The early Earth was literally bombarded by asteroids

What do you do when there are no more direct traces of impact craters on earth, but you still want to know how many celestial bodies hit here earlier? One has to infer this from analyzes of moon samples or crater counts on the moon, because the impacts were better preserved there because there are neither active volcanoes nor air or water.

This is exactly what an international team of scientists, including researchers from the University of Vienna, did and came to the conclusion that the number of asteroids that struck early Earth 3.5 to 2.5 billion years ago was ten times higher, than previously assumed. In addition, this higher number of impacts delayed the enrichment of oxygen in the atmosphere, they report in the journal Nature Geoscience.

Rock evaporates

Frequent violent impacts from sometimes large asteroids and comets shaped the geological and atmospheric development of the early Earth. There have long been scientific discussions about the strength and duration of this bombing.

For some time now, special remnants of asteroid impacts have also been providing new data: “When large asteroids or comets hit the early Earth, rock material from the earth’s crust was melted and evaporated,” explained the geochemist Christian Köberl, Professor of Impact Research and Planetary Geology at the University of Vienna. This rock vapor then condensed and solidified, as a result of which round, glass-like particles in millimeter size, so-called impact spheres, fell back to earth worldwide.

These globules were deposited and formed several thin “spherule layers” in the earth’s crust, the age of which was determined to be between 2.4 and 3.5 billion years. In recent years, numerous hitherto unknown layers of this type have been identified in drill cores and outcrops, especially in South Africa, but also in Australia.

In the current work, he and his colleague Toni Schulz from the University of Vienna examined many of these impact deposits and estimated their total number.

The lead author of the study, Simone Marchi from the Southwest Research Institute in Boulder (US state Colorado), assumes that in the late Archean, around 3.5 to 2.5 billion years ago, the number of impacts was ten times higher than previously assumed.

Air for the earth

This intensified bombardment is likely to have had a massive impact on the oxygen content of the earth’s atmosphere. Because: If objects with a diameter of more than ten kilometers strike, gases are released. This led to chemical reactions that immediately consumed the little oxygen in the atmosphere from geological and biological processes. This is what happened 3.5 to 2.5 billion years ago.

So it would be a long time before more oxygen slowly collected in the atmosphere and the development into today’s oxygen-rich planet began.

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