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Giant impact added to Earth's core
2005 10 31

By Anna Salleh |

The Earth's core was formed in two stages, which could explain why scientists have been coming up with two different dates for its age (Image: Universe Today/NASA)
The planetary collision believed to have created the Moon may help to explain why scientists can't agree how old the Earth's core is.

Professor Bernard Wood of Macquarie University in Sydney, Australia and Professor Alex Halliday of the University of Oxford in the UK report their research in today's issue of the journal Nature.

"You basically had two episodes of core formation," says Wood.

When the solar system formed 4.5 billion years ago, the Earth was a molten mixture of rock and metal. Over time, as the Earth cooled, the heavier metal gravitated towards the centre, or core.

Scientists have worked out when this happened by using 'clocks' based on the decay of radioactive isotopes found in the rocky part of the Earth.

These clocks were set ticking when the Earth's metal separated from its rock.

"You can calculate how far back core formation took place," says Wood.

But scientists came up with two different dates of when the core formed, depending on which isotopes they studied.

Studies of tungsten isotopes, formed from hafnium, suggest the core was formed 30 million years after the birth of the solar system.

But studies of lead isotopes, formed from uranium, suggest a later date of 80 million years.

Now, geologists Wood and Halliday have come up with a model they think explains this discrepancy.

Giant impact changes Earth's chemistry

The researchers argue that while 99% of the core was formed when the Earth was just 30 million years old, a last bit of core was added later.

This late addition happened during an event, believed to have created the Moon, involving the collision of a Mars-sized object with the Earth at around 45 million years.

"This impact had a huge effect on the chemistry of the Earth, we believe," says Wood.

He and Halliday argue that the collision would have caused the metal in the Earth to mix with sulfur.

Wood says this in turn would have changed the ratio of uranium and lead, resetting the uranium-lead clock, giving the date of 80 million years for core formation.

The hafnium-tungsten clock was already dead by then, he says, so could not record the late addition to the core.

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When Did the Earth\'s Core Separate From Its Shell?
2005 10 31

By Mitch Battros | ECTV

Is it me, or is science turning its attention to the Earth’s core? If yes, why? Perhaps the following information is yet another piece of the puzzle.

Some of the following is based on recent studies from Bristol University. New research allows geologists to estimate the time at which the Earth\'s core separated from its rocky outer shell. A paper in this week’s scientific journal “Nature” shows how the problem can be resolved by considering the effect of a giant impact with the Earth.

The collision of a Mars-sized object with the Earth is thought to have contributed to the last ten percent of the Earth\'s mass, as well as forming the Moon. Lead researcher Professor Bernard Wood, who completed this research while at Bristol University, and Professor Alex Halliday from Oxford University, propose that the impact would have also changed the conditions of core formation. They put forward a model that explains the discrepancy between the two isotope clocks if the effects of the oxidation state of the mantle are taken into account.

Previous research, using two different types of radioactive ‘clocks’ (hafnium-tungsten and uranium-lead), appeared to give conflicting core formation times of about 35 and 80 million years, respectively, after the origin of the solar system. Professor Wood states: “The explanation may be that the hafnium-tungsten clock represents the initial phase of core formation, sometime before 35 million years after the origin of the solar system, whereas the uranium-lead clock, that gives a younger age of about 80 million years after the origin of the solar system, has been reset by the upheaval introduced by the giant impact.”

Dr. Tom Van Flandern, former US Naval Observatory Chief of Celestial Mechanics who is a frequent guest on ECTV, was the first to propose such an “Exploded Planet” hypothesis. Abstract. The hypothesis of the explosion of a number of planets and moons of our solar system during its 4.6-billion-year history is in excellent accord with all known observational constraints, even without adjustable parameters. Many of its boldest predictions have been fulfilled. In most instances, these predictions were judged highly unlikely by the several standard models the ‘exploded planet hypothesis’ would replace. And in several cases, the entire model was at risk to be falsified if the prediction failed.

The successful predictions include: (1) satellites of asteroids; (2) satellites of comets; (3) salt water in meteorites; (4) “roll marks” leading to boulders on asteroids; (5) the time and peak rate of the 1999 Leonid meteor storm; (6) explosion signatures for asteroids; (7) strongly spiked energy parameter for new comets; (8) distribution of black material on slowly rotating airless bodies; (9) splitting velocities of comets; (10) Mars is a former moon of an exploded planet.

Exploded Planet Hypothesis:

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