Oldest known fragment of earth lends weight to ‘cool earth’ theory

Scientists have confirmed a near-microscopic zircon crystal measuring 200 by 400 microns – roughly twice the diameter of a strand of hair – is the oldest known part of our planet, dating back 4.4 billion years ago.

Extracted from a sheep ranch in Western Australia back in 2001, the crystal suggests that Earth’s crust formed relatively soon after the planet was made 4.5 billion years ago.

Thus, as the research’s leader John Valley, University of Wisconsin geoscience professor notes, the crystal suggests the earth early years may not have been such a harsh place after all.

Dating methods

Writing in the journal Nature Geoscience, Valley states that the scientists first used the uranium-lead dating method to determine the age of the zircon fragment; which measures the radioactive decay of uranium as compared to lead to ascertain a mineral’s age.

Yet due to the crystal’s supposed age, experts hypothesised that the possible movement of lead atoms over time could lead to false findings.

The researchers then turned to atom-probe tomography, a sophisticated technique which identifies and measures the mass of individual lead atoms, which confirmed the 4.4 billion year-old birthdate.

Scientists believe earth was originally formed as a ball of molten rock 4.5 billion years ago, meaning the findings indicate the earth’s crust was formed only a short while – a mere 100 million years – later.

Supporting a cool notion

“One of the things that we’re really interested in is: when did the Earth first become habitable for life? When did it cool off enough that life might have emerged?” Valley said.

As the oldest recognised terrestrial material, Valley states the crystal supports the cool early Earth (CEE) theory, which states that the earth’s temperatures were low enough to support oceans and perhaps even life earlier than previously thought.

Named after the Greek underworld of Hades, the Hadean was the first geologic eon of earth, and the discovery of the zircon crystal’s age suggests that earth may have been able to sustain microbial life during this time.

“We have no evidence that life existed then. We have no evidence that it didn’t,” Valley states.

“But there is no reason why life could not have existed on earth 4.3 billion years ago.”

Stromatolites produced by cyanobacteria, an archaic form of bacteria, are the oldest fossil records of life, and date back to around 3.4 billion years ago. The cyanobacteria fossils were also found in Western Australia.

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