EARLY Earth was bombarded by a series of “city-sized” asteroids like the one that wiped out the dinosaurs , according to a new study.
Scientists know that our planet was struck by huge objects not long after its formation, but a new analysis suggest that the number of such impacts may have been ten times higher than previously thought.
They say that translates into a barrage of collisions, similar in scale to that of the asteroid strike which wiped out the dinosaurs.
On average, such impacts struck our pale blue dot every 15million years between 2.5 and 3.5billion years ago.
Some of the individual impacts may have been even bigger – possibly ranging from city-sized to even small province sized, say scientists.
The research team is also considering what effect the impacts may have had on the Earth’s evolving near-surface chemistry.
Researcher Dr Simone Marchi explained that Earth’s early years were “unimaginably violent” compared to today.
Scientists believe that Earth was struck by a “significant” number of large asteroids – more than six miles in diameter.
The collisions would have had a major impact on the Earth’s near-surface chemistry and ability to support life.
Dr Marchi said that the effect of just one such collision was shown comparatively recently by the Chicxulub impact 66million years ago, which led to the extinction of the dinosaurs.
However, he explained that the early Earth was very different to the planet at the time of the Chicxulub impact, and so were the effects of collisions.
Dr Marchi said impact craters from similar collisions can be seen on the Moon and other rocky planets.
Atmospheric weathering and plate tectonics have tended to mask any direct evidence for ancient impact craters on Earth.
However, he said echoes of these distant impacts can be seen in the presence of “spherules” found in ancient rocks.
The huge impacts threw up molten particles and vapours which then cooled and fell to earth to be embedded in rock as small spherical glassy particles.
The greater the impact, the more these particles would have spread from the impact site.
Global distribution of a thick spherule layer shows a huge impact.
Dr Marchi, of the Southwest Research Institute in the United States, said: “We have developed a new impact flux model and compared with a statistical analysis of ancient spherule layer data.
“With this approach, we found that current models of Earth’s early bombardment severely underestimate the number of known impacts, as recorded by spherule layers.”
Dr Marchi added that the true number of impacts could have been 10 times higher than previously thought in the period between 3.5 and 2.5billion years ago.
“This means that in that early period, we were probably being hit by a Chicxulub-sized impact on average every 15million years. Quite a spectacle,” he said.
Dr Marchi presented his work at the Goldschmidt geochemistry conference.
Dr Rosalie Tostevin, of the University of Cape Town who was not involved in the study, said of the findings: “These large impacts would certainly have caused some disruption.
“Unfortunately, few rocks from this far back in time survive, so direct evidence for impacts, and their ecological consequences, is patchy.
“The model put forward by Dr Marchi helps us to get a better feel for the number and size of collisions on the early Earth.”
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