Scientists in Australia have unearthed 3.48 billion-year-old rock fragments that may be the earliest evidence of a meteorite crashing into Earth.
The fragments, known as spherules, may have formed when the meteor slammed into the ground, spraying melted rock into the air. This melted rock then cooled and hardened into pinhead-size beads that became buried over the eons.
Researchers presented this discovery, which has not been peer-reviewed, at the 54th Lunar and Planetary Science Conference (opens in new tab) in Texas last week. In a summary of their results (opens in new tab), the scientists concluded that the spherules, which they drilled up from a group of volcanic and sedimentary rocks called the Dresser Formation of the Pilbara Craton in Western Australia, are "the oldest evidence of a potential bolide impact in the geologic record of Earth." (A bolide is a large meteor that explodes in the atmosphere while falling to Earth.)
Until now, the oldest evidence of meteor impacts were 3.47 billion-year-old spherules, also from Pilbara Craton, and 3.45 billion-year-old fragments found in Kaapvaal Craton, in South Africa.
"This new research documents ejecta in slightly older rocks, which have an age of 3.48 billion years old (about 10 million years older than previously found)," Chris Yakymchuk (opens in new tab), a geologist at the University of Waterloo in Canada who was not involved in the research, told Live Science in an email. The results appear robust, he said, but access to the entire dataset would be necessary to confirm their significance.
Related: What are the largest impact craters on Earth?
The scientists discovered the spherules in 2019 in cores of sedimentary rock and dated them using isotopes, versions of the same chemical element that have different masses due to the numbers of neutrons in their nuclei. "This is a robust and reliable dating technique," Yakymchuk said. "We have a good idea of their age based on isotope dating of the mineral zircon."
The team concluded that the spherules were almost certainly of alien origin due to their chemical composition. They detected platinum group elements like iridium in much higher amounts than normally found in terrestrial rocks, as well as minerals called nickel-chromium spinels and isotopes of osmium within the range typical for most meteorites. They also noted that the fragments had the characteristic dumbbell and teardrop shapes of impact spherules and contained bubbles, which tend to form when splashes of melted rock solidify after a meteor strike.
Evidence of meteor collisions with Earth is difficult to find and often controversial. Plate tectonics and erosion wear down the planet's crust and can erase traces of ancient strikes, such as impact craters. A 2012 study (opens in new tab) claiming to have uncovered the world's oldest meteor crater triggered a heated debate among scientists. But when geological forces wipe out a crater, 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 an impact crater preserved — the oldest known one 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 through an impact, but they have been ejected from the impact crater and are now found in rocks."
The team is now studying the rocks which encased the spherules and analyzing the different layers of sediment they drilled up to refine their understanding of the meteor strike. Ancient bombardments like this one shaped the conditions on early Earth and hold rare clues about the history of our planet.
