GRAIL provides some insights into lunar impact history

Nature : More than a half century ago, the Soviet Union’s Luna 3 probe provided the first images of the Moon’s dark side, which appeared to be significantly more heavily cratered than the side visible from Earth. Until recently, no explanation for the difference had gained wide acceptance. Now, a team led by Katarina Miljković of the Paris Institute of Earth Physics has used NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission to accurately map the thickness of the Moon’s crust. The mapping revealed that the visible side of the Moon has eight basins—craters filled with lava—with diameters greater than 320 km, while the hidden side has only one. That asymmetry could have occurred if the visible side’s crust was much warmer and therefore much softer than the crust on the hidden side: Both sides would have received roughly equal numbers of asteroid strikes, but impacts on the warmer side would have resulted in larger craters because of the softness of the crust. Other evidence suggests that the two sides of the Moon did experience a difference in surface temperatures, though the cause is still uncertain.