The moon was blitzed by two different sets of asteroids or comets in its youth, according to new results from NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft.
NASA scientists have created the first-ever comprehensive catalog of large craters on the moon using results from a detailed topographic map of the moon created using LRO's Lunar Orbiter Laser Altimeter (LOLA).
"Our new LRO LOLA dataset shows that the older highland impactor population can be clearly distinguished from the younger population in the lunar 'maria' - giant impact basins filled with solidified lava flows," says James Head of Brown University.
"The highlands have a greater density of large craters compared to smaller ones, implying that the earlier population of impactors had a proportionally greater number of large fragments than the population that characterized later lunar history."
Using the crater counts from the different impact basins and examining the populations making up the superposed craters, the team deduced that the transition occurred about the time of the Orientale impact basin, about 3.8 billion years ago.
The results have implications for Earth, which would have been subjected to the same impacts - which seem to have disrupted the initial origin of life and later to have altered life's evolution.
But it's difficult to reconstruct the meteorite bombardment history of Earth because impact craters are eroded by wind and water, or destroyed by the action of plate tectonics. The moon gives a much clearer picture.
"The moon is thus analogous to a Rosetta stone for understanding the bombardment history of the Earth," said Head. "Like the Rosetta stone, the lunar record can be used to translate the 'hieroglyphics' of the poorly preserved impact record on Earth."