Moon

• 1. Introduction

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  The Moon is the only natural satellite of Earth, and it orbits at an average distance of 384,399 km. That gap is roughly thirty times the width of Earth. The Moon completes one orbit relative to Earth and the Sun every 29.5 days. It is a world held captive. Tidal forces have pulled it to always show the same near side to us, locking its spin to its orbit. Why does only one face look back at Earth? Where did a body this large come from, when its mass is just 1.2% of Earth's? How did the dark plains we see from the ground come to be, and what is hiding in its permanently shadowed craters? This is a place humans have touched. The first spaceflights to any body beyond Earth went to the Moon, beginning in 1959. Yet much about it remains unsettled, starting with the question of how it formed at all.

• 2. Born From A Collision

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  Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. Several explanations were tried and discarded. A fission of the Moon from Earth's crust by centrifugal force would have demanded an impossibly fast spinning Earth. Gravitational capture of a ready-made Moon needed an unfeasibly extended atmosphere to bleed off the passing body's energy. Co-formation in the primordial accretion disk failed to explain why the Moon is so depleted in metals. None of these accounted for the high angular momentum of the Earth-Moon system.

  The prevailing theory points to a Mars-sized body named Theia striking the proto-Earth about 4.51 billion years ago. The oblique impact blasted material into orbit, and that debris accreted into the Moon just beyond Earth's Roche limit. Giant impacts are thought to have been common in the early Solar System. Computer simulations of such an event match both the mass of the lunar core and the angular momentum of the system, with most of the Moon derived from the impactor.

  Earth and the Moon have nearly identical isotopic compositions, unlike Mars and Vesta, whose meteorites differ sharply in oxygen and tungsten. That similarity may come from post-impact mixing of vaporized material, though this is debated. The impact released enough energy to liquefy both the ejecta and Earth's crust, forming a magma ocean. The newly formed Moon had its own magma ocean, estimated between about 500 km and 1,737 km deep. On the 1st of November 2023, scientists reported that remnants of Theia could still be present inside the Earth.

• 3. A Layered Interior

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  Crystallization of the global magma ocean built the Moon from the inside out, shortly after its formation 4.5 billion years ago. As minerals like olivine, clinopyroxene, and orthopyroxene precipitated and sank, they formed a mafic mantle more iron-rich than Earth's. After about three-quarters of the magma ocean had crystallized, lighter plagioclase minerals floated up to form a crust atop. Geochemical mapping from orbit suggests that crust is mostly anorthosite, averaging about 50 km thick.

  The core is small and divided. A solid iron-rich inner core may have a radius as little as 240 km, wrapped by a fluid outer core of liquid iron roughly 300 km in radius. Around that sits a partially molten boundary layer about 500 km across. Analyses of the Moon's time-variable rotation suggest the core is at least partly molten, with the pressure there estimated at 5 GPa.

  The Moon's shape betrays its past. It is a slightly scalene ellipsoid, its long axis displaced 30° from facing Earth because of gravitational anomalies from impact basins. This 'fossil bulge' is more elongated than today's tidal forces can account for. It indicates the Moon solidified while orbiting at half its current distance, and is now too cold to relax back into hydrostatic equilibrium.

• 4. The Dark Seas

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  The dark, relatively featureless plains called maria are vast solidified pools of ancient basaltic lava, named from the Latin for 'seas' because they were once believed to hold water. Lunar basalts carry more iron than terrestrial ones and contain no minerals altered by water. Most of these lava deposits flowed into depressions tied to impact basins. The largest expanse, Oceanus Procellarum, does not match any obvious basin. As the maria cooled and contracted, they raised wrinkle ridges that can run for hundreds of kilometers.

  Almost all the maria sit on the near side, covering 31% of that hemisphere against just 2% of the far side. This likely reflects a concentration of heat-producing elements under the near-side crust, which warmed the mantle, partly melted it, and let lava erupt. Most mare basalts erupted during the Imbrian period, 3.3 to 3.7 billion years ago, though some are as young as 1.2 billion years and as old as 4.2 billion years.

  In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, dust-free features that appeared only about 2 million years old. Evidence of recent volcanism has turned up at 70 irregular mare patches, some less than 50 million years old. Sinuous rilles such as Schroter's Valley and Rima Hadley, likely extinct lava channels or collapsed tubes, run longer, wider, and deeper than anything comparable on Earth. The lighter highlands, or terrae, were radiometrically dated to 4.4 billion years ago.

• 5. A Pockmarked World

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  Roughly 300,000 craters wider than 1 km mark the Moon's near side alone, formed when asteroids and comets struck the surface. Their forms scale with size, from simple bowl-shaped craters with upturned rims, to complex craters with terraced walls and central peaks, up to multi-ring basins with concentric rings of peaks. Most are circular, but some, like Cantor and Janssen, take polygonal outlines, while the Messier pair, Schiller, and Daniell are elongated.

