The oldest evidence of a meteorite impact on Earth has been found in Western Australia, inside a group of sedimentary and volcanic rocks called the Dresser Formation.
It is difficult to find and date old rocks on Earth because the constant geological and biological activity wears down the planet鈥檚 crust. 鈥淚f we go back about 3.5 billion years, there is only a very, very small percentage of the earth鈥檚 crust that is of that age,鈥� said at the University of Vienna in Austria during a presentation at the Lunar and Planetary Science Conference in Texas on 14 March.
Despite this, K枚berl and his colleagues have now found evidence of a meteorite impact that occurred 3.48 billion years ago, making it the oldest impact evidence on Earth. The previous oldest was two impact deposits that are 3.47 billion and 3.45 billion years old.
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The new evidence came in the form of tiny spherules of rock, each less than a millimetre across, found in layers across a series of drill cores taken in Australia. Spherules can form in a few different ways, but one is when a meteorite hits the ground and creates a splash of melted rock, which solidifies into tiny droplets.
To determine whether these particular spherules came from a meteorite impact, K枚berl and his colleagues used a series of cutting-edge techniques to analyse their textures and chemical compositions. 鈥淓xtraterrestrial components dominate the composition of these spherule layers,鈥� he said.
Those components include high amounts of iridium, some isotopes of osmium and minerals called nickel-chromium spinels. They also had characteristic dumbbell and teardrop shapes and bubbles inside them, common in impact spherules due to how they solidify after the meteorite strike. These newfound grains are nearly identical to the slightly younger ones researchers have found in Australia and South Africa.
Finding old meteorite impacts is important because they help us reconstruct the history of our planet 鈥� the conditions on early Earth depended heavily on how many meteorites were bombarding it at any given time. 鈥淪everal of these spherule layers were found in a number of these drill cores鈥� they probably represent at least two, maybe three different individual events,鈥� said K枚berl. The researchers are now working on refining their understanding of the layers and figuring out how they affect our understanding of meteorite bombardment billions of years ago.
