The Earth, together with the Moon and the rest of the inner planets of the Solar System, received, about 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.
In that distant time, around 3.5 billion years ago, one such meteorite struck what is now Western Australia, leaving its tracks in a group of volcanic rocks known as the Dresser Formation. And now the geologist Christian Köberl of the University of Vienna has found them. The unusual finding 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 being able to date, such extremely old rocks is something very difficult, since the constant geological and biological activity (earthquakes, eruptions, atmospheric agents, bacteria, etc.) continually wear and transform the crust of our planet. For this reason, unlike other worlds, the terrestrial surface has ‘erased’ the traces of that period of extreme violence. “If we look back about 3.5 billion years,” says Köberl, “we will only find a very, very small percentage of the Earth’s crust of that age.”
Despite which, Köberl and his colleagues have managed to find evidence of the impact of a meteorite 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 one in South Africa, that are 3.47 billion and 3.45 billion years old.
rock spherules
Evidence of the collision came in the form of a series of tiny rock spheres, each less than a mm in diameter, found in various layers in different drill cores taken in Western Australia. These kinds 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 spray around it. When solidifying, these drops give shape to the stone spheres.
To find out if this was the case and the spherules really came from an impact, the researchers analyzed them using a number of cutting-edge techniques. “The 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 spherules also had characteristic dumbbell and teardrop shapes, with bubbles inside, something common in impact spheres due to how they solidify after the meteor strike. The newly discovered grains, in fact, are almost identical to slightly younger ones that researchers have already found in Australia and South Africa.
Finding such ancient meteorite impacts is important because they help piece together the history of our planet. Without going any further, the prevailing conditions on the early Earth depended, to a large extent, on the number of meteorites that bombarded it at any given time. “Several of these spherule layers,” Köberl concludes, “were found in several of these drill cores…they probably represent at least two, maybe three different individual impact events.” Now, the researchers are working to better understand the significance of the distribution of those layers and to discover how they affect our understanding of meteorite bombardment billions of years ago.