The oldest thing on Earth is smaller than a piece of human hair. Researchers from the University of Wisconsin, Madison, have determined that blue zircon crystals found in a remote part of Australia are 4.4 billion years old, just 160 million younger than the formation of our solar system. The crystals were dug up in 2001 in an area called Jack Hills in Western Australia; researchers believed they were about 4.4 billion years old, but they couldn't prove it at the time.

"This confirms our view of how the Earth cooled and became habitable."

Now using two different methods, researchers have been able to definitively age the crystals. They first analyzed how much uranium in the crystals had decayed into lead. But atoms of lead can move around inside a crystal which could lead to inaccurate measurements, so the team tried a new method of aging called atom-probe tomography. This technique identifies and maps out individual atoms, showing their distribution inside the crystal. They found out that the lead atoms did move around, but not enough to affect age calculations. With both methods, they were able to place the crystal's age at about 4.4 billion years, with a margin of error of 6 million years.

This new data supports the theory that the crystals formed after a fiery period when an early proto-Earth was hit by a large, Mars-sized object and then melted. After that, the Earth's crust congealed and temperatures dropped, allowing liquid water, oceans, and a hydrosphere to form. According to the team's geochemist John Valley, the study not only reinforces the theory of the existence of a hydrosphere before 4.3 billion years ago, but also suggests life could have been sustained shortly after.