Underestimated bombardment: Large asteroids and comets hit the earth ten times more often than previously thought, 3.5 to 2.5 billion years ago, researchers report. Their models also show how these impacts shaped the development of the earth’s atmosphere: They delayed the enrichment of the air with oxygen. The new models can now help us understand the processes by which our planet evolved into the world we know.
We are still in danger from space: the “billiards” between the celestial bodies in our solar system can set asteroids and comets on a collision course with the earth and ultimately cause catastrophic impacts. But today’s probability of such a collision is low compared to the early days of the earth: In the age of the Archean, about 4 to about 2.5 billion years ago, there were extremely frequent and violent crashes. This is evident from the dating of lunar craters as well as from finds of earthly traces of the impacts. But it still seems unclear how often the earth was hit by objects more than ten kilometers in size and to what extent the fallout of such impacts affected the atmosphere, especially its oxygen content.
On the trail of the bombing
Researchers led by Simone Marchi from the Southwest Research Institute in Boulder are now providing new insights. Their results are based on new information on the earthly traces that caused impacts in the late Archean Era 3.5 to 2.5 billion. These are so-called impact spheres (spherules) that were created during the infernal collisions. Rock material melted and evaporated in the earth’s crust and shot upwards in huge clouds. Small droplets of the molten rock then solidified and fell back to the surface of the earth as particles the size of a grain of sand. There they formed thin layers that can be detected in some places on earth.
As the researchers report, a number of new layers with impact globules have been identified in drill cores and geological structures in recent years, which has significantly increased the total number of known impact events in the early history of the earth. This enabled the team to update existing bombing models. According to the results, the extent of the impacts in the late Archean was clearly underestimated: “It is becoming apparent that the impactor flow at this time was up to ten times higher than previously assumed,” says Marchi.
Impacts consumed oxygen
Based on this new assessment, the researchers then modeled how all of these impacts might have affected the atmosphere. “Our results show that the impactors that struck early Earth were an important oxygen sink. The bombardment probably delayed the oxidation of the earth’s atmosphere significantly, ”says Marchi. As the researchers explain, the oxygen content in the earth’s atmosphere is due to a balance between production and degradation processes. “The late Archaic bombardment of objects with a diameter of more than ten kilometers would have generated enough reactive gases to completely consume the small amounts of atmospheric oxygen,” explains co-author Laura Schaefer from Stanford University.
These results, in turn, fit geological evidence of so-called “whiffs” – relatively steep but transient increases in atmospheric oxygen that occurred particularly about 2.5 billion years ago. “We believe these climbs were interrupted by impacts that removed oxygen from the atmosphere. This also agrees with indications of large impacts that were detected in the spherule layers of the Australian Bee Gorge and Dales Gorge, ”says Schaefer.
About 2.4 billion years ago – at the end of the intensive bombardment – there was, however, a significant increase in the atmospheric oxygen content, which is known as the “Great Oxidation Event”. “Before this development, the impact vapors caused episodic low oxygen levels over long periods of time,” says Marchi. “But over time, the collisions became increasingly rare and too small to significantly change the oxygen content after the Great Oxidation Event. The earth thus embarked on the path of development that made it the world we know today, ”said the scientist.
Source: Harvard University, Southwest Research Institute, article: Nature Geoscience, doi: 10.1038 / s41561-021-00835-9