Between 2.5 and 4 billion years ago, a time known as the archaic eon, the probability of a comet or asteroid hitting Earth was significantly higher than it is today. In fact, the largest ones, over nine kilometers wide, altered the oldest chemistry on the planet.
Although these facts have been known for a long time, it is still not well known how often there would be impacts of this type and what their consequences were on our atmosphere, specifically on oxygen levels, which allows a good part of life such as we know it on Earth can exist. A team of researchers now believe they have some of the answers.
A team of scientists analyzed the remains of ancient asteroids and modeled the effects of their collisions to show that collisions were more frequent than previously believed, and that they began to decline once oxygen began to accumulate on the planet. ” Free oxygen in the atmosphere is essential for any living thing that uses respiration to produce energy, ” explains Nadja Drabon, assistant professor of Earth and Planetary Sciences at Harvard. “Without the accumulation of oxygen in the atmosphere, we probably would not exist.”
A collision rate higher than estimated
The new collision rate found suggests that meteorites and comets would impact the Earth approximately every 15 million years, a value about ten times higher than current models estimate. Scientists realized this after analyzing records of what appear to be ordinary chunks of rock. They are actually ancient evidence, known as impact spherules, that formed in fiery collisions every time large asteroids or comets hit the planet. As a result, the energy from the impact melted and vaporized the rocky materials in the Earth’s crust, shooting them into a giant column. 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 into the Earth’s crust. These ancient markers are difficult to find, as they form layers in the rock that are generally 2-3 centimeters at most.
These new layers of spherules increased the total number of impact events known during early Earth. This allowed the Southwest Research Institute team to update their bombing models to find that the collision rate had been underestimated.
The researchers then modeled how all of these impacts would have influenced the atmosphere. Basically, they found that the cumulative effects of meteorite impacts by large objects likely created an oxygen sink that sucked up most of the oxygen from the atmosphere.
The findings align with the geological record, which shows that oxygen levels in the atmosphere varied but remained relatively low in the early Archaic eon. This was the case until about 2.4 billion years ago, during the end of this time period, when the bombardment slowed down. Then the Earth underwent a major change in surface chemistry brought on by rising oxygen levels known as the Great Oxidation Event.
“As time passed, the collisions became less and less frequent and too small to be able to significantly alter oxygen levels,” Marchi said in a statement. “Earth was on its way to becoming today’s planet.”
Drabon said the next steps in the project include putting their modeling work to the test to see what they can model in the rocks.
