Zircon crystals, the oldest pieces of rock on the planet, were likely formed in craters left behind when asteroids hit a young Earth, instead of through plate tectonics as earlier believed, according to researchers.

As rocks form over a period of time, they let geologists track Earth's history via inferences about climate variations, where water first appeared and even where life sprouted from. However, just how far back researchers can go is determined by the zircon crystals they have.

Zircon crystals that naturally occur are about as wide as a human hair and more than 4 billion years old. The planet itself is slightly a little older than that at 4.5 billion years old. Some 15 years ago, researchers revealed that there was water on the Earth's surface during the crystal formation, but 10 years ago, researchers argued that tectonic plate movement was probably responsible for the formation of zircon crystals.

Now, evidence points out that plate tectonics did not occur when the Earth was young. So, where did zircon crystals come from?

According to geologists, from space. Specifically, they formed in massive impact craters left behind when asteroids peppered the planet.

For a study published in the journal Geology, researchers gathered zircon crystals from the Sudbury impact crater in Ontario, Canada, collecting thousands of samples from the largest impact crater on the planet and the second crater to be confirmed nearly 2 billion years old.

At the Swedish Museum of Natural History, the samples were analyzed and were found to be indistinguishable in composition from older, more ancient zircon crystals.

"There's a lot we still don't fully understand about these little guys but it looks like we may now be able to form a more coherent study of Earth's early years," said Gavin Kenny, the study's first author and doctoral researcher at Trinity College Dublin's School of Natural Sciences.

He recently presented their findings at the yearly Lunar and Planetary Science Conference held in Texas, meeting a research group, which two years ago studied the timing of the asteroid impacts on a young Earth and suggested that the impacts may be able to explain how old ancient zircon crystals are. Needless to say, the other research team was pleased to hear that they were in the right direction.

Photo: Ryan Somma | Flickr