The formation of Earth's moon was thought to have occurred when a Mars-sized object called Theia slammed into proto-Earth over 4.4 billion years ago. It blasted out material which later coalesced into this artist's illustration.
A new study shows that the massive impact that created the moon was really a two-punch.
Scientists believe that the moon, the planet's only natural satellite was created in violence by the material that was blasted into space from Theia, a Mars-sized body, which crashed into the proto-Earth 4.4 billion years ago.
The authors of the new study pointed out that the canonical collision scenario invokes one catastrophic event. However, there are problems.
"The standard model of the moon requires a slow collision relative to the speed of light. It creates a moon composed mostly of the proto-Earth and not the impacting planet. This is a problem because the moon has almost identical isotopic chemistry to Earth," Erik Asphaug (lead author), a professor at University of Arizona's Lunar and Planetary Laboratory, said in a statement.
Related: How did the moon form?
A giant impact is believed to have caused the moon's formation. A new theory suggests that there were two large impacts in succession, separated by 100,000 to 1,000,000 years. They involved a body of Mars-sized, Theia, and proto-Earth. This image shows the hit-and run collision that was simulated in 3D. It is shown approximately one hour after impact. The iron cores are visible in a cutaway view. Theia, or most of it, barely escapes so a follow on collision is probable. (Image credit: A. Emsenhuber/University of Bern/University of Munich)
Double whammy to the moon
Asphaug and his associates performed computer simulations of the long ago giant impact and came up a better match: Theia and proto-Earth collided at higher speeds than originally thought, creating an initial "hit-and-run" collision that set the stage to a slower, more accretionary encounter about 100,000 to 1,000,000 years later.
Asphaug stated that "the double impact mixes up more than one event, which could explain Earth's isotopic similarity to the moon and how the second, slower, merging collision would occur in the first instance."
In those early days, hit-and-run collisions were not limited to the Earth-moon system. According to a second study, these bouncing collisions are probably as common as accretionary merging in the ancient inner sun system.
Venus Blocking Space Rocks
The second paper was a model of giant impacts in the inner Solar System, including how these collisions affect planet formation and how they impact the orbits over time. The scientists found that Earth acted as a shield for Venus by absorbing the impactors' first hits. These initial collisions helped slow down the impactors, setting up for later accretionary merging with Venus.
Alexandre Emsenhuber (lead author of the second study) stated in the same statement that "it doesn't really matter whether planets collide and don’t merge immediately because they will run into each other again and merge then."
Emsenhuber said, "But that's not what we find." He was completing the research as part of a postdoctoral fellowship at LPL in Asphaug and is now at Ludwig Maximilian University, Munich. We found that they are more likely to become part of Venus than return to Earth. It is easier to travel from Earth to Venus than vice versa. This is due to Venus's proximity to the sun, which has a strong gravity that draws objects in.
These results indicate that Venus and Earth may have different compositions than scientists thought.
Asphaug stated that you would assume Earth is more made of material from outer systems because it is closer than Venus to the outer solar system. "But in reality, Earth's role as a vanguard makes it more likely that Venus will accrete material from the outer solar system."
Both the Emsenhuber and Asphaug new studies were published online on Thursday, September 24, in The Planetary Science Journal.
