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Moon Formed In 'Massive Collision With Proto Earth'

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The Moon was created by a giant collision between two planet-sized objects, two new studies have concluded.

The titanic event occurred around 4.5 billion years ago, scientists said.

It has long been thought that the Moon was formed from the debris of a collision with a planet-sized object, but those theories did not explain why the Earth and the Moon are made out of the same type of rock.

Previous models suggested that the Earth was not able to throw off enough material to form the Moon.

The new studies differ in some details, but both conclude that the satellite was made when massive amounts of rock and minerals were blasted into space after the impact.

In one of the studies, published in Science, Matija Cuk of the SETI (Search for Extraterrestrial Intelligence) Institute said that the Earth previously rotated around its axis much more quickly than it does today. If Earth's days were just two to three hours long, it would have been possible to throw off at least 1.2% of its mass - enough to form the Moon.

The SETI study says that the planet was previously on the threshold of throwing itself apart - but was slowed over time because of a gravitational interaction with the Sun and the resultant Moon.

Under this model the Earth collided with a body about the size of Mars but 5 to 10% of Earth's mass.

Another study, also published in Science by the Southwest Research Institute, posits that the impact was actually between two objects of a similar mass - each about 50% as massive as the Earth as it is today.

In this model, the collision created a disk of mixed material which was almost identical to that which formed the Earth.

Details about the Moon's formation are still to be proven, both studies conclude - but point to us being closer than ever to understanding how it - and the ground beneath our feet - came to exist in their current form.