by Australian scientists have discovered 3.48 billion-year-old rock fragments that may be the first evidence of a meteorite crash on Earth.
The fragments, known as spherules, may have formed when the meteoroid slammed into the ground, blasting molten rock into the air. This molten rock then cooled and hardened into pinhead-sized pearls that were buried over the eons.
The researchers presented this finding, which has not been peer-reviewed, at the 54th Lunar and Planetary Science Conference (opens in a new tab) in Texas last week. In a summary of their results (opens in a new tab), the scientists concluded that the spherules, which they drilled from a group of volcanic and sedimentary rocks called the Dresser Formation of the Pilbara craton in Western Australia, are “the earliest evidence of a potential bolide impact in the Earth’s geological records”. (A fireball is a large meteoroid that explodes in the atmosphere upon falling to Earth.)
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So far, the oldest evidence of meteoroid impacts were 3.47 billion year old spherules, also from the Pilbara craton, and 3.45 billion year old fragments found in the Pilbara craton. Kaapvaal, South Africa.
“This new research documents ejecta in slightly older rocks, which are 3.48 billion years old (about 10 million years older than previously),” University of Waterloo geologist Chris Yakymchuk in Canada who was not involved in the research, Space.com’s sister publication Live Science told in an email. The results appear robust, he said, but access to the full data set would be needed to confirm their significance.
Scientists discovered the spherules in 2019 in sedimentary rock cores and dated them using isotopes, versions of the same chemical element that have different masses due to the number of neutrons in their nuclei. “It’s a robust and reliable dating technique,” Yakymchuk said. “We have a good idea of their age based on isotopic dating of the mineral zircon.”
The team concluded that the spherules were almost certainly of foreign origin due to their chemical composition. They detected platinum-group elements like iridium in much higher amounts than normally found in Earth’s rocks, as well as minerals called nickel-chromium spinels and osmium isotopes in the range typical of most meteorites. They also noted that the fragments had the characteristic barbell and teardrop shapes of impact spherules and contained bubbles, which tend to form when splashes of molten rock solidify after a meteoroid strike.
Evidence of meteor collisions with Earth is hard to come by and often controversial. Plate tectonics and erosion wear down the earth’s crust and can erase traces of past impacts, such as impact craters. A 2012 study (opens in a new tab) claiming to have discovered the The world’s oldest meteor crater has sparked heated debate (opens in a new tab) among scientists. But when geological forces obliterate a crater, the spherules are sometimes all that’s left of the event.
“There are two groups of impact-related rocks,” Yakymchuk said. “The first group is where there is still a preserved impact crater – the oldest known is the 2.23 billion year old Yarrabubba structure in Western Australia. The second group is where we have fragments of rock and minerals that were created by an impact, but they were ejected from the impact crater and are now in the rocks.”
The team is currently studying the rocks that covered the spherules and analyzing the different layers of sediment they drilled to refine their understanding of the impact of meteoroids. Ancient bombardments like this shaped conditions on early Earth and hold rare clues to our planet’s history.
The story originally appeared on Live Science.
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