BOSTON — A new study suggests that, up to 2.5 billion years ago, Earth was bombarded by ten times more comets and asteroids than previously thought. Some of these were over six miles wide. The ferocity of the repeated strikes was so severe that they may have delayed the evolution of the atmosphere by stalling oxygen growth.
Scientists say that Earth’s weather between 2.5 and 4 billion years ago, a period known as the Archean eon, could often be described as “cloudy with a chance of asteroid.”
While it is generally accepted by geologists that asteroid or comet strikes were not uncommon then, what hasn’t been as well understood is how often large objects would hit. It also hasn’t been clear how exactly the fallout from the impacts affected Earth’s atmosphere, specifically oxygen levels. Now, a team of researchers believes they have some of the answers.
Earth was struck by asteroids more frequently than thought
After analyzing remnants of ancient asteroids and then modeling the effects of the collisions, they have shown that the strikes took place much more often than previously thought. “Free oxygen in the atmosphere is critical for any living being that uses respiration to produce energy. Without the accumulation of oxygen in the atmosphere, we would probably not exist,” says study co-author Dr. Nadja Drabon, an assistant professor of Earth and planetary sciences at Harvard University, in a statement.
The research team found that existing planetary bombardment models underestimate how frequent asteroids and comets would hit Earth. The new, higher collision rate suggests impacts struck Earth roughly every 15 million years, about ten times more than current models.
The scientists made the discovery after analyzing records of what appeared to be ordinary bits of rock. They are actually ancient evidence, known as impact spherules, that formed in the fiery collisions each time large asteroids or comets struck the planet.
As a result, the energy from the impact melted and vaporized the rocky materials in the Earth’s crust, shooting them up in a giant plume. Small droplets of molten rock in that cloud would then condense and solidify, falling back to Earth as sand-sized particles that would settle back onto the planet’s crust. The ancient markers are hard to find since they form layers in the rock that are usually only about an inch or so thick.
“You basically just go on long hikes and you look at all the rocks you can find because the impact particles are so tiny. They’re really easily missed. Over the last couple of years, evidence for a number of additional impacts has been found that hadn’t been recognized before,” says Dr. Drabon.
She explains that the new spherule layers increased the total number of known impact events in Earth’s early days. That allowed the research team to update their bombardment models to find the collision rate had been underestimated. They then modeled how all the impacts would have influenced the atmosphere.
‘The Great Oxidation Event’
The team found that the accumulated effects of meteorite impacts by objects larger than six miles wide probably created an “oxygen sink” that sucked most of the oxygen out of the atmosphere. The findings align with the geological record, which shows that oxygen levels in the atmosphere varied, but stayed relatively low in the early Archean eon.
That was the case until around 2.4 billion years ago when the bombardment slowed down. The researchers say that Earth then went through a major shift in surface chemistry triggered by the rise of oxygen levels, known as the Great Oxidation Event.
“Current bombardment models underestimate the number of late Archean spherule layers, suggesting that the impactor flux at that time was up to ten times higher than previously thought,” says said study lead author Dr. Simone Marchi, of the Southwest Research Institute. “What’s more, we find that the cumulative impactor mass delivered to the early Earth was an important ‘sink’ of oxygen, suggesting that early bombardment could have delayed oxidation of Earth’s atmosphere.”
“As time went on, collisions become progressively less frequent and too small to be able to significantly alter post-GOE oxygen levels. The Earth was on its course to become the current planet,” notes Dr. Marchi.
The findings are published in the journal Nature Geoscience.
South West News Service writer Stephen Beech contributed to this report.