Hidden in the vast mountain ranges of western Australia and South Africa, a few ancient slabs of stone offer curious insights into Earth’s early days.
These rocks, billions of years in age, are the basis of a new study published last month in the journal Nature Geoscience and co-authored by a Harvard researcher that suggests that asteroids, some of nearly unfathomable proportions, crashed into Earth’s surface far more frequently than was previously believed. It further concludes that the frequency of those impacts may have delayed the evolution of oxygen in the atmosphere.
“The rise of oxygen in the atmosphere has been a huge question for a really long time,” said Nadja Drabon, an assistant professor of earth and planetary sciences at Harvard. “Because without oxygen, multicellular life, animals would not exist. You need to have oxygen in the atmosphere for humans to form in the first place.”
The research zeros in on the Archean eon — between 2.5 and 4 billion years ago — well before life could be supported here. Among the key findings: “large impact” asteroids collided with the Earth once every 15 million years, give or take.
Large is perhaps an understatement. These were hulking bolides, ranging anywhere from 12 to 60 miles wide and hurtling into Earth at alarming speeds. To put that into perspective, the asteroid believed to have wiped out the dinosaurs around 66 million years ago was 6 miles wide. The bolide that entered the atmosphere above Chelyabinsk, Russia, in 2013, and injured more than 1,000 people when it combusted miles above the ground measured just 20 meters wide.
And while by our standards 15 million years seems like an eternity, compared to Earth’s 4.5 billion year lifetime, those millions are mere blips. In fact, the impacts were frequent and significant enough that they effectively sucked oxygen out of the atmosphere, the study determined. As the impacts tapered off about 2.5 billion years ago, oxygen began building up.
“Up until about 2.4 billion years ago, the abundance of oxygen in the atmosphere was relatively low,” said Drabon. “But right after these impacts started to decrease is when the level of oxygen started to rise pretty quickly.”
So how do researchers today piece together such broad views of prehistoric times? In this case, the clues lie within ancient rocks found thousands of miles apart that have survived the tremendous tumult of a growing planet and for which Drabon and other researchers have searched tirelessly.
“Finding evidence of these meteorite impacts, it’s really hard,” said Drabon, who has scoured mountain ranges in South Africa for the rock slabs. “You hike over these mountain terrains 100 kilometers in size looking for one specific piece of rock. They’re very thin, and it’s hard to see, but they have these tiny little grains.”
Those grains are chunks of asteroid. When they fell to Earth, some bolides impacted the planet’s surface with such force that they almost entirely vaporized, sending into the atmosphere a massive cloud of vapor particles and molten rock, Drabon said. The clouds were large enough to surround the Earth, and eventually the particles solidified and fell back to the surface, imprinting on rock layers in the form of tiny bubbles known as impact spherules.
Under the right conditions, the layers were preserved, each of them a DNA print of sorts for the asteroid they belonged to.
“These layers are global, they covered the earth entirely,” said Simone Marchi, a staff scientist at the Southwest Research Institute and lead author of the paper. “But of course, they’re only preserved in very few localities where conditions were just perfect for them to remain. The others were completely obliterated by geological evolution.”
The layers that remain today amount to a sort of geological puzzle for researchers to put together, each piece contributing to a larger picture of the early Earth.
Find a piece of the rock layer, and researchers can date it based on radioactive decay and identify what specific asteroid it belonged to by testing its chemical makeup and matching it to other similar layers of rock from the same impact, Drabon said.
They can measure the asteroids too. The bigger the bolide, the larger the plume of vapor was, and the thicker the layer of rock that appears today, according to Drabon. The asteroid that killed the dinosaurs left behind a layer just 1 centimeter thick. Some of the layers studied for this research measure 20 centimeters thick.
Using the rocks that have been recovered in recent years and the spherules within them, Marchi developed a model that approximates how often large asteroids struck the planet. And based on the timeline he formed, he was able to reasonably tie the newly discovered frequency of the impacts to the lack of oxygen in the atmosphere.
As Marchi explains it, in those early years of the planet’s existence, there were sources of oxygen, like tiny bacteria, and sinks of oxygen, like volcanic eruptions.
Asteroids were likely massive sinks, the study determined.
“If a meteor hits the Earth and vaporizes, the gases that are produced — that could be methane, that could be hydrogen — those would combine with any oxygen in the atmosphere and effectively remove it,” said Marchi.
The findings, said Drabon, scratch the surface of what is left to be discovered about the early Earth. And while they answer some questions previously unanswered, they raise more. How did these asteroid strikes impact early life? What organisms could withstand such a harsh environment?
For now, it’s not clear. There may be more answers in the rocks, Drabon says.