“Baaaaa!” The unmistakable sound of life echoes from a sheep ranch in Australia’s Jack Hills. Beneath their hooves lie bits and pieces of the oldest rocks on the planet. A tiny zircon crystal twice the width of a human hair found there has been dated at 4.4 billion years old. That’s 300 million years older than the previously oldest dated rocks.
John Valley, a University of Wisconsin-Madison professor of geoscience who led the research, wrote in the journal Nature Geoscience Sunday that the discovery indicates Earth’s rocky crust, along with the zircon crystal, were forged soon after the planet itself formed 4.5 billion years ago.
Our planet was originally a ball of molten rock bombarded by meteorites and heated by gravitational contraction. Heavier materials like iron and nickel sank to form a core, while lighter silicates floated on top as a global magma ocean. Welcome to Earth during the Hadean Era, named for Hades and the hellishly hot conditions at the time.
Yet just 100 million years later – a week’s vacation in geologic time – the zircon discovery indicates Earth had cooled enough for a solid surface to form and water to begin accumulate into the early oceans.
But was our hot-headed planet cool enough yet for life? Some forms of life, like the bacteria that thrive in the super-heated waters of Yellowstone’s hot springs, love the heat and would perish if subjected to the “chill” of a hot summer day in the Midwest.
Valley believes there’s no reason why life couldn’t have existed on Earth 4.3 billion years ago 100 million years after the crust hardened. The most ancient life forms discovered to date are the filo-like layers of bacteria and sediment that form the pillowy stromatolites, some of which 3.4 billion years old. Life’s been around a l-o-n-g time.
To date the zircon, Valley and team used the tried and true technique of measuring the radioactive decay of uranium into lead. Uranium is an unstable element that sheds nuclear particles, slowly transforming into lead at a known rate. While highly accurate as a dating method, scientists were concerned that lead atoms might migrate over the great age of the crystal, concentrating in certain areas and leading to inaccurate age estimates.
To verify their results, the team used a more sophisticated method called atom probe tomography. Here an electric pulse removes atoms from a sample one layer at a time while a computer maps the precise positions and types of atoms to create a 3-D reconstruction of the specimen at the atomic scale. Good news. Lead atoms had virtually stayed put, confirming the zircon’s primeval status.
Still, nothing beats a CAI. Pale white calcium-aluminum-rich inclusions or CAIs dated at 4.567 billion years are found in a class of meteorites known as the carbonaceous chondrites. They’re the oldest matter we can get our hands on in the solar system, pre-dating the Jack Hills zircon by 167 million years. All this time travel is starting to make me feel like Alice falling down the rabbit hole …