august: ice crater

One of the most exciting discoveries in recent lunar science was that of water ice on the surface of the Moon. It is there because of a combination of two factors. Comets are essentially large dirty snowballs which usually orbit in the cold outer regions of the solar system. From time to time they are nudged onto paths that bring them much closer to the Sun, leaving behind them a trail of dust, water vapour and ice crystals. It has been suggested that the huge quantities of water scattered through the inner solar system in this way over billions of years is responsible for Earth's oceans. On the airless Moon, volatile substances such as water could never usually accumulate. However, because of the geometrical relationship between the Moon's rotation on its axis and its revolution around the Earth and in turn around the Sun, the floors of some of the craters around the lunar poles are in permanent darkness, and have been for much of the Moon's history. In such places the temperature is very close to absolute zero, and millions of litres of water ice appears to have built up over time, as shown in this NASA artist's impression.

This brings the possibility of colonisation of the Moon much closer, as the water - which would be very expensive to transport from the Earth in bulk - can be used both to support life and to make rocket fuel.

Image: NASA.