Asteroids the size of cities every 15 million years, this was the cosmic bombardment of the early Earth

The number of these impacts that the Earth suffered in its early days may have been 10 times greater than previously thought

It is believed that the planet suffered collisions like the one that killed the dinosaurs every 15 million years

Researchers talk about the impact of asteroids the size of cities or entire small provinces

The image that illustrates this article shows the impact crater of an asteroid that fell in the state of Arizona, in the United States, about 50,000 years ago. It has a diameter of 1,250 meters, a depth of 173 meters, and a circumference of almost 5 km. It was caused by a cosmic rock about 50 meters in diameter that impacted the ground at about 70,000 kilometers per hour.

The numbers are evidence of the violence of that crash, and yet the cosmic cataclysms described in new work presented by astronomers speak of impacts that could have been hundreds of times greater during the first thousand years of the history of our planet.

The scientists have long known that the Earth was bombarded by huge impactors in remote ages, but new research suggests that the number of these impacts may have been 10 times greater than previously thought, resulting in a barrage of collisions, similar in scale to the asteroid collision that wiped out the dinosaurs, on average every 15 million years between 2,500 and 3,500 million years ago.

According to this study, presented at the Goldschmidt geochemistry conference, some of these individual impacts may have been much larger, possibly dthe size of a city or small province. The researchers are also studying the effect that the impacts may have had on the evolution of the chemistry of the planet’s surface.

Earth’s early years were unimaginably violent compared to the current ones. Scientists believe that it was hit by a significant number of large asteroids (more than 10 km in diameter), which would have had a significant effect on surface chemistry close to earth and in its capacity to host life.

The effect of a single collision of this type was demonstrated relatively recently with the Chicxulub impact 66 million years ago, that caused the extinction of the dinosaurs. However, the early Earth was very different from Earth at the time of the Chicxulub impact, and so were the effects of collisions.

Impact craters from similar collisions can be seen on the Moon and other rocky planetsBut atmospheric weathering and plate tectonics have tended to mask any direct evidence of ancient impact craters on Earth.

However, echoes from these distant impacts can be seen in the presence of “spherules” found in ancient rocks; the huge impacts spewed molten particles and vapors which then cooled and fell to the ground to embed themselves in the rock as small spherical glassy particles. The larger the impact, the more these particles would have spread from the impact site, so the overall distribution of a thick layer of spherules shows an enormous impact.

Researcher Simone Marchi, from the Southwest Research Institute, in the United States, explains that they have developed “a new model of impact flow and we have compared it with a statistical analysis of the data from the old layer of spherules. With this approach, we discovered that current models of Earth’s first bombings severely underestimate the number of known impacts, as recorded by the layers of spherules “.

In this sense, he adds that “the true flow of impacts could have been up to a factor of 10 times greater than what was thought in the period between 3,500 and 2,500 million years ago. This means that in that initial period, we were probably hit by an impact the size of Chicxulub on average every 15 million years. Quite a show “, he emphasizes.

“As we deepen our understanding of the early Earth, we discover that cosmic collisions are like the proverbial elephant in the room,” he continues. “They are often neglected, as we lack detailed knowledge of their number and magnitude. but it is likely that these energetic events fundamentally altered the Earth’s surface and atmospheric evolution“.

For example, one of the outcomes they are studying is trying understand whether these impacts may have affected the evolution of atmospheric oxygen. “We found that oxygen levels would have fluctuated drastically in the period of intense impacts,” he emphasizes. “Given the importance of oxygen for the development of the Earth, and indeed for the development of life, its possible connection with the collisions is intriguing and warrants further investigation. This is the next stage of our work. ”

For her part, Dr. Rosalie Tostevin, from the University of Cape Town, in South Africa, who did not participate in the study, points out that “these great impacts would have undoubtedly caused some disorder. Unfortunately, few rocks survive from such ancient times, so direct evidence of impacts and their ecological consequences is patchy. The model proposed by Dr. Marchi helps us to have a better idea of ​​the number and size of collisions on the early Earth “, he highlights.

“Some chemical markers suggest that there were ‘odors’ of oxygen in the early atmosphere, before a permanent increase about 2.5 billion years ago – he continues -. However, there is a great debate about the importance of these ‘odors’, or even whether they occurred. We tend to focus on the Earth’s interior and the evolution of life as controls for Earth’s oxygen balance, but rock bombardment from space offers an intriguing alternative, “he concludes.

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