Have you ever wondered where all the water on the Earth came from? One likely explanation is that most of the Earth’s water arrived in asteroids during the evolution of the Solar System. Indeed, two 4.5 billion-year-old meteorites have been found that both contained water (Figure 1). (An alternative theory is described in the article ‘Earth’s water and Neptune’s magnetic fields’, PHYSICS REVIEW Vol. 33, No. 2, pp. 2–5.)
Rocks bombarded our Moon over the same period (Figure 2) and, because it lacks an atmosphere and has no active tectonic plate motions, craters remain. This means that water can be found on the surface of the Moon, albeit in its solid state and in very small amounts.
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