The Moon

Origin: the giant-impact hypothesis

The Moon formed approximately 4.5 billion years ago, roughly 60 million years after the Solar System itself coalesced from a collapsing cloud of gas and dust. The dominant explanation for its origin is the giant-impact hypothesis, sometimes called the Theia impact. In this model, a Mars-sized body — given the name Theia — collided with the proto-Earth at high speed during the chaotic early phase of planetary accretion. The catastrophic collision melted and partially vaporised large fractions of both bodies, ejecting an enormous cloud of silicate-rich debris into orbit around the young Earth. That debris rapidly accreted to form the Moon.

Several independent lines of evidence support this scenario. Oxygen isotopes and many other chemical signatures in lunar rocks match those of Earth's mantle with a precision that strongly points to a common origin. Lunar samples are also strikingly depleted in volatile elements — zinc, cadmium, indium, tin, thallium — that would have evaporated in the searing heat of the impact plume and not fully recondensed, a clear fingerprint of a high-temperature birth. Meanwhile, the Moon's internal iron core is tiny, representing only about 1.6–1.8 % of its mass, compared with roughly 30 % for Earth; this asymmetry is consistent with most of the metallic iron having remained with Earth during and after the collision, leaving the Moon to form predominantly from silicate material.

Geological and remote-sensing data indicate that shortly after its formation, the Moon was covered by a deep global magma ocean — possibly hundreds to thousands of kilometres deep. As this ocean slowly cooled, lighter plagioclase feldspar crystals floated to the surface and solidified into a pale anorthositic crust, while denser minerals sank, eventually producing a layered interior of crust, mantle, and metallic core. Recent high-resolution supercomputer simulations suggest that a significant portion of the Moon may have been assembled within hours of the initial impact, rather than over a lengthy period from a slowly dispersing debris disk — but these models operate firmly within the giant-impact framework.

Before Apollo returned samples, three rival theories competed for acceptance: the capture hypothesis (the Moon formed elsewhere and was gravitationally caught by Earth), the co-accretion hypothesis (Earth and Moon grew side by side from the same material), and the fission hypothesis (a fast-spinning early Earth flung off material that became the Moon). Each fails on one or more key constraints — the close isotopic match rules out simple capture; co-accretion cannot explain the Moon's tiny core and volatile depletion; fission cannot satisfy angular-momentum requirements. High-precision isotopic analysis of Apollo samples effectively settled the debate in favour of giant impact, though the precise mass, angle, and velocity of Theia remain active areas of research.

Surface geology: craters, maria, and regolith

The Moon's surface is a record of Solar System history, preserved in exceptional detail because the body lacks an atmosphere, liquid water, and active plate tectonics — the three processes that erase or bury Earth's ancient terrain within hundreds of millions of years. On the Moon, craters billions of years old remain sharply defined.

Impact cratering is the dominant geological process. A lunar crater forms when an asteroid or comet strikes at hypervelocity, producing an explosion that excavates material in all directions. Small impacts produce simple, bowl-shaped depressions; larger events create complex craters with terraced walls, central peaks, and flat floors. The largest structures are impact basins, exceeding 300 km across. More than 40 major impact basins scar the Moon, most of them formed around 3.9 billion years ago during the Late Heavy Bombardment — a period when the rate of large impacts throughout the inner Solar System was dramatically elevated.

The grandest basin of all is the South Pole–Aitken Basin, spanning approximately 2,500 km in diameter and descending up to 8–12 km below the surrounding terrain. It is both the largest and oldest recognised impact structure on the Moon, and the energy of the impact that created it dwarfs the event that ended the dinosaurs on Earth. This basin's interior hosts some of the Moon's most permanently shadowed regions, making it a prime target for water-ice prospecting.

Volcanism shaped the other great class of lunar terrain: the maria (singular mare, from the Latin for 'sea'). Ancient astronomers mistook these dark, relatively flat plains for actual bodies of water; they are in fact vast sheets of basaltic lava that flooded large impact basins as magma rose through fractures in the crust. Mare basalts are iron-rich and often titanium-rich, giving them their characteristic low reflectivity. Most eruptions occurred between roughly 3.0 and 3.5 billion years ago, though some samples have ages approaching 4.2 billion years. High-resolution imagery has revealed small, irregular mare patches with crater counts suggesting ages younger than 50 million years, raising the possibility that minor volcanic activity persisted into geologically recent times. Mare Tranquillitatis, the 'Sea of Tranquillity', was the destination of Apollo 11 in 1969.

