Around 4.6 billion years ago, the world was a magmatic ocean, with chunks of gargantuan rock continuously smashing into it as it increased in size. Millions of years before life emerged, the lakes of fire began to cool to form the very first rocks, minerals, and crystals.
Out of all of these, zircons are thought to be the oldest, the signature of an Earth on the verge of forming some sort of solid ground. Although geologists have long known the various high-pressure high-temperature processes that can cause them to form – certain types of volcanic activity, asteroid impacts, and tectonic plates colliding into each other – no-one could agree as to which of these formed the oldest zircons.
A new study in the journal Geology has come to a rather definitive answer: It reveals that a heavy, prolonged bombardment by asteroids forged the very first zircons in the apocalyptic conflagration at the beginning of the world. By looking carefully at the chemistry of far younger impact-formed zircons and comparing them to some of the oldest ever found, the international team noticed that they were essentially identical, meaning that they had to have been formed by the same process.
“There's a lot we still don't fully understand about these little guys,” Gavin Kenny, a PhD student at the University of Dublin’s Trinity College and lead author of the study, said in a statement. “But it looks like we may now be able to form a more coherent story of Earth's early years – one which fits with the idea that our planet suffered far more frequent bombardment from asteroids early on than it has in relatively recent times.”
A scanning electron microscope image of a zircon crystal from the Sudbury impact crater. Gavin Kenny
Zircons are little resilient time capsules that capture the state of the planet as it was when they formed. They can reveal the presence of water, a breathable atmosphere, the global temperature, and even the existence of biological material. Consequently, geologists of all kinds are keen to find as many of them as possible. They are known to form during processes that involve huge spikes in pressure, and there aren’t many things that can do that.
As this new study points out, the consensus has been that the elder zircons were formed during the collision of colossal tectonic plates, but there was always a huge question mark hovering over this hypothesis: Plate tectonic motion didn’t begin until around three billion years ago – one billion years after the oldest zircons formed.
Asteroid impacts can also generate zircon crystals, so the researchers thought they would compare the oldest zircons with some far younger. If the compositions matched, it was probable that they formed via the same process.
The researchers visited Sudbury impact crater, the best preserved impact crater on Earth and the planet’s second oldest at nearly 2 billion years old. After analyzing the internal characteristics of the crystals, and comparing the chemical compositions of the two, they found that the two sets were essentially indistinguishable.
The Sudbury impact crater, roughly 30 by 60 kilometers across, is the elliptical feature in the center of the image. A smaller impact crater, now a lake, can be seen just to the top right of it. NASA
“Many people thought the very ancient zircon crystals couldn't have formed in impact craters, but we now know they could have,” Kenny added.
This all but confirms that the early Earth was indeed a truly violent place, one that would have looked far more like the pockmarked surface of the far side of the Moon than the pale blue dot it resembles today.