The Moon

At a glance

The Moon is Earth’s only natural satellite, with a mass about 1/81 that of Earth. Because it has no plate tectonics, almost no atmosphere and very little erosion, its surface preserves a record of the first billion years of the inner Solar System that has largely been erased on Earth.

Values from the NASA NSSDCA Moon Fact Sheet (page last updated 11 January 2024), accessed 29 October 2026.

Sources: [1]

Origin: the giant-impact hypothesis and its alternatives

The prevailing explanation for the Moon’s origin is the giant-impact hypothesis. About 4.5 billion years ago, a Mars-sized body, usually called Theia, struck the young Earth; molten and vaporised debris thrown into orbit then gathered to form the Moon. Radiometric ages of lunar rocks suggest this happened roughly 60 million years after the Solar System began to form.

The strongest constraint comes from the Apollo samples. Oxygen isotopes and other elements in lunar rocks match those of Earth too closely to be a coincidence, and basalts from the lunar mantle closely resemble Earth’s mantle basalts. These results ruled out two older ideas: capture of a body formed elsewhere, and co-formation of the Earth and Moon side by side from the same material. Yet the very same similarity raises a puzzle: simple impact models predict a Moon built largely from the impactor, which should carry a different isotopic signature. Variants in which several smaller impacts, rather than a single one, built the Moon are among the alternatives still discussed.

The newly formed Moon was largely molten and covered by a deep magma ocean for tens to hundreds of millions of years. Light anorthosite crystals floated to the top of this ocean and formed the first crust about 4.4 billion years ago.

Sources: [2]

Interior structure: crust, mantle and core

The Moon’s mean density (3,344 kg/m³) is well below Earth’s, pointing to a small metallic core. Two main lines of evidence reveal the interior: seismic recordings from the Apollo missions and the high-resolution gravity field from NASA’s GRAIL mission.

The Apollo Passive Seismic Experiment comprised four stations deployed on the near side by Apollo 12, 14, 15 and 16, and operated until mid-1977. Re-analysing these data in 2011, Weber and colleagues stacked signals from repeating deep moonquake sources and reported a solid inner core of about 240 km radius, a fluid outer core extending to about 330 km, and a partially molten layer above it to roughly 480 km. The fluid outer core’s seismic velocity is consistent with a liquid iron alloy, with sulphur content estimated at 6 wt% or less. The authors note that these radii are not tightly constrained.

GRAIL (launched 2011) mapped the Moon’s gravity field with two spacecraft flying in formation. Comparing the gravity field with topography gave a bulk density of 2,550 kg/m³ for the highland crust; set against its remotely measured composition, this implies an average porosity of about 12%, a crust heavily fractured by impacts. The mean crustal thickness came out at 34–43 km, 10–20 km thinner than several earlier estimates.

Sources: [1] [3] [4]

Surface: maria, highlands, regolith and craters

To the naked eye the Moon’s surface divides into two units. The bright highlands are dominated by anorthosite and represent remnants of the earliest crust. The dark maria are large impact basins later flooded by lava; NASA dates these fillings to between 4.2 and 1.2 billion years ago. Craters of every size, from bright young ones such as the 85-km Tycho to ancient basins, record the Solar System’s impact history; sample-calibrated crater counts are used to date surfaces.

Almost the entire surface is covered by regolith, a loose layer of dust and rock fragments produced by repeated impacts. Beneath it lies fractured bedrock called the megaregolith. The Moon has only a very thin exosphere and an extremely weak magnetic field, so the surface is directly exposed to the solar wind, cosmic rays and micrometeorites. Temperatures swing from about 127 °C in full sunlight to about −173 °C in darkness.

The anchor of lunar chronology is returned samples. Between 1969 and 1972, six Apollo missions collected 382 kg of material in around 2,200 separate samples, and three robotic Soviet Luna spacecraft returned about 300 g from three other sites.

Sources: [5] [6]

Tides, synchronous rotation, libration, phases and eclipses

Because the Moon pulls more strongly on Earth’s near side than on its far side, the oceans form two tidal bulges, one facing the Moon and one opposite. As Earth rotates, coastlines pass through these bulges, so most shores see two high and two low tides a day. The Sun’s tide-raising force is a little less than half the Moon’s: when the two line up near new and full Moon, spring tides occur; near the quarters, neap tides.

Tidal interaction works in the other direction as well. The Moon rotates on its axis in the same time it takes to orbit Earth, so the same hemisphere always faces us (synchronous rotation). Tidal friction also transfers angular momentum to the Moon’s orbit, and the Moon is receding from Earth by about 3.8 cm per year. Owing to the eccentric orbit and the Moon’s axial tilt of about 6.7°, we see slightly more of its edges at different times; thanks to this libration, about 59% of the surface can be seen from Earth over time (Hong Kong Observatory).

The cycle of phases repeats every 29.53 days (the synodic month). Because the lunar orbit is tilted by 5.145° to the ecliptic, eclipses do not occur every month: a solar eclipse needs a new Moon, and a lunar eclipse a full Moon, near the points where the orbit crosses the ecliptic.

Sources: [1] [5] [7]

Interactive lab: the Moon, the Sun and tidal force

The lab opens on the Moon → Earth preset: the Moon’s tidal acceleration at Earth’s surface is about one ten-millionth of surface gravity. Switch to Sun → Earth and the ratio to the Moon’s tide drops to 0.46, so the Sun’s tidal effect is a little under half the Moon’s. Then pick Jupiter → Io: Io orbits Jupiter at roughly the same distance as the Moon orbits Earth, yet it is stretched thousands of times harder.

