Earth younger than previously thought, say scientists

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maandag 12 juli 2010

A new geological study has set a more accurate age for planet Earth, according to scientists.

Researchers say their investigation shows the Earth is 70 million years younger than the 4.537 billion-year-old planet "we had previously imagined".

To confirm Earth's age, the team compared elements in its mantle to those in meteorites that are the same age as the Solar System.

The group reports its findings in the journal Nature Geosciences.

The crux of its conclusion was that the formation of the planet took much longer than previously thought.

The scientists studied this timescale by looking at how long Earth took to "accrete", or grow, as smaller "planetary embryos" smashed together to form it.

"The collisions caused part of the planet to melt, and allow metal to segregate to the centre of the Earth to form the core," explained Dr John Rudge, from Cambridge University, UK, who led the research.

"So [during this process], the planet differentiated into its molten metal core and outer-lying mantle."

The longer this process took, the later the Earth was "born" in its current size and geological form.

Planetary clock

To shed light on this, the scientists looked at two "isotopes" - chemical elements in the Earth's mantle called 182-hafnium and 182-tungsten. Over a set period of several million years, hafnium decays to become tungsten. And tungsten "loves" metal, so while the planet's core was still forming, it became incorporated into that.

This left a "signature" in the mantle that revealed how long the Earth took to differentiate.

By comparing the amount of 182-tungsten in the mantle to the amount found in meteorites, the researchers could work out how long it took for Earth to fully differentiate into mantle and core.

The team compared the results from this technique with a similar method using two different isotopes. And instead of assuming that one method was more accurate than the other, and that the Earth formed at a steady rate, they modelled all of the different ways that the process could have happened.

Dr Rudge explained that, for these two methods to agree, the formation of the Earth would have had to have been a "rapid early on, then there was some hiatus and more gradual accretion".

This meant, he said, that instead of Earth forming over 30 million years, it took closer to 100 million years.

He explained that the end of the "hiatus" could have been the giant impact that is believed to have formed the Moon.

"If correct, that would mean the Earth was about 100 million years in the making altogether," Dr. Rudge said. "We estimate that makes it about 4.467 billion years old - a mere youngster compared with the 4.537 billion-year-old planet we had previously imagined."

Bron: news.bbc.co.uk

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