Early in Earth's history, a violent collision with another planetary body created a huge mess, but just how much of a mess has been debated. What we do know, however, is that this cataclysmic collision created the moon and plenty of debris that formed a planet-encompassing disk.
Now, in a December 2017 study published in the journal Nature Geoscience, researchers have simulated this ancient smashup to figure out just how much of the disk debris rained back down on Earth's surface as planetesimals, or small, planet-building celestial objects. This period of disk debris pelting the planet is known as "late accretion." What they found adds to our understanding of how early Earth formed and may have implications for how life emerged from our molten and battered young planet.
Seeding Earth With Rare Elements
During late accretion, differentiated planetesimal-sized chunks of debris with metal cores bombarded Earth's surface. These objects solidified from the debris of the moon-forming collision and so contained a mix of materials, including rare elements such as gold, platinum and iridium. These so-called "siderophile elements" (heavy elements that mix easily with iron) were integrated into our planet's mantle. The fact that we find these elements near Earth's surface is a key piece of evidence that late accretion occurred. Scientists had thought that Earth gained approximately 0.5 percent of its total mass during this time.
"After the formation of the moon, the Earth was fully molten for a while, and it's very likely that these elements were segregated into the core of the Earth," says Simone Marchi, of the Southwest Research Institute (SwRI), who led the study. "So, under this assumption, there shouldn't be any gold, platinum and other elements left behind in the mantle or the crust of the Earth, but the very fact that we see a significant amount of these elements, that would imply that they were delivered to the Earth via planetesimals."
After carrying out computer simulations on siderophile elements permeating through a young Earth's interior, Marchi's team found that even the materials delivered by planetesimals would have been assimilated in our planet's core over time, removing them from Earth's upper layers. The simulations also predict that substantial quantities of these planetesimals would have been blasted into space after the moon-forming collision, thus preventing them from falling back to Earth's surface at all.
More Planetesimals, More Gold
So how can we explain the abundances of these rare elements that are clearly present today at Earth's surface? To find out, the researchers took a closer look at the process of delivery by these large collisions and tracked the fate of the impactor materials to see how they got mixed into the mantle.
"We then realized that to be able to explain the amount of these elements we see in the mantle, we needed to increase the total mass accreted by the Earth between a factor of two and five," Marchi explains.
In other words, previous estimates of the amount of material delivered to Earth via late accretion are too low. To explain the abundances of rare elements at or near the surface, between 1 and 2.5 percent of Earth's mass must have been delivered by planetesimals after the moon-forming collision.
"In the aftermath of the formation of the moon that was caused by a massive collision, there seems to have been a protracted time of bombardment of the early-Earth," says Marchi. Although this was generally understood to be the case before this research, "what we are saying now is that you need to have a much higher [rate of] bombardment in order to explain the quantity of these elements."
The Question of Life
Marchi provides another way to think about this far higher rate of bombardment during the late accretion period.
"If you were to spread that mass as a layer over the surface of the Earth, you'd get a layer of the order of tens of kilometers," Marchi says. "In this regard, you also get a visual idea that the delivery of this mass is potentially very important for the surface."
These collisions would have had a tremendous impact on Earth's surface, the chemistry of the primordial atmosphere and may have even had a significant role to play in early biology. After all, the oldest record for the origin of life is about 4 billion years ago, and that was around the time when these collisions took place.
"This is important as it would imply that these collisions were really important in the early evolution of the Earth," he concludes. "They were a primary engine, so to speak, that would affect how the surface of the Earth works. This has tremendous implications for early life on Earth."