  The Moon's most extensive feature is the far-side South Pole-Aitken basin, some 2,240 km across, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. At 13 km deep, its floor holds the lowest point on the surface. The highest point, the Selenean summit at 10.629 km, lies directly to the northeast and may have been thickened by that same oblique impact.

  Radiometric ages of impact-melted Apollo rocks cluster between 3.8 and 4.1 billion years old, which has been used to propose a Late Heavy Bombardment of increased impacts. High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a present-day crater-production rate well above earlier estimates. Distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years, about 100 times faster than micrometeorite models alone predict. Fault scarp cliffs reveal that the Moon has shrunk by about 90 meters within the past billion years as it loses heat.

• 6. Bound By The Tides

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  The gravitational pull between Earth and the Moon is slightly stronger on the sides facing each other, and that imbalance produces tidal forces. Ocean tides are the most familiar result. There are two bulges in Earth's oceans, one beneath the Moon and one opposite, giving two high tides and two low tides in about 24 hours. High tides arrive about every 12 hours and 25 minutes, the extra 25 minutes owing to the Moon's own motion in its orbit.

  The Sun contributes too, adding as much as 40% of the Moon's tidal force and producing the spring and neap tides in interplay. The lunar crust itself flexes by around 10 cm over 27 days. The cumulative stress from these forces produces moonquakes, which can last up to an hour because seismic waves scatter in the dry, fragmented upper crust. Their existence was an unexpected discovery from seismometers placed by Apollo astronauts from 1969 through 1972.

  Delays in the tidal peaks create a torque that drains angular momentum from Earth's rotation and hands it to the Moon. This tidal acceleration lifts the Moon into a higher, slower orbit. Laser reflectors left during the Apollo missions show the Moon receding by 38 mm per year, about the rate human fingernails grow. Atomic clocks show Earth's day lengthening by about 17 microseconds each year, slowly increasing how often UTC needs a leap second.

• 7. The Hunt For Water

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  Liquid water cannot persist on the lunar surface, since solar radiation breaks it apart through photodissociation and it is lost to space. Since the 1960s scientists have suspected that water ice might survive in cold, permanently shadowed craters at the poles, delivered by comets or made when solar-wind hydrogen reacts with oxygen-rich rocks. Computer simulations suggest up to 14,000 km2 of the surface may lie in permanent shadow. In 1998, the neutron spectrometer on Lunar Prospector found high hydrogen concentrations in the first meter of regolith near the poles.

  The 2008 Chandrayaan-1 spacecraft confirmed surface water ice with its Moon Mineralogy Mapper, detecting absorption lines of hydroxyl in reflected sunlight, with concentrations possibly as high as 1,000 ppm. In 2009, LCROSS sent a 2,300 kg impactor into a shadowed polar crater and detected at least 100 kg of water in the plume, with a later analysis putting the figure closer to 155 kg. In October 2020, several spacecraft, including the Stratospheric Observatory for Infrared Astronomy, reported molecular water even on the sunlit surface.

  The ice is more abundant at the South Pole, where permanently shadowed craters shield it from the Sun. These cold traps reach the lowest temperatures ever measured by a spacecraft in the Solar System. The Lunar Reconnaissance Orbiter recorded just 26 K near the winter solstice in the north polar crater Hermite, colder even than the surface of Pluto. The promise of usable water is what makes lasting lunar habitation look affordable, since hauling it from Earth would be prohibitively expensive.

• 8. Watched And Visited

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  The oldest cave paintings, from up to 40,000 years ago, may have tracked the phases of the Moon, and tally sticks 20,000 to 30,000 years old may have kept time by its waxing and waning. The earliest astronomer known by name, Enheduanna, was the Akkadian high priestess to the lunar deity Nanna/Sin and a daughter of Sargon the Great, and she had the Moon tracked in her chambers. The Nebra sky disc, from about 1800 to 1600 BCE, shows the Moon beside features like the Pleiades. The Greek philosopher Anaxagoras reasoned that the Sun and Moon were giant spherical rocks, and that the Moon reflected the Sun's light.

  The telescope changed the view. Thomas Harriot made the first rough telescopic map of the Moon in early summer 1609 but never published it. Galileo Galilei observed the same year and published his conclusions in 1610 in Sidereus Nuncius, showing the Moon was not smooth but carried mountains and craters. Those craters were thought volcanic until Richard Proctor proposed impacts in the 1870s, a view supported in 1892 by geologist Grove Karl Gilbert.

  Then humans reached it. Spaceflight to the Moon began in 1959 with the flyby of Luna 1, sent by the Soviet Union, followed by the deliberate impact of Luna 2. Luna 9 made the first soft landing in 1966, and Luna 10 the first orbital insertion. Apollo 8 carried humans into lunar orbit on the 24th of December 1968, and Apollo 11 set them on the surface on the 20th of July 1969. By 1972, six Apollo missions had landed twelve people, some staying up to three days. The renewed search for water has fueled plans to return, beginning with the Artemis program scheduled for the late 2020s.