spacetoday.net: Lunar impact history more complex than first thought

Posted: Fri, Sep 17, 2010, 7:40 AM ET (1140 GMT)
Lunar cratering map (NASA)

The Moon was struck by two distinct populations of asteroids and comets in its history, according to the analysis of data from NASA's latest lunar mission. In a paper published in the journal Science on Thursday, researchers said that the cratering record of the Moon, as analyzed by a laser altimeter on NASA's Lunar Reconnaissance Orbiter, showed that older highland regions had proportionally more large craters than small ones when compared to craters in younger maria. That suggests that impactors that struck the Moon early in its history had a larger fraction of large objects than those that his after the maria formed, with the transition occurring around the time the large Orientale basin formed 3.8 billion years ago. The study was published the same week that NASA transitioned operations of LRO from its Exploration Systems Mission Directorate to the Science Mission Directorate to continue its mission.