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A Partner of the Earth Fundamentally linked to the origin of the Earth. Earth-Moon pair an extraordinary double planet. Possible formation by giant meteorite impact on primordial Earth. Effects on the Earth and its history?

How did the Moon Form? How is its formation related to formation of Earth? Is the currently favored Earth-Moon formation mechanism (a "great impact" on Earth) unique, rare, or common in the solar system?

A Museum of Planetary History Preserved records of planetary formation, early history. Period of intense meteorite bombardment from formation of solar system (4.5 billion years) to about 4.0 billion years ago. Early widespread melting, formation of outer "magma ocean," chemical separations; traces still preserved in rocks. Widespread basaltic volcanic eruptions: a common planetary process. Information no longer accessible on active planets (Earth, Venus, Mars). Only other terrestrial planet with absolutely calibrated geological history (from ages measured on returned samples). Standard for measuring surface ages by crater counts on all other planets and moons (Mercury to Neptune). Preserved records of large-body impact flux in Earth-Moon system over geologic time. Easily accessible field laboratory for fundamental planetary processes. Meteorite impact phenomena: mechanics of large and small crater formation; impact-produced rocks. Development of surface regolith on airless planet; relevant to studies of Mercury, asteroids, and other bodies. Crustal formation, melting, and volcanism.

What is the Moon's Internal Structure? How did its Crust and Mantle Form? Primordial melting ("magma ocean")—total or partial? Heat sources? Role of early large meteorite impacts? What is the Moon's Magnetic History? Does a lunar metal core exist? How big is it? Did the Moon ever have a planetary magnetic field? If so, what shut it off? What produced the "fossil magnetism" found in lunar rocks? Did the primordial Moon interact with an early intense solar magnetic field? What has been the Long-Term History of the Sun and Cosmic Rays? How much information about solar and cosmic processes can be extracted from lunar materials? How did the observed variations in solar-wind nitrogen over time originate? Can the Moon Provide Better Estimates of the Recent Flux of Large Impacts in the Earth-Moon System? Can we improve estimates of the impact rate over geological time? Can we obtain better data about the rate of present and future impacts and the hazards they pose to Earth?

A Space Probe and Time Machine Direct atomic samples from Sun (solar wind, solar flares). Rocks and soil contain historical records of solar behavior over millions to billions of years. Evidence for historical variations in solar wind. Historical data on cosmic-ray activities, flux levels. Historical records of impact flux rates, especially for small and dust-sized particles.	How will Human Beings Live and Work Permanently on the Moon? What would they do? Where would be the best places to do it? How can people and robots work together effectively?
A Place for People Most accessible planet for future human activity. Candidate for permanent human presence. Scientific base potential: lunar/planetary science, astronomy, and other sciences. Resource potential (oxygen, metals, volatiles) to support future human space activity beyond Moon.

WHAT DID WE LEARN ABOUT THE MOON FROM THE APOLLO PROGRAM?

WHAT DID WE LEARN ABOUT THE MOON FROM CLEMENTINE AND LUNAR PROSPECTOR?

Not primordial—it is an evolved terrestrial planet. Ancient—it still preserves an early history (the first billion years), which must be common to all terrestrial planets. Shows evidence for wholesale primordial melting, formation of outer "magma ocean"—large-scale chemical separations within Moon; traces still remain in lunar rocks. Dark lunar maria ("seas") and light-colored highlands made of chemically and mineralogically different rocks. Maria made of dark volcanic lavas (basalts) poured out in huge volcanic eruptions 3-4 billion years ago. Lifeless—no life, no fossils, no organic chemicals. Chemically similar to Earth, but significantly different in details. It has no indigenous water, is poor in volatile elements. Preserves effects of catastrophic early meteorite bombardment—common to all terrestrial planets, including Earth, but traces no longer preserved on active planets like Earth. Not uniform throughout; it is divided (like Earth) into an outer crust, inner mantle, and (possibly?) small metal core. Globally asymmetric (slightly egg-shaped); thicker crust on farside, most maria deposits (lava flows) on nearside. Has no magnetic field, little or no metallic core like Earth's, but "fossil" magnetism is preserved in lunar rocks. Large-scale (100-1000 kilometers) magnetic anomalies preserved on lunar surface. Lunar surface covered by powdery fragmental layer ("lunar soil" or regolith), produced by shattering of bedrock by prolonged meteorite bombardment. History of Sun and cosmic rays determined from actual atoms of Sun and stars trapped in lunar rocks and soil. Unexplained nitrogen anomalies detected in ancient solar wind trapped in lunar regolith. Pre-Apollo hypotheses about lunar origins shown to be inadequate. Scientists now believe that the Moon formed as a result of a collision between the early Earth and a former, smaller planet about 4.6 billion years ago. The giant impact sprayed vaporized material into a disk that orbited the Earth. This vapor later cooled into droplets that coagulated into the Moon.

Work to understand the data returned from two recent unmanned missions (Clementine 1994; Lunar Prospector 1998) is ongoing. However, we do have new knowledge about the Moon and some first-order conclusions from these two missions. The Global Surface Composition of the Moon Crust is highly enriched in aluminum on a global basis, supporting its origin by early global melting (the magma ocean). Incompatible trace elements (i.e., those that do not go easily into rock-forming minerals) are concentrated within an elliptical zone on the western nearside (the Imbrium-Procellarum region). Mafic (i.e., magnesium and iron-rich) zones are found within the lunar highlands, usually associated with large impact basins. Mare basalts rich in titanium (returned in abundance by the Apollo 11 and 17 missions) are rare in the global mare lava inventory. The Topography of the Moon The Moon displays an enormous range of global relief (16 kilometers), as big a range as the more active and diverse Earth. The dominant cause of high relief on the Moon is the presence of large, multiring impact basins. Lunar multiring basins appear to preserve their original topography for most of geological time. The South Pole-Aitken Basin on the farside of the Moon is the largest (2600 kilometers diameter) and deepest (over 12 kilometers) impact basin known in the solar system. The Internal Structure of the Moon The Moon shows many areas of excess subsurface mass (mascons) that cause the gravity field of the Moon to be very "lumpy," requiring constant adjustments for orbiting spacecraft. The mascons are always found beneath the floors of large impact basins and probably represent plugs of dense, uplifted rocks from the lunar mantle. The Poles of the Moon Areas are found near the lunar poles that are in permanent darkness; some areas may be in permanent sunlight. The south pole appears to have more dark area than the north pole, mostly as a result of its location just inside the rim crest of South Pole-Aitken Basin. Water ice, derived from impacting comets, is found in the dark areas near both poles.