Ancient crystal questions Earth's history

A false colour image of the orldest zircon ever found

By BBC News Online science editor Dr David Whitehouse

A tiny crystal, little thicker than two human hairs, is the oldest Earth material ever found and it may force a reappraisal of what our planet was like early in its history.

This is an astounding thing to find from 4.4 billion years ago

Professor John Valley

The grain, technically known as a zircon, has been dated to be 100 million years older than any solid sample previously discovered.

It casts doubt on the accepted state of the Earth over four billion years ago: a boiling ocean of molten magma, devoid of life.

The new picture emerging is of a world cool enough to support water and continents, and possibly the first stirrings of life.

Cool Earth

Zircon is a durable crystal made of silicon, oxygen and zirconium, among other elements. The record-breaking sample was one of a dozen specimens extracted from the Jack Hills section of northwestern Australia. It was dated in two separate studies at 4.4 and 4.3 billion years old.

One of the researchers involved, Professor John Valley, from the University of Wisconsin-Madison, US, said: "This is an astounding thing to find from 4.4 billion years ago. At that time, the Earth's surface should have been a magma ocean. Conventional wisdom could not have predicted a low-temperature environment."

The conditions prevalent at the time the zircon grain formed suggest that the Earth cooled faster than anyone thought.

"Previously, the oldest evidence for liquid water on Earth, considered by most to be a precondition for life, was from a rock estimated to be 3.8 billion years old," said Professor Valley.

Accepted view

Both research teams dated the zircon grains by analyzing their uranium isotopes - types of uranium atoms with different masses.

Uranium decays into lead. By calculating the ratio of uranium to lead, they determined that a dozen of the grains in their possession were very old indeed.

The zircon analysis also involved looking at the ratio of oxygen isotopes. This showed the crystals could only have originated in a wet, low-temperature environment. This does not fit in with the accepted view that shortly after the Earth formed 4.5 to 4.6 billion years ago, the planet was a ball of molten rock.

Scientists had thought that it took several hundred million years for the Earth to cool enough for oceans to condense out of a thick atmosphere.

Catastrophic impacts

Evidence from the Moon also suggests that about 500-600 million years after formation, Earth was subjected to intense bombardment from meteoroids.

An intriguing question the new research raises is whether or not life could have arisen so early in the Earth's history.

Professor Valley said that the zircon analysis was consistent with the suggested idea that life began but was completely extinguished in catastrophic impacts that melted the Earth's surface.

Both research papers are published in the journal Nature. Commenting on the work, Professor Alex Halliday, of the Swiss Federal Institute of Technology in Zurich, said: "It represents a significant advance in reconstructing Earth's Dark Ages."