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Wednesday, March 17, 1999 Published at 16:56 GMT


Sci/Tech

Earth smash spawned Moon

The Earth and Moon were once one

By BBC News Online Science Editor Dr David Whitehouse

The Moon was blasted away from the early Earth by a massive interplanetary collision, according to an idea which has received strong new support.

The new data comes from the Lunar Prospector spacecraft and was presented at a scientific meeting in Houston, Texas.

"This is a critical finding in helping scientists determine how the Earth and the Moon formed,'' said Dr Alan Binder, principal investigator of the Lunar Prospector mission.


[ image: Lunar Prospector]
Lunar Prospector
The new results show that the lunar core contains less than four percent of its total mass. This is a very small ratio compared to the Earth - its core has about 30 percent of the planet's mass.

The size of the core is critical because everything in the solar system was made from the same starting materials. As the early Earth formed, the heavy iron sank to the core, leaving a rocky layer above.

If the Moon had started as an independent planet, it too would have a large iron core. However, the Moon's small core suggests it is a chunk of rock knocked out of the early Earth, after most of the iron there had separated out.

The collision theory suggests that a planet about the size of Mars struck the ancient Earth. It would have almost disintegrated both planets and left huge clouds of debris orbiting what remained of the Earth.

"This impact occurred after the Earth's iron core had formed, so it ejected rocky, iron-poor material from the outer shell into orbit,'' Dr Binder said, "It was this material that collected to form the Moon.''


[ image: A small core reveals the Moon's origin]
A small core reveals the Moon's origin
When the Earth was formed 4.5 billion years ago, the solar system contained many more planets and large objects than it does today. This would have lead to many titanic collisions.

The new Lunar Prospector data comes from gravity measurements which indicate that the radius of the Moon's core is between 220 and 450 km (140 and 280 miles).

"Further analysis of Lunar Prospector data to refine the exact size of the lunar core is required. We also need to determine the amounts of elements like gold, platinum and iridium in lunar rocks, all of which are concentrated with metallic iron,'' said Dr Binder.



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