Polar volatiles and water ice

Near the lunar poles, the Sun stays close to the horizon, and the floors of some craters never see sunlight. These permanently shadowed regions are cold enough to trap water ice over long periods, whereas on sunlit surfaces water evaporates quickly.

The evidence has built up step by step. In 2009, NASA’s LCROSS mission sent a projectile into a shadowed crater and detected grains of water ice in the ejected plume. The full analysis of data from the M³ instrument on India’s Chandrayaan-1, published in 2018, identified multiple locations of exposed water ice in permanently shadowed regions. In 2020, the SOFIA airborne observatory detected molecular water (H₂O) even on the sunlit surface in Clavius crater, in an amount roughly equivalent to a 355 ml bottle per cubic metre of soil. The Lunar Reconnaissance Orbiter (LRO), in orbit since 2009, continues to map hydrogen and other resources.

The quantity, purity, depth distribution and origin of polar ice (comets, asteroids, the solar wind or volcanic outgassing) remain uncertain.

Sources: [8]

Exploration history: from Luna to Artemis

The Soviet Luna 2 impacted the Moon in 1959, and later that year Luna 3 returned the first images of the far side. Luna 9 made the first soft landing in 1966. Apollo 11 landed the first crew in July 1969; the programme ended with Apollo 17 in December 1972. Luna 16, 20 and 24 brought back robotic samples between 1970 and 1976. In the twenty-first century China’s Chang’e programme took the lead: Chang’e-4, launched in December 2018, made the first soft landing on the far side, and Chang’e-5 (launched 2020) brought back near-side samples.

Chang’e-6, launched on 3 May 2024, landed on 1 June 2024 in the Apollo basin within the South Pole–Aitken basin (The Planetary Society) and on 25 June 2024 delivered 1,935.3 g of material, the first sample ever returned from the far side. In a study published in Nature in November 2024, 107 of 108 basalt fragments analysed were dated to about 2.8 billion years and one to about 4.2 billion years, indicating far-side volcanism spanning at least 1.4 billion years; a parallel study in Science reported an age of about 2.83 billion years for low-titanium basalts.

India’s Chandrayaan-3 soft-landed on 23 August 2023 in the southern high latitudes, at 69.37° S (ISRO). Japan’s SLIM landed on 20 January 2024 about 10 m from its target point, achieving the first “pinpoint landing”; JAXA ended its operations on 23 August 2024. On the commercial front, Intuitive Machines’ Odysseus landed near the crater Malapert A on 22 February 2024, becoming the first successful soft landing of NASA’s CLPS programme (NASA); Firefly’s Blue Ghost touched down near Mons Latreille in Mare Crisium on 2 March 2025, carrying 10 NASA instruments (NASA).

In the Artemis programme, Artemis II launched on 1 April 2026 with Reid Wiseman, Victor Glover, Christina Koch and Jeremy Hansen (CSA) aboard and splashed down off San Diego on 10 April 2026 (US Pacific time). The crew flew within about 6,545 km of the lunar surface and reached about 406,770 km from Earth, breaking the Apollo 13 distance record. In February 2026 NASA redefined Artemis III as a test flight in low Earth orbit (NASA). As of October 2026 this roughly 14-day mission, with Randy Bresnik, Frank Rubio, Andre Douglas and ESA’s Luca Parmitano, is planned for 2027 to rehearse rendezvous and docking with test versions of the SpaceX and Blue Origin landers; crewed landings near the south pole are targeted to begin in 2028.

Sources: [9] [10] [11] [12] [13] [14]

Data archives for researchers

The main gateway to lunar orbital data is NASA’s Planetary Data System (PDS). The PDS Geosciences Node hosts the LRO Diviner, LEND, LOLA, Mini-RF and radio science archives, plus pages for GRAIL, Lunar Prospector, Clementine, Kaguya and Apollo. The Lunar Orbital Data Explorer offers map-based search and download for data from LRO, GRAIL, Clementine, Lunar Prospector, Lunar Orbiter and Chandrayaan-1.

For high-resolution imagery, the LROC team site at Arizona State University provides a data portal, the PDS archive, the Lunar QuickMap tool and an image target request system. Researchers seeking physical samples can turn to NASA’s lunar sample curation at Johnson Space Center, which distributes nearly 400 samples a year for research and teaching.

Data from other agencies are also open: ISRO released Chandrayaan-3 science data on 23 August 2024 through the PRADAN portal (registration required), and China’s Lunar and Planetary Data Release System publishes data from Chang’e-4, Chang’e-5 and earlier missions after their protection periods end.

Sources: [6] [11]

Known vs open questions

Established

• The Moon formed about 4.5 billion years ago, and its isotopic composition is strikingly similar to Earth’s; capture and co-formation models have been ruled out.
• The early Moon had a magma ocean; the anorthositic highland crust formed about 4.4 billion years ago.
• The Moon has a small iron-rich core; seismic data point to a solid inner core and a fluid outer core.
• The highland crust averages 34–43 km thick and is roughly 12% porous.
• Water ice is present in permanently shadowed polar craters; molecular water exists even on sunlit surfaces.
• Far-side volcanism continued until at least about 2.8 billion years ago.

Open questions

• How can Earth–Moon isotopic similarity be reconciled with a single giant impact? What were Theia’s size and composition, or were there multiple impacts?
• What explains the stark contrast between the near and far sides in crustal thickness, maria coverage and volcanic history?
• What are the exact size, composition and state of the core, and how long did the ancient magnetic dynamo operate?
• How much polar ice is there, how is it distributed with depth, where did it come from, and is it usable as a resource?
• Did the impact rate change sharply in early lunar history, and how reliable is the sample-calibrated crater chronology?

Sources: [2] [3] [4] [8] [10]