Scientists believe that about 4 billion years ago, during a period called the Late Heavy Bombardment, the moon took a severe beating, as an army of asteroids pelted its surface, carving out craters and opening deep fissures in its crust. Such sustained impacts increased the moon’s porosity, opening up a network of large seams beneath the lunar surface.Now scientists at MIT and elsewhere have identified regions on the far side of the moon, called the lunar highlands, that may have been so heavily bombarded — particularly by small asteroids — that the impacts completely shattered the upper crust, leaving these regions essentially as fractured and porous as they could be. The scientists found that further impacts to these highly porous regions may have then had the opposite effect, sealing up cracks and decreasing porosity.