Virtually the entire lunar surface is blanketed by regolith: a layer of pulverised rock, glass beads, and dust produced by billions of years of micrometeorite bombardment. Unlike terrestrial soil, it contains no organic material and was created entirely by physical processes. Grains are sharp and angular, making the regolith highly abrasive — a challenge for both equipment and spacesuits. Depth varies with terrain age: about 2 m over relatively young maria, up to 20 m over the oldest highland surfaces. Apollo and Soviet Luna missions returned approximately 382 kg of rocks and soil, which remain the primary physical archive of lunar history.

The bright, heavily cratered highlands represent the Moon's ancient anorthositic crust — the solidified rind of the primordial magma ocean. Many of the mountain ranges that rim the maria are not the product of tectonic compression (as on Earth) but are uplifted walls of enormous impact basins. Linear features called rilles cut the surface: sinuous rilles are channels carved by ancient lava flows, while straight rilles are fault structures associated with basin formation and crustal cooling.

The Moon's influence on Earth

The Moon's relationship with Earth extends far beyond the visual spectacle of moonrise and lunar phases. Two effects in particular have shaped Earth's environment profoundly: tidal forcing and axial-tilt stabilisation.

Tides arise because the Moon's gravity pulls more strongly on the side of Earth facing it than on the opposite side. This differential force stretches the oceans into two bulges — one pointing toward the Moon, one pointing away — and as Earth rotates beneath them, coastlines experience roughly two high tides and two low tides every 24 hours. The Moon is the dominant driver of this cycle; the Sun also exerts a tidal pull, but it is less than half as strong as the Moon's. Without the Moon, only the weaker solar tides would remain, drastically reducing tidal ranges and transforming the intertidal ecosystems that depend on strong, regular tidal cycles. Over geological time, tidal dissipation has been gradually slowing Earth's rotation and pushing the Moon slowly further away.

Earth's rotation axis is currently tilted at approximately 23.4° relative to its orbital plane, an angle that produces the seasons. The Moon's gravitational torque on Earth's equatorial bulge acts as a stabiliser: it restricts the axial tilt to a slow, gentle wobble of about 2–3° over a roughly 26,000-year precession cycle. Models and comparisons with other planets — most notably Mars, whose axial tilt has varied chaotically over millions of years in the absence of a large moon — suggest that without the Moon, Earth's obliquity could range from near 0° to very large angles. Such extremes would produce dramatic, rapidly shifting climate regimes: periods with virtually no seasons giving way to periods where the poles seasonally overheat and equatorial regions cool. The Moon's stabilising presence is therefore widely regarded as one of the factors that has helped sustain relatively moderate, predictable climates over the billions of years during which life evolved on Earth.

The new lunar race: programmes and ambitions in the 2020s

After a prolonged lull following the Apollo era, the 2020s have become the busiest decade in lunar exploration since the 1960s and 1970s. Multiple space agencies and commercial operators are pursuing an overlapping set of goals: mapping and characterising water ice for potential in-situ resource use, demonstrating precision landing technologies, returning samples from previously unvisited terrain, and laying the groundwork for sustained human presence.

NASA's Artemis programme is the centrepiece of the United States' return to crewed lunar exploration. Its long-term objective is to land the first woman and the next man on the Moon, targeting the south polar region because of its concentration of water ice in permanently shadowed craters. The 2022 Artemis I uncrewed test flight successfully demonstrated the Space Launch System rocket and the Orion capsule on a trajectory around the Moon. Artemis II is designed to carry a crew on a lunar flyby, and Artemis III aims to deliver astronauts to the surface — the first crewed lunar landing since Apollo 17 in 1972.

China has mounted the most consistently successful robotic programme of the modern era, with Chang'e 3 (2013), Chang'e 4 (2019, far side), Chang'e 5 (2020, sample return), and Chang'e 6 (2024, far-side sample return) each achieving their primary objectives. China and Russia have announced plans for a joint International Lunar Research Station. Russia's own Luna-25, the first Russian lunar mission since 1976, crashed in August 2023, but subsequent Luna missions remain part of long-term planning, including Luna-26 as a polar-mapping orbiter.

India joined the small club of successful south-polar landers with Chandrayaan-3 in August 2023. Japan demonstrated pinpoint landing capability with SLIM in January 2024. Looking further ahead, a joint ISRO–JAXA mission called LUPEX (also referred to as Chandrayaan-5) is approved to drill and directly characterise subsurface water ice at the south pole, targeting the kind of in-situ resource assessment that could support future crewed outposts.