Earth-Moon

The Moon (NASA).

As an integral part of the Earth-Moon system, the Moon is a witness to more than 4.5b.y. of solar system history, and it is the only planetary body except Earth for which we have samples from known locations. The Moon’s simple composition and its restricted geological activity provide insight into elementary planetary processes. The Moon is thought to be the product of an early planetary collision of a Mars-sized body with Earth.
Earth and Moon build a celestial System with its common centre of mass to be located at about 1,700km beneath the Earth’s surface. With a diameter of 3,474km the moon is the fifth largest satellite in the Solar System. Its volume is about 2% and its mass only 1.2% of that of Earth. Its equatorial surface gravity is 1.622m/s2, about 1/6 (17%) of that on Earth.
The lunar rotation slowed down early in its history due to frictional effects associated with tidal deformations and became locked into a synchronous orbit. A complete orbit around the Earth needs 27.3days (the orbital period), whereas the periodic variations in the geometry of the Earth–Moon–Sun constellation are responsible for the phases of the Moon, which repeat every 29.5days (the synodic period). Its density is 3.34g/cm3 and its rocks are of relative simple mineralogical composition.
Our knowledge about the moon is based on telescopic observations from Earth, observations by spacecrafts from the lunar orbit, measurements on the lunar surface by manned and unmanned landing missions and the analyses of lunar sample material brought to Earth by the Apollo (382kg) the Luna (326g) missions and about 120 lunar meteorites (48kg) that have been collected on Earth to date. Remote sensing with passive sensors in the optical wavelength range as well as active laser and radar detection supported by in-situ field work yield information about the geological processes that formed the Moon throughout its history with respect to an impact-related, volcanic, tectonic and space weathering evolution.
The lunar crust is composed of distinct rock types developed from geochemical differentiation as well as mechanical destruction and mixing. The uppermost lunar curst consist of an unconsolidated debris layer called regolith of fine-grained pristine crystalline rock and mineral fragments and agglutinates - aggregates of small particles welded together by glass produced in micrometeorite impacts. On a macroscopic scale the lunar surface is dominated by three major forming processes: impact, volcanism and tectonics. The Moon is stratigraphically divided into five time units: Pre-Nectarian (>4.1b.y.), Nectarian (4.1-3.85b.y.), Imbrian (3.85–3.2b.y.) Eratosthenian (3.2–0.8b.y.) and Copernican (<0.8b.y.).
Lunar volcanism was active for almost 3 b.y., starting at about 4.0-3.9b.y. and ceasing at ~1.2b.y. Geomorphologic features of volcanic origin include lava flows, volcanic domes, cones and lava tubes (sinuous rills). Tectonism is related to both impact and volcanism and expresses extensional and compressional features such as faults, graben, dikes and wrinkle ridges.
The lunar interior is split in three major zones: the crust, the mantle and the core. Although the existence of a lunar core is still debated, geophysical  data are consistent with a core being composed of either a molten Fe-FeS-C metallic alloy of less than 375km radius or a slightly larger molten Fe- and Ti-enriched silicate composite. The Moon has no global magnetic field, however, all measurements indicate that the rocks of the lunar crust are magnetized. Figure2 Conceptual cross section of the lunar crust an interior.
Numerous models have been proposed to explain the origin and formation of the Moon including fission from Earth, co-accretion with Earth, gravitational capture and giant impact. The most striking constraints for the formation of the Moon are its bulk composition and the high angular momentum of the Earth-Moon-System, which can best be explained by a giant collision of the proto-Earth with a large object in the early solar system history.
Recently H2O and OH have been confirmed to be existent in the south polar crater Cabeus. Due to the fact that the lunar axis is almost perpendicular to the ecliptic, larger polar craters are permanently in shadow. These cold traps accumulate water-molecules from impacts or either produced by hydration processes in the lunar surface or by continuously creation when solar-wind protons interact with the oxygen-rich surfaces during the formation of impact-induced lunar soil particles. Although these discoveries demonstrate the Moon to contain water, the amounts, which are in the order of parts per million (ppm) still keep the Moon to be extreme dry.
As the Moon might be responsible for the 24-hour rotation period of Earth, stabilize its obliquity, which otherwise would be chaotic causing large variation and by rising substantial tides in the Earth’s oceans, it acts as a climate regulator and supports the habitability of our planet.