Water

Water, H2O, is an ubiquitous molecule in the universe and very common in our Solar System. There is water in the atmosphere of Venus, drenching the Earth, as permafrost and polar caps on Mars, and it is the major component of several moons of the outer Solar System, as well as much of the debris farther out in the Kuiper Belt and Oort Cloud. So it came as quite a surprise in the 1960s, when samples were brought back from the Moon for the first time, that the Moon was anhydrous, or without water. This was more profound than the lack of groundwater or permafrost; the water that is incorporated in many minerals on Earth was completely absent from lunar minerals.

The absence of water on a body that shared Earth's orbital zone around the Sun came as a profound revelation, and somewhat of a shock. Some of the most active theories about formation of the Moon (for example, co-accretion and fission) were instantly discredited. There was a period of a few years where theorists and researchers were left scratching their heads.

The theory of lunar formation that has now become generally accepted, that accounts not only for the lack of water but also the lack of heavier elements and lighter elements in the lunar makeup, posits that roughly 4 billion years ago, when the Solar System was still in formation and much more chaotic than it is today, a body about the size of Mars (but not Mars) struck the primordial Earth obliquely. The body became incorporated into the Earth, but the "splash", composed of a mix of proto-crustal material from Earth and the so-called "giant impactor", shot out into space. Some of it created a ring around the planet which, over the ages, was swept back together to form the Moon. This is called the "giant impactor hypothesis".

Because it was a glancing blow against the Earth, which had already started differentiating with lighter elements near the surface and heavier elements settling to the core, the debris was mostly composed of the lighter, proto-crustal material. Because so much energy was imparted to the blowout debris it became very hot and the lightest elements, up to and including carbon and nitrogen, were lost to space. When the remaining light, but refractory (high melting point), material condensed and consolidated to become the Moon, it lacked both heavy and light elements.

Water and Life

Water is of course a primary component of life as we know it. The near total lack of water on the Moon struck quite a blow to lunar settlement plans. One of the components of water, oxygen, is abundant on the Moon, since many lunar rocks are oxides. It will take energy and machines to win this oxygen. Some hydrogen is also trapped in the lunar regolith, deposited by the solar wind, but it is very thin (Blacic, ref. below, states 100 ppm by weight) (ppm = parts per million). Finding hydrogen deposits in the cold trap areas of the lunar poles, presumably water ice but possibly some other ices as well, such as methane (CH4), has caused many planners to regard the poles as initial base candidates. Such hydrogen as can be found there, in whatever form, should probably not be squandered, but kept religiously in the lunar economy, being circulated and recirculated as water, carbohydrates, etc.

Water and Fuel

Some people see the hydrogen deposits at the lunar pole cold traps as a source of cheap rocket fuel. This would be extremely wasteful of a vital life support resource, at least until such time as replacement hydrogen can be had from other extraterrestrial sources such as comets and outer moons. Other substances, such as aluminum or magnesium and oxygen can be used for rocket fuel. We have plenty of those.

Water and Glass

In Lunar Bases and Space Activities of the Twenty-first Century (W.W. Mendell, ed., 1985), James D. Blacic of Los Alamos National Laboratory wrote about "Mechanical Properties of Lunar Materials Under Anhydrous, Hard Vacuum Conditions: Applications of Lunar Glass Structural Components" (p.487). He states that, "Hydrolysis of Si-O bonds at crack tips or dislocations reduces the strengh of silicates by about an order of magnitude in Earth environments." This means that lunar anhydrous glass is about an order of magnitude (10x) stronger than Earth glass we are familiar with, and can be useful as a structural component. Experiments confirm this. Anhydrous lunar glass or glass composites can be made into "a lightweight structural material with several hundred thousand psi tensile strength."
This also has implications for geology and material handling on the Moon. The glass fraction of regolith will be much harder than we would otherwise expect, and this will make tools and machines wear more quickly. In geology, it implies that the glass matrix component of lunar basalts (about 52% [1]) is much stronger on the Moon than on Earth, and this may translate to a much wider span being supportable than the roughly 340m theoretical maximum based on simple extrapolation of Earth basalt to the Moon (an order of magnitude, Earth maximum being approximately 30 meters). This possibility is supported by circumstantial evidence (Coombs & Hawke, 1992) that lunar lavatube caves may reach a kilometer or more in span (diameter). Note that due to the evidence of flowing water on Mars, its basalt may be no stronger than Earth basalt, and maximum size of its lavatubes may be roughly 150 meters (back of envelope calculation).