Y:        Hey Don, do you know what the oldest rock on Earth is?

D:        Well Yaël, I think I remember reading that it was zircon found in Australia. Haven't scientists found zircon crystals there that are over four billion years old?

Y:        Yup! And since the Earth is just over 4 and half billion years old, those zircon crystals are the closest thing we have to a time capsule from Earth in its early years. One problem scientists have been trying to solve to help understand what that period looked like is figuring out how the earliest zircon crystals formed.

D:        Didn’t scientists think they formed because of plate tectonics ‑‑ giant chunks of earth colliding with each other.

Y:        They did think that at first, but current evidence suggests that plate tectonics the way we understand them today weren't in play back then. Now, researchers believe that zircon formed in impact craters, which are indentations in the Earth created by asteroids crashing into it. The asteroid hits Earth at such a high speed that it melts the surface beneath it. That creates a perfect condition for zircon formation.

D:        Which ties in well with the idea that asteroids were really pummeling the Earth in its early years.

Y:        Right. So to test this idea, scientists studied zircon from the Sudbury impact crater in Canada -- one of the best preserved, and the second‑oldest impact crater on Earth. They found that the compositions of those crystals were indistinguishable from the four billion‑year‑old ones, implying that it's likely the older crystals formed in an impact crater as well.