Exploring the Moon: Origin, Far Side and Apollo
A World That Never Turns Away
The Moon is Earth's only natural satellite and our closest neighbour in space. One of its most striking features is that it always shows us the same face. Synchronous rotation means the Moon spins on its axis in exactly the same time it takes to complete one orbit of Earth — approximately 27.3 days. The result is that the near side is permanently visible from Earth, while the far side remained completely unknown until the Soviet Luna 3 spacecraft photographed it in 1959.
Inside the Moon
Like Earth, the Moon has a layered internal structure, though it is far less geologically active. The outermost layer is the crust, which is thicker on the far side (averaging around 60 km) than on the near side (around 30 km). Beneath this lies the mantle, composed mainly of silicate rock. At the centre is a small, partially molten core approximately 350 km in radius, thought to be iron-rich. Evidence for this internal structure comes from seismometers left on the lunar surface by Apollo astronauts, which detected moonquakes and tracked how seismic waves travelled through different layers.
The Moon's interior has three layers: a crust (thicker on the far side), a silicate mantle, and a small iron-rich core of around 350 km radius. Apollo seismometers provided direct evidence of this layered structure.
Near Side vs Far Side
The near side and far side of the Moon look strikingly different. The near side is covered with large dark maria (singular: mare), which are vast plains of solidified basaltic lava. In contrast, the far side has very few maria and is dominated by rugged, heavily cratered highland terrain. The reason lies in the Moon's asymmetric crust: the thinner near-side crust allowed ancient volcanic eruptions to reach the surface more easily, flooding impact basins with lava. The thick far-side crust suppressed volcanism, leaving craters unfilled.
Our knowledge of the far side has expanded dramatically through missions such as the Soviet Luna 3 (1959, first photographs), Apollo 8 (1968, first crewed orbit of the Moon, whose crew were the first people to see the far side directly), and NASA's Lunar Reconnaissance Orbiter (LRO), launched in 2009, which has produced the most detailed maps of the entire lunar surface to date.
Shortly after the Moon formed, Earth's heat source may have preferentially warmed the near side, keeping its crust thinner and more volcanic. One leading explanation involves the positioning of the Moon as it solidified: the near-side faced a hotter, newly formed Earth, delaying crustal solidification, while the far side cooled and solidified faster, building a thicker crust. This asymmetry is consistent with the Giant Impact Hypothesis timeline.
How Did the Moon Form?
Several theories have been proposed for the Moon's origin. The currently favoured explanation is the Giant Impact Hypothesis. Around 4.5 billion years ago, a Mars-sized protoplanet named Theia struck the early Earth at an oblique angle. The collision ejected a vast disc of debris into Earth orbit; this debris gradually clumped together under gravity to form the Moon. Evidence supporting this model includes:
- The Moon's composition is strikingly similar to Earth's mantle, not its core — consistent with the impact excavating mantle material.
- The Moon has a relatively small iron core compared to its size, because Theia's iron core merged with Earth's core during the impact and the ejected debris came from the two mantles.
- Apollo samples showed lunar and terrestrial rocks share almost identical oxygen isotope ratios, implying a common origin.
The Giant Impact Hypothesis is supported by three key evidence strands: the Moon's composition matches Earth's mantle, the Moon has a disproportionately small iron core, and Apollo rock samples share Earth's oxygen isotope fingerprint.
Capture Theory proposes that the Moon formed elsewhere in the solar system and was gravitationally captured when it passed close to Earth. A major problem is that capture requires the Moon to slow down dramatically at precisely the right moment — the gravitational physics make this very unlikely without a mechanism to dissipate energy. Co-accretion Theory suggests the Moon and Earth formed simultaneously from the same disc of material orbiting the young Sun. This struggles to explain the Moon's low density and small iron core compared to Earth. The Giant Impact Hypothesis addresses both problems and remains the dominant scientific model.
Escape Velocity and Reaching the Moon
Any spacecraft launching from Earth must first overcome Earth's escape velocity, which is approximately 11.2 km/s (about 40,000 km/h). The energy requirement is enormous — hence the size of the Saturn V rocket used in the Apollo programme. Once free of Earth's gravity, spacecraft must also deal with the Moon's own escape velocity (~2.4 km/s). Slowing down to enter lunar orbit and again to land requires engines firing in the reverse direction, consuming additional fuel. The Apollo lunar module carried its own descent and ascent engines for this reason.
The Apollo programme (1961–1972) achieved the first crewed lunar landings, with Apollo 11 landing at the Sea of Tranquility on 20 July 1969. The missions returned 382 kg of lunar rock and soil, deployed scientific instruments, and fundamentally changed our understanding of the Moon's composition and history. Apollo 8 (1968) was the first crewed mission to orbit the Moon, producing iconic photographs and proving the journey was achievable. The programme demonstrated that the Moon's surface was stable enough to support a lander and that humans could survive the radiation environment during transit.
A 4-mark comparison question requires you to address BOTH sides of each property you discuss. Don't say only "the near side has maria". Instead: "The near side is covered with extensive dark volcanic plains called maria, whereas the far side is predominantly rugged, heavily cratered highland terrain with very few maria." One well-constructed comparison sentence earns two marks — one for each side of the comparison.