The Moon is made of rock and metal—just like the Earth and the other rocky planets (Mercury, Venus and Mars). The crust, the Moon’s outer shell, is covered by lunar soil, also called regolith: a blanket of fine rock particles, varying between three and 20 metres (10–65 feet) deep. The footprint of the first person to walk on the Moon, Neil Armstrong, is preserved in regolith. This layer was produced partly by the action of the Sun, breaking down surface rocks, and partly by the arrival of ​​​​​​​meteorites, most of them tiny (less than a gram), over millions of years. The Moon’s crust itself is about 50 kilometres (30 miles) deep. Its rocks are mostly composed of the elements oxygen and silicon. The crust also contains the metals magnesium, iron, calcium, titanium and aluminium—of interest to nations involved in future lunar exploration.

Water on the Moon

With no atmosphere to protect it from the Sun’s rays, water in its liquid form cannot exist on the Moon’s surface. Water does, however, exist as solid ice in cold, permanently shadowed craters known as "cold traps" near the lunar poles. Water has now also been discovered in flat, sunlit regions near the Moon's south pole by scientists using an infrared telescope.
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Strewn across the Moon’s surface are countless tiny glass beads, each containing water. (This "glass" is a naturally occurring solid material with no regular pattern in its molecules.) The beads, which measure less than a millimetre across, form when meteoroids crash into the Moon and send up showers of molten rock droplets. On cooling rapidly, the droplets solidify and fall into the soil. The water inside them forms when high-energy particles streaming from the Sun—solar wind—strike the rock droplets while they are still molten. The solar wind contains hydrogen which combines with oxygen in the droplets to produce H₂0: water.

Unlike frozen water found in steep-walled craters, which would be difficult to access, water in glass beads should be far easier to obtain by astronauts or robots working on the Moon. The water can be extracted by heating the beads above 100°C. It could then be used by astronauts on future crewed missions to the Moon and, later, Mars.