New ‘Collision Chain’ Model Challenges Great Impact Theory

Great minds have spent many years trying to puzzle out the deep history of our solar system. Far from resembling Plato’s perfect spheres, the current thinking is that while Earth and Venus were clearing their orbits, the Moon was formed by a Mars-sized impactor named Theia smashing into Earth. But that may not be the whole story. In a pair of reports in The Planetary Science Journal, a team of scientists have published a new “collision chain” model for the Great Impact hypothesis of our rocky inner planets’ formation, and it challenges our narrative of how Venus, Earth and the Moon spent their youth.

The authors demonstrate the idea that great impacts may not be the efficient mergers we believe them to be, explained team lead Erik Asphaug. “We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision,” Asphaug said. “To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”

To demonstrate their ideas, the authors focus on Venus and Earth. Alexandre Emsenhuber, who worked on the first of the two papers during a postdoctoral fellowship, says in the report that the young Earth would have served as a kind of kinetic shield that slowed down inbound impactors. Earth would have robbed them of their momentum, slowing them down. “We think that during solar system formation, the early Earth acted like a vanguard for Venus,” said Emsenhuber.

To explain the vanguard effect, Emsenhuber uses the analogy of a bouncing ball. A body coming in from the outer solar system is like a ball bouncing down a set of stairs, with each bounce representing a collision with another body.

“Along the way, the ball loses energy, and you’ll find it will always bounce downstairs, never upstairs,” he said. “Because of that, the body cannot leave the inner solar system anymore. You generally only go downstairs, toward Venus, and an impactor that collides with Venus is pretty happy staying in the inner solar system, so at some point it is going to hit Venus again.”

A second paper, published in tandem with the first, uses machine learning to build predictive models from 3D simulations of giant impacts. The team tested their predictive powers on both hit-and-run and merging collisions, to simulate terrestrial planet formation over a period of 100 million years. The authors further propose and demonstrate their hit-and-run-return scenario on the formation of the Moon.

“The standard model for the moon requires a very slow collision, relatively speaking,” said Asphaug, “and it creates a moon that is composed mostly of the impacting planet, not the proto-Earth, which is a major problem since the moon has an isotopic chemistry almost identical to Earth.”

In the team’s new scenario, a protoplanet roughly the size of Mars hits the Earth, just like the standard model. Instead of the Earth simply accreting Theia in one shot, the impactor bounces off in a big sloshy mess. It returns in about a million years for another giant impact, moving slower this time — and that second pass could be the key to better aligning our models with what we see.

“The double impact mixes things up much more than a single event,” Asphaug said, “which could explain the isotopic similarity of Earth and moon, and also how the second, slow, merging collision would have happened in the first place.”

It may also explain the different chemical compositions of Earth and Venus. Because impactors that hit Earth with a glancing blow would have been flung away deeper into the Sun’s gravity well, Asphaug added, “Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus. Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus.”

“You would think that Earth is made up more of material from the outer system because it is closer to the outer solar system than Venus. But actually, with Earth in this vanguard role, it makes it actually more likely for Venus to accrete outer solar system material.”