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Our young Earth may have collided with a body similar to the planet Mercury, says a new paper.

The dramatic event could explain why our planet has a hot core that gives it its magnetic field.

And it could also explain why certain rare-earth elements in Earth's mantle do not seem to originate from the planet's other building blocks - meteorites.

Oxford scientists say a Mercury-like body struck the young Earth (artist's illustration shown). The Mars-sized object would have been the heat source for our planet's core. The same object could have been responsible for creating the moon. It also explains where some rare-Earth elements came from

The study, by two scientists from the University of Oxford, was published in the journal Nature.

Our current theory of Earth's formation involves basically more and more asteroids and other objects accumulating into one body.

However, this model cannot explain where the heat source for Earth's core - and ultimately its magnetic field - come from.

In addition, Earth's crust and mantle appear to have a higher ratio of the rare-Earth elements samarium and neodymium than most meteorites - suggesting there was more at play in Earth's formation than thought.

To address this issue, the scientists think that Earth was hit by a body that was the size of Mars, but like Mercury in its composition, early in its life.

This likely would have happened about 4.5 billion years ago. The body would have been rich in sulphur but very poor in Oxygen - like Mercury.

HOW THE MOON FORMED: THE GIANT IMPACT HYPOTHESIS 

Many researchers believe the moon formed after Earth was hit by a planet the size of Mars billions of years ago.

This is called the giant impact hypothesis.

The hypothesis claims the moon is debris left over following an indirect collision between our planet and an astronomical body approximately 4.5 billion years ago.

The colliding body is sometimes called Theia, after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

But one mystery has persisted, revealed by rocks the Apollo astronauts brought back from the moon - why are the moon and Earth so similar in their composition?

Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon.

Perhaps the impact created a huge cloud of debris that mixed thoroughly with the Earth and then later condensed to form the moon.

Or Theia could have, coincidentally, been isotopically similar to young Earth.

A third possibility is that the moon formed from Earthen materials, rather than from Theia, although this would have been a very unusual type of impact. 

Our current theory of Earth's (left) formation involves basically more and more asteroids and other objects accumulating into one body. Now scientists think that Earth was hit by a body that was the size of Mars, but like Mercury (right) in its composition, early in its life.

In the young solar system 4.5 billion years ago, illustrated, various planetesimals and other small bodies coalesced together to form larger bodies like Earth

As it collided with Earth, it would have produced radioactive uranium and thorium, generating the heat needed to drive the 'dynamo' of molten iron in Earth's core.

And they do not rule out the possibility that this is the same Mars-sized body that formed the moon.

'We think that that is quite conceivable,' said study co-author Bernard Wood, reported the LA Times.

'It's kind of exciting to think that this reduced body could actually be the thing which caused the moon.'

In an accompanying News and Views article for Nature, Dr Richard Carlson of the Carnegie Institution for Science in Washington, DC said the theory could open up new avenues for thought..

For example, if true, it would raise further questions about 'how Earth ended up in its present oxidised state, which it has apparently retained for more than three billion years.'