When it comes to space rocks slamming into Earth, two stand out. There’s the one that killed the dinosaurs 65 million years ago (goodbye T-rex, hello mammals!) and the one that formed Earth’s Moon. The asteroid that hurtled into the Yucatan peninsula and decimated the dinosaurs was a mere 10 kilometers in diameter. The impactor that formed the Moon, on the other hand, may have been about the size of Mars. But between the gigantic lunar-forming impact and the comparatively diminutive harbinger of dinosaurian death, Earth was certainly battered by other bodies.
At the 2023 Fall Meeting of the American Geophysical Union, scientists discussed what they’ve found when it comes to just how our planet has been shaped by asteroids that impacted the early Earth, causing everything from voluminous melts that covered swaths of the surface to ancient tsunamis that tore across the globe.
Modeling melt
When the Moon-forming impactor smashed into Earth, much of the world became a sea of melted rock called a magma ocean (if it wasn’t already melted). After this point, Earth had no more major additions of mass, said Simone Marchi, a planetary scientist at the Southwest Research Institute who creates computer models of the early Solar System and its planetary bodies, including Earth. “But you still have this debris flying about,” he said. This later phase of accretion may have lacked another lunar-scale impact, but likely featured large incoming asteroids. Predictions of the size and frequency distributions of this space flotsam indicate “that there has to be a substantial number of objects larger than, say, 1,000 kilometers in diameter,” Marchi said.
Unfortunately, there’s little obvious evidence in the rock record of these impacts before about 3.5 billion years ago. So scientists like Marchi can look to the Moon to estimate the number of objects that must have collided with Earth.
Armed with the size and number of impactors, Marchi and colleagues built a model that describes, as a function of time, the volume of melt this battering must have produced at the Earth’s surface. Magma oceans were in the past, but impactors greater than 100 kilometers in diameter still melted a lot of rock and must have drastically altered the early Earth.
Unlike smaller impacts, the volume of melt generated by objects of this size isn’t localized within a crater, according to models. Any crater exists only momentarily, as the rock is too fluid to maintain any sort of structure. Marchi compares this to tossing a stone into water. “There is a moment in time in which you have a cavity in the water, but then everything collapses and fills up because it’s a fluid.”
The melt volume is much larger than the amount of excavated rock, so Marchi can calculate just how much melt might have spilled out and coated parts of the Earth’s surface with each impact. The result is an astonishing map of melt volume. During the first billion years or so of Earth’s history, nearly the entire surface would have featured a veneer of impact melt at some point. Much of that history is gone because our active planet’s atmospheric, surface, and tectonic processes constantly modify much of the rock record.
Balls of glass
Even between 3.5 and 2.5 billion years ago, the rock record is sparse. But two places, Australia and South Africa, preserve evidence of impacts in the form of spherules. These tiny glass balls form immediately after an impact that sends vaporized rock skyward. As the plume returns to Earth, small droplets begin to condense and rain down.
“It’s remarkable that we can find these impact-generated spherule layers all the way back to 3.5 billion years ago,” said Marchi.
There are about 35 known layers between 3.5 and 2.5 billion years old, derived from 16 confirmed, independent impacts (some of which show up more than once in the geological record due to folding of rock layers). By studying these layers and those that came before and after, we can learn just what the impact (pun intended) of these impactors might have been.
Nadja Drabon, a geologist and assistant professor at Harvard University, studies rocks in South Africa’s Barberton Belt, a region that records many Archean impact events, including evidence of a 3.26 billion-year-old impact called S2. “We don’t know where the impactor hit,” she said. Because less land existed back then, “wherever the impactor hit, it’s likely going to be an ocean.”
Modeling suggests that the offending impactor was 40 to 60 kilometers in diameter—four to six times bigger than the dino killer. When a rock that could have been as wide as the island of Puerto Rico careens into the ocean, a tsunami will propagate around the world. Drabon sees direct evidence of this in the rocks.
Before any impact, cherts—rocks made of microcrystalline quartz that form in low-energy environments—were being deposited, with their pure white layers alternating with carbon-rich black layers. These black and white banded cherts formed on a calm oceanic shelf, Drabon said. When the tsunami came through, the massive current ripped up these seafloor cherts, she explained. Those chunks are mixed into the spherule layer that marks the impact.
Modeling work by others suggests that intense heat that persisted for a few years caused the ocean to partially evaporate. Drabon and her team found direct evidence for this colossal exodus of water atop the tsunami-born sedimentary rock in layers of chert with evaporites—minerals that form in shallow waters via evaporation.
What about life?
Billions of years ago, there weren’t any dinosaurs around to die; life consisted of single-celled organisms. The somewhat regular bombardment of Earth would have sculpted the surface and resulted in physical and chemical changes in the environment that could have hindered life. But it could possibly have helped as well.
In the case of the S2 impact, Drabon said that photosynthetic life would have had a hard time for at least the first few years after the impact because of the darkening effects of dust. Residents of shallow oceans would have struggled to survive this combination of tsunami, evaporated ocean, and blotted sun.
“When I started the study a few years ago, I was mostly imagining disastrous effects,” Drabon said. However, an uptick of phosphorous—an element crucial to life as we know it (it’s a component of DNA’s backbone)—appears in the impact layers and above. Models suggest that within a decade, the partially evaporated ocean, heated off into the atmosphere, would have rained back down, she explained. This rain would have weathered phosphorous from the land and sent it seaward. Moreover, Drabon found that the spherules are phosphorus-rich, suggesting that the asteroid’s arrival also ferried this key element from space to Earth’s surface.
Other lines of evidence point to additional post-impact benefits, including the tsunami bringing iron-rich water up from the deep, along with chemical signatures in organic matter that imply a shift in the nascent biosphere. For instance, along with the iron spike, Drabon sees a strong shift in the carbon isotopic composition of organic matter. “Life prefers lighter carbon,” Drabon said. “We see a really strong shift in the carbon isotopes that clearly indicates that something in the biosphere was happening, something was changing.” These positive effects on life in the aftermath of such catastrophe, said Drabon, “was honestly really a surprise.”
Alka Tripathy-Lang is a freelance science writer with a PhD in geology. She writes about earthquakes, volcanoes, and the inner workings of our planet.