They found that when a slow enough impact happened, at speeds of less than 27,000 miles per hour (43,000 kph), the rock that struck doesn't necessarily vaporize. Instead, it gets shattered into a rain of debris that is then swept back down the crater sides and piles up in the crater's central peak.
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In the case of craters like Copernicus (pictured top), the foreign material stands out because it contains minerals called spinels. These only form under great pressure -- in the Earth's mantle, for instance, and perhaps in the mantle of the moon. But spinels are also common in some asteroids, said Melosh, which are fragments of broken or failed planets from earlier days in the formation of our solar system.
The team has concluded, therefore, that the unusual minerals observed in the central peaks of many lunar impact craters are not lunar natives, but imports.
That conclusion could also explain why the same minerals, if they were instead from the interior of the moon, are not found in the largest impact basins -- as would be expected if the impact event was larger and penetrated deeper into the moon.