The other major geologic process that has affected the Moon’s surface is impact cratering,[74] with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon’s near side alone.[75] Some of these are named for scholars, scientists, artists and explorers.[76] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale, structures characterized by multiple rings of uplifted material, typically hundreds to thousands of kilometres in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon.[77] The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.[77] The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment of impacts.[78]

Blanketed on top of the Moon’s crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder.[79] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 km (6.2–12.4 mi) in the highlands and 3–5 km (1.9–3.1 mi) in the maria.[80] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometres thick.
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly survive in cold, permanently shadowed craters at either pole on the Moon.[82][83] Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow.[84] The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.[85]

In years since, signatures of water have been found to exist on the lunar surface.[86] In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters.[87] In 1998, the neutron spectrometer located on the Lunar Prospector spacecraft, indicated that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions.[88] In 2008, an analysis of volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water to exist in the interior of the beads.[89]

The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000 ppm.[90] In 2009, LCROSS sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material.[91][92] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb).[93]

In May 2011, Erik Hauri et al. reported[94] 615–1410 ppm water in melt inclusions in lunar sample 74220, the famous high-titanium “orange glass soil” of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth’s upper mantle. Although of considerable selenological interest, Hauri’s announcement affords little comfort to would-be lunar colonists—the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.
he gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill these basins.[95][96] These anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.[97]
Magnetic field
Main article: Magnetic field of the Moon

The Moon has an external magnetic field of about 1–100 nanoteslas, less than one-hundredth that of Earth. It does not currently have a global dipolar magnetic field and only has crustal magnetization, probably acquired early in lunar history when a dynamo was still operating.[98][99] Alternatively, some of the remnant magnetization may be from transient magnetic fields generated during large impact events, through the expansion of an impact-generated plasma cloud in the presence of an ambient magnetic field—this is supported by the apparent location of the largest crustal magnetizations near the antipodes of the giant impact basins.

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tons (9.8 long tons; 11 short tons).[103] The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, the release of atoms from the bombardment of lunar soil by solar wind ions.[9][104] Elements that have been detected include sodium and potassium, produced by sputtering, which are also found in the atmospheres of Mercury and Io; helium-4 and neon[105] from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle.[106][107] The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood.[106] Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith.[108] These gases can either return into the regolith due to the Moon’s gravity or be lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind’s magnetic field.[106]
Dust

A permanent asymmetric moon dust cloud exists around the Moon, created by small particles from comets. Estimates are 5 tons of comet particles strike the Moon’s surface each 24 hours. The particles strike the Moon’s surface ejecting moon dust above the Moon. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising to 100 kilometers above the surface. The dust measurements were made by LADEE’s Lunar Dust EXperiment (LDEX), between 20 and 100 kilometers above the surface, during a six-month period. LDEX detected an average of one 0.3 micrometer moon dust particle each minute. Dust particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon, pass through comet debris. The cloud is asymmetric, more dense near the boundary between the Moon’s dayside and nightside.

The Moon’s axial tilt with respect to the ecliptic is only 1.5424°,[111] much less than the 23.44° of Earth. Because of this, the Moon’s solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects.[112] From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of Peary Crater at the Moon’s north pole may remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters,[84] and these dark craters are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F)[113] and just 26 K (−247 °C; −413 °F) close to the winter solstice in north polar Hermite Crater. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto.[112] Average temperatures of the Moon’s surface are reported, but temperatures of different areas will vary greatly depending upon whether a spot is in sunlight or in shadow.
The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days[g] (its sidereal period). However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days[h] (its synodic period).[56] Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane than to the planet’s equatorial plane. The Moon’s orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon’s orbital motion gradually rotates, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini’s laws.[115]
Relative sizeThe Moon is exceptionally large relative to Earth: a quarter its diameter and 1/81 its mass.[56] It is the largest moon in the Solar System relative to the size of its planet,[i] though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto’s mass.[j][116] Earth and the Moon are nevertheless still considered a planet–satellite system, rather than a double planet, because their barycentre, the common centre of mass, is located 1,700 km (1,100 mi) (about a quarter of Earth’s radius) beneath Earth’s surface