'Earth much younger than previously thought'

Geologists at Cambridge University have calculated that the planet could have taken far longer to form following the birth of the solar system 4.567 billion years ago than it was previously believed.

By comparing chemical isotopes from the Earth's mantle with those from meteorites, the geologists have claimed that Earth reached its current size around 4.467 billion years ago, 'The Daily Telegraph' reported.

Scientists have in the past estimated that the Earth's development, a process known as accretion where gas, dust and other material clumped together to form the planet, happened over just 30 million years.

But the new study claims this process may have taken up to 100 million years -- more than three times.

The geologists, however, claim that while the Earth probably grew to 60 per cent of its current size relatively quickly, the process may well have then slowed, taking about 100 million years in all.

"The whole issue hinges on working out how long it took for the core of the Earth to form, which is one of the big unknowns in this area of science. "One of the problems has been that scientists usually presume Earth's accretion happened at exponentially decreasing rate. We believe the process may not have been that simple and that it could well have been a much more staggered, stop-start affair," said lead author Dr John Rudge.

The accretion of the Earth involved a series of collisions between large pieces of debris, known as planetary embryos. The huge levels of heat created by the impacts caused the interior of the growing planet to melt, creating molten metal core at the Earth's centre and the mantle above it.

The team used measurements of the levels of chemical isotopes created during the accretion of the Earth, providing a form of geological clock. The Earth's isotope levels were then compared with samples taken from meteorites that have hit our planet in modern history. These meteorites are a kind of time capsule that have isotope levels similar to those present in original material that clumped together when the solar system formed.

Differences in the isotopic values of Earth tungsten and that taken from the meteorites was able to provide the geologists with information about how long accretion took. Dr Rudge and his colleagues used computer models to calculate how the Earth could have formed to match the levels of isotope decay found in the planet's mantle.

They showed that the Earth almost certainly could not have formed within 30 million years but instead grew very quickly, reaching two-thirds of its size within about 10 to 40 million years. The accretion process then slowed and took up to another 70 million years to complete. "If correct, that would mean the Earth was about 100 million years in the making altogether. We estimate that makes it about 4.467 billion years old -- a mere youngster compared with the 4.537 billion-year-old planet previously imagined," he wrote in the 'Nature Geoscience' journal.

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