EARTH × ATMOSPHERE
The Jack Hills Zircons
~4.4 billion years ago
The Jack Hills zircons, from Western Australia, are grains of a resistant mineral that are about 4.4 billion years old and are the oldest solid pieces of the Earth, older than any surviving rock.
The Jack Hills zircons are crystals of zircon, a mineral that resists erosion and metamorphism, the transformation of a rock by heat and pressure. Found in the Jack Hills of Western Australia, they are the oldest solid pieces of the Earth in our possession, older than any surviving rock. A zircon crystal retains the composition of the melt, the molten rock, in which it grew. The melt of the Jack Hills zircons had incorporated older material, which liquid water had previously altered at low temperature. Oxygen exists in a light form, oxygen-16, and a heavier form, oxygen-18, and this alteration raises the ratio of oxygen-18 to oxygen-16. The crystals still have that raised ratio. Read at face value, the ratio indicates that liquid water, and possibly oceans, existed at the surface within 150 million years of the formation of the planet.
Why it matters
The earliest period of the Earth's history is ordinarily represented as a field of lava. The Jack Hills zircons indicate instead a planet with rain, with persisting water and possibly seas almost from the outset. Each million years by which that period of lava is shortened is a further million years during which life could have appeared.
Dating & uncertainty
Each grain is an isolated crystal. Erosion removed it from the rock in which it formed, and it was later buried in a far younger rock. Its age is therefore only the date of its own crystallisation. Reading the oxygen of the grain as the mark of cool surface water remains an inference, because no part of that surface survives against which to check the reading.
Earth history as one day
00:44:2423 h 16 min before midnight
if Earth’s 4.54 billion years were compressed into a single day