A series of city-sized asteroids bombarded the early Earth between 2.5–3.5 billion years ago — striking with a frequency of one impact every 15 million years.
This means that the onslaught may have been some ten times more intense than was previously estimated, experts from the Southwest Research Institute have reported.
These impactors, the team explained, would have been significantly larger than the asteroid believed to have killed off the dinosaurs, which was only seven miles wide.
While the craters formed by these ancient impacts have long been weathered away or destroyed, traces of them can still be form of glassy particles called spherules.
These spherules formed as a result of the impacts throwing up molten particles and vapours in to the atmosphere, where they cooled and fell back to Earth.
Their distribution can also reveal information on the size of an impactor — the larger the colliding body, the more further afield spherules end up being deposited.
A series of city-sized asteroids (like illustrated) bombarded the early Earth between 2.5–3.5 billion years ago — striking with a frequency of one impact every 15 million years
A series of city-sized asteroids bombarded the early Earth between 2.5–3.5 billion years ago — striking with a frequency of one impact every 15 million years. Pictured: an artist’s impression of the surface of the Archean Earth, the period during which simple life first emerged
‘We have developed a new impact flux model and compared with a statistical analysis of ancient spherule layer data,’ said paper author and physicist Simone Marchi of the Southwest Research Institute in Colorado.
‘We found that current models of Earth’s early bombardment severely underestimate the number of known impacts, as recorded by spherule layers. The true impact flux could have been up to a factor of 10 times higher than previously thought.’
This, Dr Marchi explained, means that in the period between 3.5–2.5 billion years ago, ‘we were probably being hit by a Chicxulub-sized impact on average every 15 million years. Quite a spectacle!’
‘As we deepen our understanding of the early Earth, we find that cosmic collisions are like the proverbial elephant in the room,’ Dr Marchi continued.
‘They are often neglected as we lack a detailed knowledge of their number and magnitude, but it is likely these energetic events fundamentally altered the Earth’s surface and atmospheric evolution.’
‘For example, one outcome we are looking at is to try to understand if these impacts may have affected the evolution of atmospheric oxygen. We find that oxygen levels would have drastically fluctuated in the period of intense impacts.’
‘Given the importance of oxygen to the Earth’s development, and indeed to the development of life, its possible connection with collisions is intriguing and deserved further investigation. This is the next stage of our work.’
‘These large impacts would certainly have caused some disruption,’ said geologist Rosalie Tostevin of the University of Cape Town, in South Africa, who was not involved in the present study. Pictured: the aptly-named Meteor Crater, in Arizona — the result of an impactor just 164 feet in diameter, far smaller than the bodies that bombarded the Earth during the Archean
‘These large impacts would certainly have caused some disruption,’ said geologist Rosalie Tostevin of the University of Cape Town, in South Africa, who was not involved in the present study.
‘Unfortunately, few rocks from this far back in time survive, so direct evidence for impacts, and their ecological consequences, is patchy,’ she added.
‘The model put forward by Dr Marchi helps us to get a better feel for the number and size of collisions on the early Earth.’
‘Some chemical markers suggest there were “whiffs” of oxygen in the early atmosphere, before a permanent rise around 2.5 billion years ago.’
‘But there is considerable debate surrounding the significance of these “whiffs”, or indeed, whether they occurred at all.’
‘We tend to focus on the Earth’s interior and the evolution of life as controls on Earth’s oxygen balance,’ Dr Tostevin noted.
‘But bombardment with rocks from space provides an intriguing alternative.’
The full findings of the study were presented at the 2021 Goldschmidt Conference, which is being held virtually this year from July 4–9.
