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Birth of the Earth pushed back in time


Jacobsen: Earth formed much earlier than we thought before (Pic: Harvard University)

The Earth turned from a lump of cosmic sediment into the crusty layered life-supporting planet we know and love today much earlier than previously thought, according to new calculations.

Professor Stein Jacobsen of Harvard University in Boston describes the find, based on a detailed new computer model of the birth of Earth, in the latest issue of the journal Science.

Scientists believe the Earth formed from lumps of cosmic debris known as chondrites some 4.6 billion years ago - around the time our solar system was forming. But the question of when this cosmic sediment differentiated into the core and mantle of today's Earth has been the subject of some debate.

"The big thing about Earth's structure is its core and mantle, and it would be very nice to find out when that actually formed," Dr Trevor Ireland of Macquarie University in Sydney told ABC Science Online.

For many years, scientists believed the mantle and core must have formed very early on to give life enough time to develop, said Ireland, a 'cosmochemist' or specialist in planetary formation. But a 1995 report in the same journal by U.S.-based researchers Dr Der-Chuen Lee and Dr Alex Halliday shocked the scientific world with calculations indicating the mantle and core could not have formed until at least 60 million years after the solar system was formed.

They also claimed the event that is believed created our Moon - an impact with Earth by a Mars-sized object - must have occurred at this time or after.

Not only did this mean there was less time for life to arise, but the question of how a mass of very hot and heavy lumps of rock could remain stable for 60 million years without melting and collapsing into a dense core had scientists stumped.

Lee and Halliday made their original calculations using an isotope 'clock' based on the conversion of the radioactive element hafnium 182 to tungsten 182. Hafnium converts to tungsten over millions of years, and the relative amount of tungsten on Earth versus meteorite samples can be used as a clock to establish when the core and mantle separated.

The U.S. duo found that the amount of tungsten in samples from Earth's mantle was the same as that in the meteorite samples - which meant that separation of the mantle and core happened a long time after the clump of cosmic sediment first appeared. Their estimate that the formation of the crusty Earth occurred 60 million years after the solar system formed was much longer than anyone expected.

"This was a big thing in 1995, to come out and say 'hey, the core formed late'," said Ireland, adding that scientists have been unsure of the claim since it was first made.

But in his article in the current Science, Jacobsen points out that in the last year, there have been three papers - including one from a team led by Dr Ronny Schoenberg from the University of Queensland in Brisbane - which have found different levels of tungsten in Earth and meteorite samples. This supported a much earlier date of core and mantle formation.

While some studies have suggested Earth formed 30 million years after the solar system, Jacobsen claims new calculations supports a figure of 10 million. "It is important that several groups continue to perform such measurements and challenge each other's results," he wrote.

According to Ireland, because the measurements of tungsten involved are very tiny and subject to huge error margins "it all comes down to reproducibility". With three different groups challenging their findings, Lee and Halliday's claim just hasn't stood up, he said.

Ireland said Lee and Halliday's theory was "not a happy scenario" because it would have given life such little time to develop. Jacobsen's estimate means that Earth formed at roughly the same time as the other planetary bodies: "So it seems that everything is okay now."

Tags: environment, astronomy-space, biology, geology