few iron particles from the cores (shown in red) being lost as the impactor is disrupted into a few distinct new bodies.

We're used to looking at collisions from the point of view of the target. In the target frame, we see a big collision and the planet oscillates for a while, trying to attain a new equilibrium. But in the impactor frame, we see it escaping as a new family of small planets, many of them greatly enriched in iron core material, and a spray of escaping mantle material.

Collision in Target Frame (MPEG - 0.57 MB) (Animated GIF - 1.2 MB)

Collision in Impactor Frame (MPEG - 0.57 MB) (Animated GIF - 1.2 MB)
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