A 500-million-year long storm of asteroids pummeled the infant Earth soon after it was formed, scientists believe. The smaller asteroids were the length of 15 football fields. The big ones were the size of Maine.
These collisions, which occurred during the Hadean eon, 4 to 4.5 billion years ago, were crucial to the planet’s evolution and may have paved the way for life. Those 500 million years of intense asteroid activity make up approximately 10 percent of the Earth’s history, said Simone Marchi, a research fellow at the Southwest Research Institute in Boulder, Colorado.
“It’s not a negligible amount of time,” Marchi said. “It’s really a critical time for understanding the evolution of the Earth. Much of what we see today, including ourselves — that is, life — is due to that early evolution. It’s very important to understand what the conditions of the Earth were during that time.”
Marchi and his colleagues have just released a new model to show how those collisions shaped the planet during its first 500 million years. Their study appears in Thursday’s edition of the journal Nature.
Their model shows where and when thousands of asteroids struck the Earth, churning up hot magma under the newly-formed surface. That mixing and melting destroyed rocks from Earth’s infancy, Marchi said. The oldest rocks recovered on Earth are 3.8 billion years old, too young to answer questions on the timing or magnitude of the asteroid strikes. An ancient mineral called zircon is the only material on Earth that appears to have survived the asteroid storm, but it is scattered throughout younger rocks, Marchi said, leaving few answers on its own.
The model simulates all of those collisions over millennia, and shows scientists how the asteroids may have shaped the Earth’s crust.
“When you have a large collision, you basically dig a large hole in the ground, and that means mixing and melting of the rocks. The heat from the impact can melt rocks in the proximity of collision,” Marchi said. “Mixing, melting and burial of rocks must have been extremely important back then, and we need to understand how the crust formed.”
The young solar system was full of debris during the Hadean eon. To figure out when and how often asteroids hit Hadean Earth, Marchi and his colleagues studied the moon’s craters and rock composition. The moon preserved a better record of ancient collisions, but it still takes detective work, he said.
“You have craters on top of craters on the moon, so you have to decode the information written in there.”
This animation shows where asteroids likely struck Earth over time.
Our planet was hit thousands of times by “small” space debris, approximately 9 miles wide, Marchi’s team found. But occasionally, supersized asteroids — asteroids larger than 300 miles wide — struck the Earth’s surface. Those large impacts would have vaporized the Earth’s oceans, filling the atmosphere with steam.
Compared to the Hadean eon, we live in a quiet time, Marchi said. He thinks the plethora of asteroid strikes helped create the conditions for life. But how they did it is still a mystery, he said.
“The oldest traces for life on Earth have been found in old rocks — isotopic traces of life. Those rocks date to 3.9 billion years old,” Marchi said, not long after the Hadean asteroid activity. “Is that just a coincidence or is there a more profound link to what’s going on in the Hadean and the present? It’s a very difficult problem to address, and there’s a lot of work to be done in that regard. This paper is just a step toward that goal.”