A telescopic inspection of the Moon reveals that both types of ground are replete with formations commonly called craters. Their number is immense, and they range in size from the South Pole -Aitkin basin, more than 2,500 km (1,550 mi) across, down to tiny pits etched on crystalline rocks brought to the Earth by the Apollo missions. They arise from impacts of celestial bodies ranging from asteroids and comets to interplanetary dust.
Because the surface of the Moon is not protected by any atmosphere, all bodies that happen to be in a collision course with it will impact with cosmic velocities of several km/sec. A particle moving at a relatively slow speed of 3 km/sec (1.9 mi/sec) possesses a kinetic energy equivalent to that released by an explosion of an equal weight of TNT. When such kinetic energy is dissipated on impact, the outcome is a surface scar commonly called a crater. Craters of small or moderate size were excavated in such a way in the rocky layers on the point of impact by removal of material. For those of large size - approximately 100 km (60 mi) or larger - the amount of heat liberated by impact was sufficient to flood the floor with molten material. Moreover, in the case of the largest impact formations encountered on the Moon, the excavation of the initial basin appears to have been followed by its lava flooding only after a few hundred million years.
Bright rays up to 16 km (10 mi) wide extend outward for hundreds of kilometers from some of the very largest lunar craters. These rays consist of lighter-colored materials ejected by the force of the impact forming the craters. Over geologic time such materials will most likely darken and fade into the rest of the Moon's surface. Other notable features of the Moon's surface include ridges and rilles, both of which may also extend for several hundred kilometers. Rilles are narrow lunar valleys, sometimes deep enough to be considered canyons. Some rilles are fairly straight; others wander sinuously like riverbeds. Rilles may possibly have been formed in the Moon's earlier years by subsurface flows of molten rock.