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Thursday, 30 November, 2000, 19:38 GMT
Lunar meteorites reveal life's troubles
Rock Nasa
A rock from the lunar highlands picked up in the Libyan desert
By BBC News Online science editor Dr David Whitehouse

A new study of Moon meteorites provides fresh evidence that the Earth and its satellite underwent an intense period of cosmic bombardment just under four billion years ago.

An analysis of four of the 20 or so known lunar meteorites suggests that the Moon's surface was melted by a torrent of impacts.

It is estimated that this lunar catastrophe would have lasted only about 200,000 years but, in that time, nearly 2,000 large craters would have been formed as well as many of the Moon's giant impact basins.

Scientists say that the Earth would have been bombarded to a far greater extent and that the frequent impacts could have delayed the emergence of life on the primitive world.

Blasted off Moon

The Apollo astronauts brought back 382 kg of lunar rock and three unmanned Soviet probes returned another 300 g. They have answered and raised many questions about the birth and history of our Moon.

Monn Nasa
The Moon's entire surface could have been molten
But scientists know that these samples are drawn from particular places on the lunar surface that do not necessarily represent the Moon as a whole.

Other types of rock are needed to fill in some of the gaps in our knowledge: the lunar meteorites. These would have been blasted off the face of the satellite by large impacts and travelled through space for about a million years before falling to Earth.

About 20 lunar meteorites are known. Some have been collected from deserts, some from the icy wastes of Antarctica.

Unlike the rocks carried back to Earth by astronauts and probes, the lunar meteorites come from all over the Moon. Some of them may even come from the far side of the satellite.

Earth bombarded as well

Barbara Anne Cohen and colleagues, from the University of Arizona, report in the journal Science that their analysis of four lunar meteorites suggests that the Moon underwent a particular phase of pummelling early in its history.

The ratios of isotopes - different types of the same atoms - in the rocks suggest that the samples were almost completely melted about 3.9 billion years ago. This is about 750 million years after the Solar System was formed - the Moon itself would have been about 700 million years old at the time.

Torrential impacts from space debris would have been the norm when the Solar System was young but by four billion years ago it had started to decline. So the new data suggests that there was a brief period of resurgence.

The researchers say that because the Earth is a bigger target than the Moon our planet would have sustained at least 10 times the number of punishing impacts.

These would have melted the surface, vaporised any oceans that were trying to form and filled the atmosphere with superhot clouds of rock vapour. Any life that may have got started would have been destroyed.

Very little evidence of this past cataclysm remains on the Earth - 70% of the surface is covered by lavas erupted in the last 200 million years. It is only by looking at rocks from the relatively unchanging Moon that scientists have thrown new light on the common past of these space companions.

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See also:

05 Oct 00 | Sci/Tech
Rocks reveal ancient tides
10 Mar 00 | Sci/Tech
Lunar rock reveals life's clues
13 Jun 00 | Sci/Tech
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