The early Earth was bombarded by asteroids the size of a city

Crater caused by a meteorite in Arizona (United States).

Crater caused by a meteorite in Arizona (United States).

Scientists know that the earth was bombarded by huge meteorites in distant times, but new research suggests that the number of these impacts may have been 10 times greater than previously thought, which translates into a barrage of collisions, similar in scale to the asteroid collision that wiped out the dinosaurs, on average every 15 million years between 2.5 and 3.5 billion years ago.

According to this study, presented at the Goldschmidt geochemistry conference, some of these individual impacts they may have been much larger, possibly the 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 today. Scientists believe it was hit by a significant number of large asteroids (over 10 km in diameter), which would have had a significant effect on the near-Earth surface chemistry and its ability to host life.

The effect of a single collision of this type was demonstrated relatively recently with the impact of Chicxulub 66 million years ago, which 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 planets, but 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 earth 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.

The investigator Simone marchi, from the Research Institute of the Southwest, 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 found 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 previously 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. ”

Relationship with the evolution of oxygen

For example, one of the results they are studying is try to understand if 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 collisions is intriguing and deserves further investigation. This is the next stage of our work. ”

For her part, the doctor Rosalie tostevin, from the University of Cape Town, in South Africa, which did not participate in the study, points out that “these large impacts would have undoubtedly caused some disruption. Unfortunately, few rocks survive from such distant times, so direct evidence of the impacts and their ecological consequences are irregular. The model proposed by Dr. Marchi helps us to have a better idea of ​​the number and size of collisions on the primitive 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 ‘smells’ or even whether they were produced. We tend to focus on the interior of the Earth and the evolution of life as controls for the Earth’s oxygen balance, but rock bombardment from space offers an intriguing alternative“, he concludes.


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