Crystal clear: How one tiny grain of zircon can tell us so much about our planet

The discovery in Australia of a tiny grain of zircon dating back to 4.374 billion years ago tells us much about the planet’s formative years

Science Editor

A tiny grain of crystal barely visible to the naked eye has confirmed that the Earth is a very ancient place indeed.

Scientists estimate that the crystal of zircon is 4.374 billion years old – give or take six million years – which lies very near to the date when the planet itself formed during the birth of the solar system about 180 million years prior to this.

The zircon grain came from the Jack Hills mountain range of Western Australia and now represents the oldest known piece of terrestrial rock on the planet – though some fallen meteorites and samples of lunar rock brought back to Earth may well be older. Whichever way you look at it, the Earth seems to be a very old place indeed.

Trying to gauge the age of the Earth has a long tradition. James Ussher, the Anglican Archbishop of Armagh and Primate of All Ireland, concluded in 1650 that the moment of creation occurred at nightfall preceding Sunday 23 October 4004BC. Although somewhat laughable by modern scientific standards, it was a remarkable piece of scholarship, weaving as he did the events and genealogies of the Bible with what was known about Greek and Roman history.

Archbishop Ussher concluded that the Earth was created 4,000 years before the birth of Christ and would be destroyed 2,000 years after his death. He was in good company in terms of his dates, by the way. Johannes Kepler, the great German mathematician and astronomer, and Isaac Newton, England’s father of physics, had both calculated that the Earth was created about 4,000 years before Christ.

As geological knowledge gradually replaced Biblical mythology, scholars began to realise that the Earth was far older than anyone had realised. The great geologists of the 19th century produced convincing evidence that some rocks were hundreds of millions of years old, which conveniently gave the time needed to explain the evolution of life and the origin of species by Charles Darwin’s mechanism of natural selection.

We now know that the Earth is far older than even these Victorian gentlemen had imagined. The crystal of zircon is like an ancient geological clock that began to tick some 4.4 billion years ago when the Earth’s first crust began to form above a hot ocean of molten magma. The dating work on the crystal, which was carried out by a team led by John Valley of the University of Wisconsin-Madison, shows that the Earth’s crust began to form relatively soon after the planet itself came into existence, which would extend the age of the origin of life.

Scientists believe the Earth was created about 4.56 billion years ago. This was followed by the violent Hadean eon, when the planet was bombarded repeatedly with asteroids and comets, including the massive planet-sized object that led to the creation of the Moon. It was a few million years after this moon-birth moment that the zircon from the Jack Hills crystallised into a miniature time capsule.

The dating method used to determine its age so precisely is based on the steady and predicable radioactive decay of uranium isotopes to lead. The decay acts like the tick-tock of a geological clock. Knowing what exists now within the crystal lattice allows scientists to estimate the time when the clock started “ticking”. Its timekeeping accuracy is critical because a difference in age of 100 million or 200 million years can make all the difference to knowing what was going on during this earliest phase of the planet’s long history.

The single most important implication of the finding is that the Earth began to cool far earlier than previously thought. It means there was a cool-enough crusty landscape for liquid water to form – perhaps helpfully delivered by icy comets – and so for life itself to begin.

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