As long as people have looked up at the sky and seen the Moon, they have wondered about its structure and origin. Early observations suggested that the Moon was a flat disk that emitted its own light in the night sky and changed shape at regular intervals. Later observations through ground-based telescopes revealed the Moon's spherical shape, as well as its relative size and heavily cratered surface. From these observations, scientists developed hypotheses to explain the Moon's origin. However, it wasn't until Apollo astronauts journeyed to the Moon and returned with lunar samples that scientists were able to formulate the hypothesis of lunar origin that is widely accepted today.
Before the first successful Apollo mission in 1969, scientists entertained three hypotheses for the Moon's origin. One said that the Moon was captured by Earth's gravity as it passed by on its way from some unknown part of the early solar system or galaxy. Another said that the Moon co-formed alongside Earth from the same interstellar cloud of dust and gas responsible for the solar system as a whole. A third said that while Earth was forming, the force of its rotation threw off a huge clump of material that ultimately became the Moon.
Between 1969 and 1972, six manned and three automated missions recovered 382 kilograms (842 pounds) of lunar rocks and sediment. Scientists expected that analysis of these samples would provide hard physical evidence to support one of the existing hypotheses. Instead, the rocks revealed flaws in all three explanations of the Moon's origin and suggested a fourth. The lunar materials showed that the Moon was similar in composition to Earth, but lacked the high iron content characteristic of Earth's rocks. The age of the samples suggested that the Moon formed about 4.48 billion years ago, just after Earth's formation approximately 4.56 billion years ago.
One of the most surprising and important discoveries in the lunar samples was the high concentration of an igneous rock called anorthosite. This type of rock forms from magma at particular pressures and temperatures. The fact that the lunar surface is literally covered with anorthosite suggests that the Moon was once covered by oceans of molten magma. If so, the Moon must have formed at very high temperatures. Together with other evidence, this has led astrophysicists to the most widely accepted hypothesis today: Early in its history, Earth was struck by a Mars-sized asteroid and ejected molten material that ultimately became the Moon.
To learn more about physical characteristics of the Moon, check out Explore the Moon, Extreme Temperatures on the Moon, Galileo on the Moon, and Your Weight on Other Worlds.