The oldest known piece of the Earth has been confirmed at 4.4 billion years old, perhaps as little as 100 million years after the formation of the planet. The finding sheds light on the Earth's conditions in its earliest days.
The Jack Hills region of Western Australia was one of the first pieces of continental crust to form, making it an ideal laboratory for investigating the dawn of terrestrial time.
Zircons are small crystals which incorporate uranium during their formation. As the uranium decays it produces lead as its end-product. Consequently zircons' age can be calculated by measuring the ratio of uranium to lead. In 2001 a crystal the width of a human hair from the Jack Hills was dated at 4.404 billion years old using this technique, making it the oldest ever found, although the rock in which it was found is thought to be a mere 3 billion years old.
However, tiny clusters within the crystal enriched with lead raised doubts about the accuracy of the measurement – if the lead was not evenly distributed might some of it be from another source? Nature Geoscience reports on the use of the atom-probe tomography method of examining individual atoms to test if lead movement might have skewed the results. According to lead author Prof John Valley of the University of Wisconsin, “The zircon formed 4.4 billion years ago, and at 3.4 billion years all the lead that existed at that time was concentrated in these hotspots.” A melting event concentrated the lead, according to Valley, but did not add any extra to make the zircon seem older than it is.
The finding lends support to the Cool Early Earth hypothesis. This holds that there was a period in the Earth's evolution, prior to what is known as the Late Heavy Bombardment 3.8 and 4.1 billion years ago when huge asteroids blasted the Earth, where conditions were cold enough to allow liquid oceans. Since it is not understood why asteroid impacts would diminish and then spike again the Cool Early Earth theory was initially treated with skepticism but is now gaining support.
Prof Valley has been a leading proponent of the Cool Early Earth idea, based on consistency of the oxygen isotopes found in crystals from that era, which he argues indicates “uniformity of processes and conditions”. Since the zircon shows signs of being formed under cool conditions the age confirmation supports his position and pushes back the time at which oceans appear to have existed even further.
“This confirms our view of how the Earth cooled and became habitable,” says Valley. “This may also help us understand how other habitable planets would form.”
The first 500 million year's history are known as the Hadean Era, as it was initially thought that the period resembled the ancient Greek hell, but if Valley is right it may have been suitable for life much earlier than previously thought.