What it is like on the Moon

The Moon is a hostile place, and almost everything about it follows from one fact: it has essentially no atmosphere (its gravity is too weak to hold gas).

• No air → no sound, no wind, no weather, no erosion, and a black sky even in daylight (no scattering).
• Surface gravity 1.62 m/s², about 1/6 of Earth's. Astronauts bounce; a dropped hammer and feather fall together.
• Temperature swings from roughly +120 °C in sunlight to −150 °C at night — and the lunar night lasts about a fortnight.
• No global magnetic field and no atmosphere → the surface is fully exposed to the solar wind, cosmic rays and micrometeorites.
• Regolith — sharp, abrasive dust that clung to Apollo suits, jammed seals and abraded visors.

Water ice has been detected in permanently shadowed craters near the lunar poles — a huge deal, because it could supply drinking water, oxygen and rocket fuel to a future base.

Your turn — weight on the Moon

A black daytime sky

Robots and astronauts

The Moon has been explored in two very different ways.

• Robotic probes — Luna 3 photographed the far side (1959); Surveyor landers proved the surface would take a spacecraft's weight; the Lunar Reconnaissance Orbiter has mapped the whole Moon in fine detail; Lunokhod and Yutu rovers drove across it; Luna 16 and Chang'e 5 returned samples automatically.
• Crewed landings — six Apollo missions landed between 1969 and 1972. Apollo 11 touched down in the Sea of Tranquillity (Mare Tranquillitatis) in July 1969. Later crews drove the Lunar Roving Vehicle and left experiment packages — including retroreflectors still used today to laser-range the Moon to a few centimetres.

The trade-off — a classic 6-marker. Robots: far cheaper, no risk to life, can survive years and go anywhere. Astronauts: adaptable, can choose the best samples, do field geology, fix things — but need life support, cost billions and can die.

Your turn — radio delay

Where did the Moon come from?

The Apollo samples were the key evidence. They showed lunar rock is:

• Chemically very like the Earth's mantle — in particular the oxygen isotope ratios match the Earth's almost exactly, which is not true of meteorites from elsewhere.
• Completely dry — no water-bearing minerals — and depleted in iron and other dense metals.
• Very old, and once molten (the surface was a magma ocean).

The theory that fits all of this is the giant impact hypothesis: about 4.5 billion years ago a Mars-sized body struck the young Earth a glancing blow. Debris blasted from the mantles of both bodies went into orbit and coalesced into the Moon. That explains the matching isotopes (same material), the missing iron (the impactor's core sank into the Earth) and the bone-dry rock (volatiles boiled away).

Rejected rivals: capture (hard to slow a passing body down, and the isotopes would not match), co-formation (would give the Moon an iron core like the Earth's), and fission (the Earth would have to spin impossibly fast).

The best evidence

Robot or human?

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Why go back?

What the experiments revealed

Apollo did far more than plant flags. Each crew left an ALSEP experiment package behind:

• Seismometers detected "moonquakes" and let scientists work out the Moon's internal structure — a thick crust (thicker on the far side), a deep mantle and a small, partly molten iron core. That small core is exactly what the giant impact theory predicts.
• Retroreflectors — corner-cube mirrors that bounce a laser pulse straight back. By timing the round trip, the Earth-Moon distance is now known to a few centimetres. The result: the Moon is drifting away from us at about 3.8 cm per year, and the Earth's day is very slowly lengthening (tidal friction).
• Solar wind collectors — foil sheets exposed to trap solar wind particles for analysis back on Earth.

Why the Moon has no magnetic field: its core is small and has largely solidified, so it cannot sustain a dynamo. With no magnetic field and no atmosphere, the surface is left utterly unprotected from the solar wind and cosmic rays.

Your turn — a retreating Moon

Why no magnetic field?

The big ideas to know

That is Exploring the Moon covered for Edexcel GCSE Astronomy. Press Finish to see your score.