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Moon's surface at saturation point
A new study has found parts of the lunar surface are now so badly pockmarked with impact craters, each new impact destroys existing craters.

The finding, based on research carried out by NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft is providing a better understanding of the early history of both the Moon and the Earth.

A team of researchers led by Professor James Head from Brown University in Rhode Island, used the LRO to construct a detailed record of large lunar craters, 20 kilometres in diameter and larger, based on topography measurements, rather than images.

Reporting in the journal Science, Head's team used data from LRO's laser altimeter to create a detailed catalogue of 5185 craters on the Moon's surface.
Saturation equilibrium

They found some regions have been so heavily bombarded by impacting asteroids, that they've reached states of saturation equilibrium.

This means each additional crater destroys a previous one, so that the number of craters actually remains the same over time.

Once this saturation equilibrium is reached, that region of the lunar surface can no longer be dated with crater counting techniques.

But, researchers did identify two areas of the Moon which appear to have undergone the least modification since their formation.

One region is on the southern near side, and the other is on the north-central far side.

Head and his colleagues suggest these areas are good targets for future missions, since they are likely to return the most ancient lunar samples.
Two waves

The researchers also used their new catalogue of craters to investigate the nature of projectiles that have bombarded the Moon's surface for billions of years.

Planetary scientist, Dr Andrew Prentice from Melbourne's Monash University says the paper dismisses suggestions of a uniform spread of cratering during the Moon's history, a view held by some astronomers.

"The study points to two very distinct periods of bombardment," says Prentice. "The larger craters in the lunar highlands, reflect what might be an earlier, initial population from about when the Moon formed. While the Mare regions are much younger."

He says this points to an initial heavy bombardment period soon after the Moon and Earth formed, followed by a late heavy bombardment between 3.8 and 4 billion years ago.

Having completed its first year of observations, focussed on mapping the lunar surface, the LRO spacecraft will now spend the next two years collecting scientific data.

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