Scientists Discover the Secrets of Earth and Venus' Violent Youths

Researchers have discovered that planets in the inner solar system could have been born as the result of multiple "hit and run" collisions.

Using computer modeling and simulations of tremendous impacts, the team at the Lunar and Planetary Laboratory (LPL), University of Arizona, found that Earth and Venus could have been formed when smaller planetary building blocks collided and stuck together.

The research challenges the picture scientists have built of neat, round, distinct planets forming from a swirling cloud of rugged asteroids and mini planets.

The model, which the team has dubbed the "hit, run, and return" scenario, provides a far messier and more complicated origin for some planets.

Two reports detailing the findings published in The Planetary Science Journal suggest pre-planetary bodies spend their early lives in the solar system crashing into each other and ricocheting away, only to come back together later on.

The first collision slows the bodies down, making them more likely to "stick" next time.

The major conclusion being, even giant impacts between planetary bodies aren't the efficient mergers that scientists may have believed them to be.

"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," LPL planetary sciences professor Erik Asphaug said in a press release.

"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."

The finding suggests that despite being neighbors in the solar system, Earth and Venus could have had very different "upbringings."

In one of the two studies Ludwig Maximilian University, Munich, researcher Alexandre Emsenhuber and his co-authors suggest that Earth would have slowed down bodies as they approached Venus, making them more likely to merge with that planet.

"The Earth acts as a shield, providing a first stop against these impacting planets," Emsenhuber said in a press release. "More likely than not, a planet that bounces off of Earth is going to hit Venus and merge with it."

The lead author of the paper suggests that this is analogous to a ball bouncing down a flight of stairs, with each bounce stripping it of energy and representing collisions with planetary bodies.

"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."

Because Earth has no similar shield slowing down objects that strike it, the authors suggest these bodies were more likely to bounce off our planet in its youth.

This could explain differences between the two similarly sized worlds including why Earth has a magnetic field stronger than Venus' and even why Venus lacks a moon.

"The prevailing idea has been that it doesn't really matter if planets collide and don't merge right away, because they are going to run into each other again at some point and merge then," Emsenhuber said. "But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth.

"It's easier to go from Earth to Venus than the other way around."

The 3D simulations of giant impacts created by the team could also have implications for the formation of the moon. This could eliminate some lingering issues with the current model that suggests the moon was created in giant impact between Earth and a proto-planet.

"The standard model for the moon requires a very slow collision, relatively speaking," Asphaug said, "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."

The team suggests in its paper that if a roughly Mars-sized proto-planet struck Earth at a faster speed and then bounced away only to return about one million years later, it would result in an impact that looks like the standard explanation but without the issues associated with that model.

"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."

Planetary collisions
A stock illustration of a collision between two planetary bodies. New research suggests that the inner planets of the solar system were subject to "hit and run" collisions. JPL-Caltech/NASA