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A huge space rock, estimated to be the size of four Mount Everests, slammed into Earth more than three billion years ago - and the impact could have been unexpectedly beneficial to the earliest forms of life on our planet, according to new research.
When a large space rock collides with Earth, the impacts are usually associated with catastrophic destruction, as in the case of the demise of the dinosaurs 66 million years ago, when an asteroid about 10 kilometers wide crashed. the coast of the Yucatán Peninsula in what is now Mexico.
But Earth was young and a very different place when the S2 meteorite, estimated to be 50 to 200 times more massive than the Chicxulub asteroid that wiped out the dinosaurs, crashed into the planet 3.26 billion years ago, according to Nadja Drabon , assistant professor of Earth. and planetary sciences at Harvard University. She is also lead author of a new study describing the impact of S2 and what followed in its aftermath, which was published Monday in the journal Proceedings of the National Academy of Sciences.
"No complex life had yet formed and only single-celled life was present in the form of bacteria and archaea," Drabon wrote in an email. "The oceans probably contain some life, but not as much as now, partly due to a lack of nutrients. Some people even describe the Archean oceans as 'biological deserts'. The Archean Earth was a water world with few protruding islands. It would have been a remarkable sight, as the oceans were probably green in color due to iron-rich deep waters."
When the S2 meteorite hit, it created global chaos - but the impact also brought up ingredients that might have enriched bacterial life, Drabon said. The new findings could change the way scientists understand how Earth and its young life responded to bombardment from space rocks not long after the planet formed.
Uncovering old influences
Early in Earth's history, space rocks regularly hit the young planet. It is estimated that according to the study authors, 'giant impactors', with a diameter of more than 10 kilometers, have hit the planet at least every 15 million years, meaning that at least 16 giant meteorites have hit Earth during the Archean Eon, which lasted from 4 billion to 2.5 billion years ago.
But the consequences of these impact events are not well understood. And given Earth's ever-changing geology, in which huge craters are covered by volcanic activity and the movement of tectonic plates, evidence of what happened millions of years ago is difficult to find.
Drabon is an early Earth geologist who is intrigued by understanding what the planet looked like before the first continents formed and how violent meteorite impacts affected the evolution of life.
"These consequences must have significantly influenced the origin and evolution of life on Earth. But how exactly remains a mystery," Drabon said. "In my research, I actually wanted to examine 'hard' evidence - pardon the pun - of how gigantic consequences affected early life."
Drabon and her colleagues conducted fieldwork to look for clues in the rocks of the Barberton Makhonjwa Mountains in South Africa. There, geological evidence of eight impact events, which occurred between 3.6 billion and 3.2 billion years ago, can be found in the rocks and traced by small meteorite impact particles called spheroids.
The small, round particles, which can be glassy or crystalline, are formed when large meteorites hit Earth, forming sedimentary layers in rocks known as spheroid beds.
The team collected a series of samples in South Africa and analyzed the composition and geochemistry of the rocks.
"Our days typically start with a long hike through the mountains to reach our sampling sites," Drabon said. "Sometimes we are lucky to have dirt roads that bring us closer. At the site we study the structures in the rocks over the impact event layer in detail and use sledgehammers to take samples for later analysis in the laboratory.
The tightly wedged rock layers preserved a mineral timeline that allowed the researchers to reconstruct what happened when the S2 meteorite hit.
Waves of destruction
The S2 meteorite was between 37 and 58 kilometers in diameter when it struck the planet. The effects were quick and severe, Drabon said.
"Imagine you're standing off the coast of Cape Cod, in a layer of shallow water," Drabon said. "It is an energy-efficient environment, without strong currents. Then suddenly you have a gigantic tsunami that rushes past and tears open the seabed."
The tsunami raged across the globe and the heat from the impact was so intense that it boiled off the top layer of the ocean. When oceans boil and evaporate, they form salts like those observed in rocks immediately after impact, Drabon said.
Dust injected into the atmosphere by the impact darkened the sky within hours, even on the other side of the planet. The atmosphere became warmer and the thick dust cloud prevented microbes from converting sunlight into energy. Any life on land or in shallow waters would have felt the negative effects immediately, and those effects would have lasted from several years to decades.
Eventually the rain would have brought back the upper layers of the ocean and the dust would have settled.
But the deep ocean environment was a different story. The tsunami churned up elements such as iron and brought them to the surface. Meanwhile, erosion washed coastal debris into the sea and released phosphorus from the meteorite. The laboratory analysis showed that there was a spike in the presence of single-celled organisms that feed on iron and phosphorus immediately after the impact.
Life quickly recovered and then flourished, Drabon said.
"Before the impact, there was some, but not much, life in the oceans due to the lack of nutrients and electron donors such as iron in the shallow water," she said. "The impact released essential nutrients such as phosphorus on a global scale. One student aptly called this impact a "fertilizer bomb." Overall, this is very good news for the evolution of early life on Earth, as the consequences would have been much more frequent during the early stages of life's evolution than they are today."
How the Earth responds to direct hits
The impacts of asteroid S2 and Chicxulub had different effects due to the respective sizes of the space rocks and the stage the planet was in when they struck, Drabon said.
The Chicxulub impactor struck a carbonate platform on Earth, releasing sulfur into the atmosphere. The emissions formed aerosols that caused a sharp, extreme drop in surface temperature.
And while both impacts caused significant die-offs, hardy, sunlight-dependent microorganisms in shallow waters would have recovered quickly after the S2 impact once the oceans were full again and the dust had settled, Drabon said.
"Life at the time of the S2 impact was much simpler," she said. "Consider brushing your teeth in the morning: you may eliminate 99.9% of the bacteria, but by evening they will have returned."
Ben Weiss, the Robert R. Shrock Professor of Earth and Planetary Sciences at the Massachusetts Institute of Technology, was intrigued by the paper's geological observations of the spherule beds, which he said will allow researchers to probe Earth's ancient impact on the way astronomers do it. can study the surfaces of planets such as Mars. Weiss was not involved in the investigation.
"There are no impact craters preserved on Earth today that are even close to the size that are thought to have produced the rocks studied here," Weiss said. "The special thing about our archive is of course that, however fragmentary and incomplete, it is the only archive that we can currently study in detail and that can tell us something about the effects of the impact on the early evolution of life. It is also impressive that despite the very local nature of these observations (outcrops in a small region of South Africa), we can begin to understand something about the global nature of these massive impact events."
The rocks in the Barberton Makhonjwa Mountains open up a completely new line of research into the history of terrestrial impacts for Drabon and her colleagues.
"We want to determine how often these environmental changes and biological responses occurred after other impact events in Earth's early history," she said. "Since the effect of each impact depends on several factors, we want to assess how often such positive and negative effects on life occur."
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