quick look at the full Moon reveals two main types of lunar terrain: expansive, light-hued regions and large patches of dark material. Closer study shows the bright material to be ancient highlands dominated by impact craters, while the dark regions consist of younger basaltic lava flows (maria) at lower elevations. Maria have far fewer impact craters than do the highlands, implying that by the time maria formed (mostly after 3.8 billion years ago), the rate of crater formation had greatly subsided. By coincidence, the Moon's
