Manned exploration of the moon came to an end 40 years ago, when the Apollo 17 space capsule splashed down in the Pacific Ocean on Dec. 19, 1972. The six manned Apollo missions that explored the moon from 1969 to 1972 answered many of the scientific questions we had about the origin and composition of our closest neighbor in space with the 840 pounds of rocks and regolith they returned to Earth.
After studying lunar samples, scientists believe that the moon did not form when the Earth and the other planets in our solar system formed 4.5 billion years ago. However, since moon rocks and Earth rocks clearly show common ancestry with distinctively similar oxygen isotopic compositions, scientists now believe that shortly after the Earth formed, a Mars-sized body collided with the Earth and the resulting debris from both Earth and the impactor accumulated to form the moon.
Before NASA's Apollo program, the composition of the moon was widely debated among planetary scientists. We now know that the moon is made of rocky material that has been melted, erupted through volcanoes, and crushed by meteorite impacts.
Moon rocks originated through high-temperature processes with little or no involvement with water. They are roughly classified into three types: basalts, anorthosites and breccias. Basalts are dark lava rocks that fill mare basins. Anorthosites are light rocks that form the ancient highlands. Breccias are composite rocks that formed from all other rock types through crushing, mixing and heating during meteorite impacts. The moon also has no sandstone, shale or limestone, which form in the water-borne processes on Earth.
The surface of the moon is covered by a rubble pile of rock fragments and dust, called the lunar regolith. The regolith was produced by pounding of countless meteorite impacts.
Research laboratories continue to study Apollo lunar samples, but the deepest secrets of the moon remain to be revealed.