It has been confirmed by three spacecraft now – though maybe not the ones you thought would do so – there is a form of water on the Moon.
Before you all start searching for an oasis on the dusty lunar deserts, it would be best to tell you that for the most part, the Moon is not even as wet as one of our own deserts, but even that is wetter than it was believed to be.
The story begins with a satellite better associated with the moons of Saturn than that of the Earth – Cassini. In 1999, the probe passed by the Moon and tested a few of its sensors. One of them, which has the capability of detecting the bond between oxygen and hydrogen (two of which are in H2O – water, one of which is in -OH, known as hydroxyl, which normally forms a part of other molecules) looked at the Moon and saw something quite interesting. It got a signal from all over the satellite, strongest at the poles. Cassini wasn’t hanging around, though, so the results were noted and the probe simply carried on to Saturn.
One probe that was looking for these things was ISRO‘s Chandrayaan-1 lunar satellite. Its NASA-built Moon Mineralogy Mapper (M3), stared directly at the Moon’s surface for 312 days, mapping the surface in detail. It saw the OH bond everywhere it looked, though it wasn’t as strong an instrument as Cassini’s. This meant a final observation was required.
Then came the mission Deep Impact, in which a satellite unleashed an impactor into a comet and watched to see what happened. Like Saturn’s rings, comets are icy bodies and so like Cassini, Deep Impact had sensors looking for the OH bond present in water and hydroxyl (just to see where and how much ice existed on the comet) – this time infrared sensors, which can tell the difference between the two. Following the mission, Deep Impact has been swinging round the Moon in order to slingshot towards another comet in what is known as the EPOXI mission. M3 investigators asked the EPOXI team to point their detectors at the Moon and they did – confirming the previous two detections were both hydroxyl and water and noting that the signal was strongest in the lunar morning and weakest at lunar noon. So not only was there water and hydroxyl, something is replenishing the stuff that gets destroyed during the day.
Apollo brought back quite a few lunar rocks (they did detect tiny amounts of water then, but the containers leaked, so no-one could be sure the detection wasn’t contamination). What they learnt was that the rocks were forty-five percent oxygen (indeed NASA runs a competition to extract a certain amount of it to provide air for a lunar base). They also ran a few experiments looking at the radiation environment of the Moon. The satellite lies at least most the time outside of the protection of Earth’s magnetosphere, meaning it gets directly pummeled by the solar wind. Apollo astronauts left behind sensors to measure this, whilst they blasted safely away.
It is believed that the constant rain of protons (positive hydrogen ions) due to the solar wind (plasma expanding off the surface of the Sun) striking at great energies onto the oxygen rich minerals leads to the formation of water and hydroxyl. A similar process at Mercury, where the solar wind is far stronger, blasts material off the surface and produces a transient atmosphere. The small amounts of water created this way are destroyed by the Sun’s rays, but are replaced in darker times. This is known as endogenic water – created on the Moon. Exogenic water, which was created off the Moon and arrived, would be the result of cometary impacts and is believed to be less likely a cause due to the replenishment of the lost water at lunar noon.
Two NASA led missions, LRO, which has mapped the presence of hydrogen on the Moon, adding to the suspicions above, and LCROSS, which intends to strike the surface of a dark crater, are in the process of searching for water ice present in useful amounts. It is believed that permanently shadowed craters still hold relatively large amounts of the stuff.