Science: Stranger in the night

It's as familiar as the sky itself - yet 30 years after man first walked on the moon, how much do we really know about it? By Charles Arthur

We know that it is 4.6 billion years old, with a skin composed of rocks containing oxygen, silicon and aluminium. We know that it is deficient in iron, and that the surface is scarred with craters and was once flooded by molten lava. And we know that its birth nearly destroyed the young Earth.

The Moon had its origin some 50 million years after the Sun formed, when huge chunks of rock that had not been swept up into planets were still chaotically flying around the infant Solar System. Billions of years ago, a world about the size of Mars headed straight for the primitive, lifeless Earth. There was no one to see it coming. If there had been it would have signified to them, with absolute certainty, the imminent end of their world. The planet that was to be the mother of our Moon took 10 hours to travel the final quarter of a million miles to its target.

Then it all seemed to take place in slow motion. For an instant the two worlds were poised for the catastrophe. Earth's atmosphere of volcanic gases was squeezed out between the two planets at supersonic speed. At five miles a second they ground into each other. Rock became not just molten but flash-vaporised; the ever-growing circle of the impact glowed blue-hot and sprayed fragmented, superheated molten rock in all directions.

In less than an hour the scar of the impact had spread over half of a planetary globe that was already glowing visibly redder. The primordial atmosphere boiled off into space; the mantle melted into an ocean of magma as Earth devoured the intruder. The planet now had no solid surface; the impactor's iron core fell on to the now highly deformed proto-Earth and sank to its centre.

Molten and vaporised rock plumed into space. Within hours the iron-deficient material became a ring around the Earth and within days the Moon was forming. The birth itself had taken just 15 minutes.

Thirty years ago this month, Neil Armstrong, an Apollo astronaut, tasted the fruits of this act of creation when he became the first person to walk on the Moon. The first Apollo samples that were returned to the Earth showed a lack of water and other volatile compounds. This is a by-product of the ancient impact. It heated the ejecta to high temperatures, and all volatile substances would have escaped to space as gases. Measurements taken by the seismic instruments that the astronauts left behind suggested that iron is lacking in the Moon's core.

At birth, the Moon was much closer to the Earth than it is now - perhaps as little as 14,000 miles away, just 5 per cent of its present distance. In some computer simulations of what may have happened at that time, two moons could have formed, but they would not have existed for long. Probably, in less than a thousand years, either they would have collided or one of them would have crashed back to Earth.

The Moon that survived would ensure that night-time on Earth would never be the same again. But how do we know when its birth occurred? In lunar samples brought back by the Apollo astronauts, the amounts of two elements, hafnium and tungsten, provide the crucial answers. Hafnium decays into tungsten with a half-life of 9 million years. By measuring the ratios of these elements and comparing them with those found in primitive meteorites, scientists can estimate that the Moon was born just 50 million years after the Solar System was formed.

Thanks to Apollo, we know more about the Moon than we do about any other object in space, with the exception of our own planet. We have the testimonies and photographs of those who have been there. We have some 2,000 samples from nine sites, 382kg from six Apollo landings plus 0.3kg from three unmanned Soviet missions that returned samples by automated probes.

Four billion years ago, during the so-called pre-Nectarian period, the Moon's crust solidified. Since then huge basins, and blankets of ejected material, have covered most of the surface. Old, shattered rocks brought back by Apollos 15, 16 and 17 are relics of this period. The Nectaris impact basin was formed by a giant asteroidal impact, as were the Imbrium basin and, the big one, the Orientale basin, following an event that was a watershed in the history of the Moon.

Following this period, basalt sheets from the interior of the satellite covered vast areas of the surface and stayed there. They established the pattern of light and dark regions that we can see today on the lunar surface. The Man in the Moon is 3 billion years old.

In the last billion years or so the Moon's interior has cooled, so that lava flows hardly ever occur and any modifications to the surface are by rare impacts with meteorites. Many of the most spectacular craters we can see on the surface were formed relatively recently in the Moon's history. Apollo 17's rocks tell us that the magnificent crater Copernicus was formed by an impact 810 million years ago; the crater Tycho is only 110 million years old. Almost all that happens now are a few weak Moonquakes that occur at the extreme points in its orbit around the Earth.

With the recent discovery of ice in the lunar dirt at the poles, there is in the lunar rocks everything required for supporting life and many profitable industries as well. To build the first Moon base we need take nothing with us. It's all there: aluminium, iron, hydrogen, helium and oxygen.

Test rigs have successfully extracted oxygen from simulated lunar soil, using a simple and reliable furnace. Automated oxygen factories could even be sent to the Moon in advance of the next explorers. The value of oxygen made on the Moon in life support systems is obvious. But oxygen can also be used as a rocket fuel, and its manufacture would be the life blood of the first viable lunar colony.

Aluminium can be obtained from the feldspar mineral plagioclase, which is abundant in the lunar highlands. Aluminium has potential uses for building and construction, and perhaps even as rocket fuel. However, the production of this metal is a complex process, not something that could be undertaken during the first phase of a Moon base.

The Moon presents us with a new Africa to explore. It is the eighth continent, with a land area about the same size as that of Africa. It's just as rich in minerals, and it could make fortunes for as many people as Africa did.

How long did it take the first settlers to reach Africa? The Moon is only three days away. In 100 years' time a "LunOx Corporation", based at the lunar south pole could be the biggest company on or off the Earth. Whoever controls the Moon's oxygen will control the Solar System.