Nature
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Researchers might have uncovered what is believed to be the earliest evidence of a meteorite's impact on Earth. In rocks from 3.48 billion years ago, researchers have discovered structures consistent with a collision from the heavens. These are rock spherules whose structure and chemical composition indicate that the extraterrestrial hypothesis is most likely the correct one.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Finding evidence of ancient impacts is difficult here on Earth. Tectonic plates and erosion have wiped out the evidence of these events that shaped the early history of our planet. The oldest known impact crater is the Yarrabubba crater in Western Australia, which dates from 2.23 billion years ago.
But there are regions on Earth with much older rocks, so researchers have been looking for indirect evidence of these impacts – such as the spherules. But rock spherules are not meteorite impact-exclusive. Geologists are aware of a few ways for them to come into being.
The analysis, led by graduate researcher Michaela Dobson from the University of Auckland, is consistent with the melting of rock following a high-speed impact. Basically, the space bolide struck the ground and melted rocks that flew into the air as tiny splashes, before landing back down and solidifying into these rocky droplets.
The way they are shaped, looking like teardrops and dumbbells, and with bubbles inside them, is a big indication that they formed following an impact. They look like other impact spherules that have been discovered in the same location at the Pilbara Craton in Australia and in the Barberton Greenstone Belt in South Africa, although these are slightly younger, ranging between 3.4 to 3.2 billion years old.
“Lots of similarities to other spherule layers (which I have been studying for about 30 years now), just a little bit older,” senior author Professor Christian Köberl told IFLScience.
There is also the matter of chemical composition, which is certainly alien. In particular, the team looked at isotopes. These are versions of the same elements that have slightly different masses – they have more or fewer neutrons in the nucleus. Ratios of isotopes are like a signature that can tell you where something comes from. And these rocks don’t seem to come from Earth.
“There is some admixture of platinum group elements, which are higher in abundance than would be normal for terrestrial rocks, and especially the ratio of isotopes of the element osmium are within the range typical for most meteorites,” Professor Köberl told IFLScience.
More work is being done on the sample rocks in which these spherules have been found to better understand this ancient event and what it tells us about the early history of our planet.
These results were presented at the 2023 Lunar and Planetary Science Conference in Texas.
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