Pressures after giant impacts

The Moon-forming giant impact

The last event in Earth's accretion was the Moon-forming giant impact. Another planet-sized body smashed into the proto-Earth releasing a huge amount of energy, melting and vaporizing much of Earth's rock, and leaving Earth rotating rapidly. The impact may even have been energetic enough to have transformed Earth into a different type of planetary object, a giant, donut-shaped body called a synestia.

The pressures in the body immediately after the collision are much lower than the present day, in some cases less than half that at the present day ?. The reduction in pressure is due to two factors. The rapid rotation of the post-impact body generates a large centrifugal force, the same force that throws you against the side of a car when going around a traffic circle (roundabout), that acts against gravity to reduce the pressure from the material above a point in the planet. The low density of the hot, partially vaporized body also reduces the weight pressing down, again reducing the pressure on a given parcel of material.

The pressure in the post-impact body is generally lower than in the largest of the colliding bodies even though the post-impact body is typically more massive ?. The increase in mass of the body is offset by the effects of rotation and lower density. Even in cases where the mass of the planet almost doubles, the pressure at the core-mantle boundary can still drop due to the impact.

A new paradigm for pressure changes during formation

The effect of giant impacts on internal pressures is not restricted to Earth. After any giant impact, the pressures in the post-impact body(ies) will be dictated as much by the parameters of the impact as by their mass. Planets of all masses would have their internal pressures changed due to giant impacts.

This is not what had been previously assumed. It had been assumed that the pressures after giant impacts were similar to those in solid, slowly-rotating planets. The pressure in a body would have been only dependent on its mass and composition and would have increased as the planet grew. What we have shown instead is that the pressure in planets varies stochastically in response to every impact. This is a new paradigm for pressure evolution in planets.

But why does it matter?

This new paradigm forces us to reinterpret some of the chemical tracers of planet formation. As the proto-Earth grew, each object that collided with it delivered metal into the mantle. After each impact, the metal absorbed small amounts of other elements from the mantle, and then sank to the core – dragging those elements with it. The amount of each element that dissolved into the metal was determined, in part, by the earth's internal pressures. As such, the chemical composition of the mantle today records the mantle pressure during the planet’s formation.

Studies of the metals in the earth's mantle today indicate that this absorption process occurred at pressures found in the middle of the mantle today. It is often posited that the metal and silicate equilibrated on top of a solid lower mantle before being transported rapidly to the core. However, giant impact models show that such impacts melt most of the mantle, and so the mantle should have recorded a much higher pressure – equivalent to what we now see just above the core. This anomaly between the geochemical observation and physical models is one that scientists have long sought to explain.

By showing that the pressures after giant impacts were lower than previously thought, we can reinterpret the lower-than-expected pressures of equilibration inferred from the geochemistry of the mantle. Earth's core formed in the aftermath of energetic giant impacts that temporarily lowered the internal pressures. This is a very different physical scenario for the formation of terrestrial planets and more work is needed to understand its implications for the chemical structure and subsequent evolution of Earth.

What happens after the impact?

After giant impacts the pressures don't remain constant, they evolve rapidly as the post-impact body cools and any satellites produced recede away from the planet. This gradual increase in pressure leads to previously unrecognized processes that can have a significant impact on planets chemical and physical structures.

The way forward

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