The Earth, along with the Moon and the other inner planets of the Solar System, received, around 4,000 million years ago, the impact of countless meteorites during one of the most violent episodes in our history: the ‘Great Bombardment’. The ‘scars’ of that period, which lasted several hundred million years, are still visible on worlds without an atmosphere, such as Mercury or the Moon.
At that distant time, around 3.5 billion years ago, one of these meteorites hit what is now Western Australia, leaving its tracks in a group of volcanic rocks known as the Dresser Formation. And now geologist Christian Köberl from the University of Vienna has found them. The unusual discovery was made public by the researcher himself on March 14 during his talk at the 54th Texas Conference on Lunar and Planetary Sciences.
Finding and dating rocks that old is very difficult, as constant geological and biological activity (earthquakes, eruptions, atmospheric agents, bacteria, etc.) continually wear and transform our planet’s crust. Therefore, unlike other worlds, the terrestrial surface ‘erased’ the traces of this stage of extreme violence. “If we look back about 3.5 billion years,” says Köberl, “we only find a very, very small percentage of the Earth’s crust of that age.”
Despite this, Köberl and his colleagues were able to find evidence of a meteorite impact that occurred 3.48 billion years ago, which is the oldest evidence of a collision with Earth known to date. The oldest previous impacts were found in two deposits, one in Australia and the other in South Africa, at 3.47 billion and 3.45 billion years old.
Evidence of the collision came in the form of a series of tiny rock spheres, each less than a millimeter in diameter, found in various layers in different drill cores obtained in Western Australia. These types of spherules can form in different ways, but one of them (the most interesting) is when the meteorite hits the ground and ‘droplets’ of molten rock spread around it. When solidifying, these drops form stone spheres.
To find out if this was the case and if the spherules actually came from an impact, the researchers analyzed them using several cutting-edge techniques. “O alien components -said Köberl- dominate the composition of these layers of spherules«.
These components, rare in terrestrial rocks but abundant in meteorites, include high percentages of iridium, some isotopes of osmium and also minerals called nickel-chromium ‘spinels’. Some of the spheres also had the characteristic dumbbell and teardrop shapes, with bubbles inside, something common in impact spheres due to the way they solidify after a meteor impact. The newly discovered grains, in fact, are almost identical to the slightly younger grains that researchers have already found in Australia and South Africa.
Finding these ancient meteorite impacts is important because they help piece together our planet’s history. Without going any further, conditions prevailing on early Earth depended to a large extent on the number of meteorites bombarding it at any given time. “Several of these spherule layers,” concludes Köberl, “were found in several of these drill cores… they likely represent at least two, perhaps three different individual impact events.” Now researchers are working to better understand the significance of the distribution of these layers and discover how they affect our understanding of meteorite bombardment billions of years ago.