The Moon is the Earth's only natural satellite. Though its mass is only 1/81 that of the Earth, the Moon is an unusually large satellite in terms of relative size. As a planet,the Moon is a primitive body on which the very earliest stages of crustal evolution (more than 4 billion years ago) are preserved. Its surface environment is clue to that of space itself.There is no atmosphere at all, and no global magnetic field, although there are local magnetic anomalies.The surface is like damp beach sand consisting of rock fragments produced by billions of years of meteoritic bombardment.The Moon is a fascinating body to study, and a potentially valuable site for astronomical observatories on its surface.

No matter how the Moon formed, its creation must have had dramatic effects on Earth. Although most scientist have concluded that the Moon formed as a result of an enormous impact on Earth, we do not know much about the details of that event. We do not know if the Moon was made mostly from Earth materials or mostly projectile; the kinds of chemical reactions that would have taken place in the melt vapor cloud; and precisely how the Moon was assembled from this cloud.

VIDEO OF PAUL LOWMAN

Paul D. Lowman, Jr. is a geologist with the Geodynamics Branch of the GSFC Laboratory for Terrestrial Physics, and has been with the Center since 1959. In a 40 year career, he has worked on the Mercury, Apollo, and Skylab programs. His research includes comparative planetology, global tectonics, and remote sensing. His current project is a book for Cambridge University Press, "Exploring Earth: the Impact of Space Flight on Geology and Geophysics."

View of the Moon's Copernicus crater.

View of Earth from the Moon during the Apollo 11 Mission.