At the end of 2009 NASA finally decided that there was not only water on the Moon, but quite a lot of it. This was a surprise to most people, because everybody knew that the Moon was bone dry. So where was the water hiding all this time?
There can't be any water on the surface
The smooth, dark regions that shape the “face” on the Moon are called maria (Latin for seas), because early astronomers looking through telescopes thought that they looked like seas. But even by the mid-nineteenth century astronomers realized that if there were water, there would be clouds and weather. This wasn't the case. We now know that the maria were once filled with lava that cooled to form basalt.
Physics tells us that the Moon couldn't have an atmosphere. It's too small for its gravitational pull to hold onto the gas particles. Without a blanketing atmosphere the lunar temperature goes through extremes during the lunar day. The daylight side can get as hot as 250 F (120 C) and the night side as cold as -240 F (-150 C). A lunar day is about 27 Earth days long because the Moon turns once on its axis as it orbits Earth.
In addition, without an atmosphere, liquid water would boil away. If there were ice on the surface, it would sublime when the sunlight hit it, i.e., go straight from a solid to a liquid. In fact, the Moon does have a very tenuous atmosphere, but its pressure is less than that of an industrial ultra-high vacuum on Earth.
Where the water went
The Apollo rocks confirmed the Moon's dryness, showing no evidence of water. An exception was rock 66095 (nicknamed “Rusty Rock”) which did contain minerals that had been produced by reactions with water. Unusually, this rock seems to have formed following a meteorite impact and people assumed that the water came from the meteorite. However since then some of the Moon rocks have been tested with new techniques which found small amounts of water.
The most widely-accepted idea of how the Moon was formed is that it was made from material thrown off by the Earth following a giant impact. The material was heated so strongly that it melted, releasing any water or dissolved gases present. Of course, this doesn't mean that comets and meteorites couldn't have brought water to the Moon during the billions of years of its existence.
In 1976 Soviet scientists analyzed the Luna 24 soil sample taken from 2 meters underground. Using a technique known as infrared absorption spectroscopy, they found water comprised 0.1 per cent of the mass of the sample. The returned sample was shared with scientists in other countries, but no one else reported finding water. In any case, the Cold War was still going on and the Apollo Moon rocks overshadowed the Soviet soil return missions.
Water in the shadowed craters?
Way back in 1961 three Caltech researchers suggested that there could be ice in polar craters that were permanently in shadow. They would always be cold, around -220 C and any ice could stay frozen for billions of years. The idea wasn't ignored, but at the time there was no evidence for such craters.
In 1994 the Clementine probe's radar data suggested icy areas around the poles, but not everyone could agree that this was the only way to explain the data. Nonetheless it did show that there were extensive regions around the Moon's south pole that were in permanent shadow. There were also some around the north pole.
Four years later Lunar Prospector studied the Moon from a low polar orbit with instruments designed to detect water. It found water ice crystals over a large area around both poles.
A number of people have the feeling that the discovery of water on the Moon has been announced excitedly almost as often as its discovery on Mars. What I find a bit odd is that even after the results from Clementine and Lunar Prospector, the Moon was still described as waterless. That perception changed in 2009.
Two separate missions in 2009 provided evidence of extensive water on the Moon.
(1) India's Chandrayaan-1 probe was carrying two NASA instruments. The Moon Mineralogy Mapper found extensive surface ice. Several months later radar showed over forty craters in permanent shadow and containing ice.
(2) The LCROSS mission was designed to crash into one of the shadowed craters of the south pole and find out what shows up in the cloud of debris. A spacecraft followed the impact and collected the data which showed a considerable amount of pure water ice.
Dry as a bone?
It will be interesting to find out how much water there is on the Moon and whether it could be used by lunar explorers in the future. It sounds exciting and it's worth remembering that our bones aren't dry, they're 22% water by volume.
You can see more images of The Moon on my Pinterest page.