MOON A CHIP OFF THE EARTH BLOCK

Giving birth to the Moon: it's a spin-off of a giant collision with an already-formed Earth. Pic: NASA View animation

The Moon probably formed after a Mars-sized object collided with a fully-formed Earth, new computer simulations suggest.

The calculations, presented by Robin Canup of the Southwest Research Institute in Texas and Erik Asphaug of the University of California at Santa Cruz, provide the most sophisticated scenario yet of the Moon's birth.

They appear in today's issue of the journal Nature.

The idea that the Moon formed as a by-product of a collision between Earth and another object, first proposed in the mid-1970s, is well accepted.

But the best previous models have assumed that the impact occurred when the Earth was only partly formed.

According to Ross Taylor, formerly of the Australian National University, the new Canup-Asphaug model removes the problems of the previous model by showing that the Earth was "pretty well formed" by the time of impact.

Professor Taylor, who is credited with the first geochemical analysis of a Moon rock, explains that if the collision occurred earlier as previously thought, both the Earth and the Moon would have accumulated the second half of their masses after the impact. But this would have resulted in the Moon gaining more iron-rich material than is found in the Moon today.

The new model was achieved by improving the accuracy of a technique used in computer simulations of large planetary impacts, called 'smooth particle hydrodynamics'. This involves splitting the Earth and the object it collided with into many small computational lumps, or 'particles'.

Canup and Asphaug used 20,000 particles, compared to the 300 used in the best previous simulation.

They show that an oblique impact by an object with 10 per cent of the mass of the Earth can eject sufficient iron-free material into Earth-orbit to yield the Moon, while also leaving the Earth with its final mass and correct initial rotation rate.

The question that remains, says Taylor, is whether the Moon's formation was crucial to the development of life on Earth.

The Moon "stabilises the tilt of the Earth", he says, therefore playing a vital role in producing conditions on Earth that can sustain life.

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