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Last Updated:  Wednesday, 19 February, 2003, 16:14 GMT
'Double whammy' created the Moon
By Roland Pease
BBC Science

Cosmic collision (William K Hartmann)
Image copyright William K Hartmann
The Moon could have been created in a double-whammy impact 4.5 billion years ago.

The theory is outlined in BBC Radio 4's An Earth Made For Life programme.

Like the climax in a firework display, the event was the culmination of a 100-million-year process in which the Earth and its neighbouring planets were built through cosmic collisions between sub-planetary objects.

This was the biggest blow our planet ever experienced.

According to Dr Robin Canup, a proto-planet, something like the size of Mars, collided at high speed with an Earth that was nearly fully formed.

The collision was a glancing one. It shattered our Earth, but pulverised the incoming planet.

Rain of debris

Simulations show the impactor being sprayed out into a shower of orbiting debris. But within a matter of hours, much of this had re-grouped to form a new impactor that smashed into the Earth's surface a second time.

"At this point, the impacting object was destroyed," Dr Canup, of the SouthWest Research Institute, in Boulder, Colorado, told the BBC.

Most of the impactor rained down on to, and became incorporated into, the Earth - the last major component to be integrated into our planet.

But 10% or so of the mass was spread out into an incandescent disc around the Earth - a scorching equivalent of Saturn's rings.

It was out of this material that the Moon was formed in a matter of decades.

'Time zero'

"At the time it was 15 times closer than the Moon is now," says Dr Canup. "So if you had been able to stand on the surface of the Earth then you would have seen something that appeared 15 times the size of what even today is an impressive full Moon."

Mike Drake of the Lunar and Planetary Institute calls the impact "time zero" for the Earth. Anything that had happened geologically to the Earth before that would have been erased by the impact.

The planet's surface was probably melted down to a depth of 1,000 kilometres, cloaking the Earth in a "magma" ocean that would have radiated like a red-hot furnace.

Tiny grains called "calcium-aluminium rich inclusions" have been recovered from comets, which are believed to be the oldest surviving solid pieces of the cloud of dust and gas that once encircled the forming Sun.

These have been dated to 4.566 billion years ago (give or take just a couple of million years) and are thought to represent the kind of stuff that the planets formed from.

The precise date the Moon was formed is still a matter of debate, but Dr Canup's research implies it was right at the end of the planet-building process, which could have taken up to 100 million years.

Curiously, recent chemical research has shown that the planet Earth collided with was a twin to the Earth - scientists have called it "Theia" after the mother of the Moon in Greek mythology.

Magma ocean

Details in the chemistry of the Moon show it to be almost identical in some key respects to the Earth, although it was made almost entirely from remnants of the impactor.

Cosmochemist Alex Halliday says that Theia must have been formed in an orbit almost identical to the Earth's. Time zero for Earth, it seems, was the end of time for its twin.

But it was not all destruction. As Robin Canup points out, the Moon-forming impact gave the Earth its spin on its axis that now gives us 24-hour days, and stirs up the atmosphere so that no part of the Earth is too hot or too cold for life.

And the presence of the Moon gives the Earth a kind of gravitational counterbalance that stabilises its slightly inclined axis of rotation - 23 degrees to its orbit - that gives us the congenial cycle of the seasons over a single orbit around the Sun.

And the scalding magma ocean, according to Mike Drake, was (surprisingly) the place where the water of the Earth's oceans would have been held - giving our planet one of its key ingredients for life.



How the moon was made
15 Aug 01 |  Science/Nature
Space agencies take new look at Moon
27 Jul 02 |  Science/Nature
Historic lunar impact questioned
25 Feb 03 |  Science/Nature
China sets date for the Moon
20 May 02 |  Science/Nature

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