T he Moon is the only natural satellite of the Earth and a unique member of the solar system in several respects. With a radius of 1,738 km (1,080 miles), it is approximately one-quarter of the size of the Earth and 81.3 times less massive. Although the solar system contains both larger and more massive satellites than the Moon, none except Pluto's moon differs so little from its planet in mass or size.

N aked-eye as well as more detailed telescopic and satellite observations disclose that the lunar surface consists mainly of two different types of terrain. The first is rough, relatively bright, and replete with mountains and occupies more than two- thirds of the visible hemisphere of the Moon and nine-tenths of its far side. The other type is much darker as well as smoother. Terrains of the first type are usually referred to as "continents." Those of the second type are called maria, Latin for "seas." The term highlands, sometimes used for continents, is a misnomer in the literal sense, for not all continental ground is elevated. Maria is an even worse misnomer, because the lunar surface is dry.

A telescopic inspection of the Moon reveals that both types of ground are replete with formations commonly called craters (see meteorite crater). Their number is immense, and they range in size from the far side's South Pole-Aitkin crater, which is more than 2,500 km (1,550 miles) across, down to tiny pits etched on crystalline rocks brought to the Earth by the Apollo missions. The origin of all such formations is no longer in doubt. They arise directly or indirectly 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 miles/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 miles) 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. Radiometric age-dating of rocks brought back by the 1969-72 Apollo missions from different parts of the Moon disclosed evidence of its geologic history. The oldest particles of lunar material found in every locality are 4.5 to 4.6 billion years old.

I n the past 3 billion years, more than two-thirds of the age of the satellite, nothing much has happened on the Moon. It has continued to expose its stony face to cosmic weather and keeps accumulating scars of new impacts, but at a diminishing rate. In the aeons to come, the surface of the Moon is likely to become more and more withdrawn in its petrified grimace, reflecting the state of the solar system in the days long gone by, a fossil reminder of the past that no internal convulsion can awaken from its slumber.