Bombardments from outer space shaped the atmosphere

Impact: Beyond the Night Sky, a 2020 SCINEMA International Science Film Festival documentary, explores impacts and collisions between bodies in our solar system. These events are occurring all the time, ranging from tiny collisions (like the meteors that burn up brightly in our atmosphere) to large-scale events that have shaped our cosmic backyard as we know it.

The film follows planetary scientist Katarina Miljkovic, an associate professor with the School of Earth and Planetary Sciences at Curtin University in Western Australia, and the moments that propelled her towards a career in planetary science. It touches on her fascinating contributions to the field of research; from studying past impact scars on the Moon so as to better understand its evolution, to analysing today’s meteorite impacts on Mars to learn more about the planet’s interior.

You can watch Impact: Beyond the Night Sky in full here.

Read on to learn about how early asteroid bombardment may have shaped the young Earth’s development by delaying atmospheric oxygen growth.

For the first few billion years of the Earth’s existence, the weather forecast was cloudy with a chance of asteroid. Geochemists have presumed for some time that this flurry of strikes altered the chemistry of the nascent atmosphere, but research from 2021 now offers clues as to how.

In a paper published in Nature Geoscience, a team of researchers found that early asteroid bombardment may have delayed the growth of oxygen on the planet.

“Free oxygen in the atmosphere is critical for any living being that uses respiration to produce energy,” says geologist Nadja Drabon, co-author of the study from Harvard University. “Without the accumulation of oxygen in the atmosphere we would probably not exist.”

The team – led by Simone Marchi at the Southwest Research Institute in Boulder, Colorado – focused on the Archaean eon, which stretched from 4 billion to 2.5 billion years ago.

To get a glimpse into this ancient time, they analysed asteroid remnants called “impact spherules”. These glassy, sand-sized particles are actually re-solidified drops of molten rock, formed when the energy from an asteroid or comet vapourised rocky materials in the planet’s crust. Today, they are found in thin layers on the Earth’s crust.

“In recent years, a number of new spherule layers have been identified in drill cores and outcrops, increasing the total number of known impact events during the early Earth,” Drabon says.

Rock sample showing small spherical balls in it. Early asteroid bombardment.
Spherule layers — such as the one shown in this 5-centimetre, 2.6-billion-year-old sample from Australia — are markers of ancient collisions. Credit: UCLA/Scott Hassler and Oberlin/Bruce Simonson

Using this new data of additional impact events, the team updated older models to find that they had previously underestimated how often asteroids and comets struck the infant Earth. Turns out, a cosmic crash happened every 15 million years – 10 times more frequently than previously thought.

“What’s more,” Marchi adds, “we find that the cumulative impactor mass delivered to the early Earth was an important ‘sink’ of oxygen, suggesting that early bombardment could have delayed oxidation of Earth’s atmosphere.”

Basically, when impactors struck the Earth, the vapours would have chemically consumed the oxygen that was around, essentially sucking it out of the atmosphere.

“Late Archean bombardment by objects over six miles [10km] in diameter would have produced enough reactive gases to completely consume low levels of atmospheric oxygen,” says co-author Laura Schaefer from Stanford University.

“This pattern was consistent with evidence for so-called ‘whiffs’ of oxygen, relatively steep but transient increases in atmospheric oxygen that occurred around 2.5 billion years ago. We think that the whiffs were broken up by impacts that removed the oxygen from the atmosphere.”

Around 2.4 billion years ago, around the time bombardment slowed down, the Earth went through a major shift known as the Great Oxidation Event (GOE) – though oxygen didn’t build up to current levels until within the last billion years.

“As time went on, collisions become progressively less frequent and too small to be able to significantly alter post-GOE oxygen levels,” lead author Marchi says. “The Earth was on its course to become the current planet.”

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