Moon Bases
Ideas for bases on the moon have been kicking around for decades. Being so close to Earth, and because we have been there and understand its environment, it is a natural first step into the solar system for human colonization. Until George Bush's space directive in July of 1989, however, the idea of a moon base had not reached anything approaching "a national goal".
What are the incentives for going to the moon? It is unlikely that any particular mineral that we could mine on the moon has sufficient commercial value to make it economically viable. The possible exception to this is 3He. This isotope of helium contains one less neutron than normal helium (4He). Nuclear fusion using 3He is much "cleaner" than the nuclear fusion process that uses hydrogen isotopes (though this fusion occurs at a temperature 5 times higher than current hydrogen fusion). It is extremely rare on the Earth, but more plentiful on the moon (where it is captured from the solar wind). It would have a value of approximately $1 billion per ton! A single shuttle load of 25 tons would have enormous commercial value if a fusion reactor using helium could be constructed.
The other two drivers for establishing a lunar base are a site for astronomical observatories, and as a base from which future expeditions into the more distant reaches of the solar system are launched.
Astronomy: The atmosphere on the Earth both blurs images, and absorbs/reflects most of the electromagnetic spectrum. To observe radiation that has a higher energy content than the optical light our eyes are sensitive to requires us to go into space. This also true for substantial parts of the electromagnetic spectrum with lower energy than optical light. Because the moon has no atmosphere, the entire electromagnetic spectrum makes it to the surface (this is one source of danger for "residents" however!).
Technically, these observations can be made from Earth orbit, but the near-Earth orbital environment is "dirty". There is the potential damage from space debris, and a tenuous atmosphere that makes some observations difficult. The other, bigger problem, is the fact that the telescope is free-floating in orbit around the earth. This means that the telescope suffers from vibrations and a rapid day-night cycling. Very long exposures are very difficult to obtain from orbit. By basing these telescopes on the moon, a stable surface with no real atmosphere, would be much more efficient. With its lower gravity, larger telescopes could be constructed on the moon. The lower rotation speed of the moon means the telescopes would be simpler in design. It does mean, howver, that the sun would be above the horizon for two weeks at a time!
Launch Base: The Moon's gravity is about 1/4 that of the Earth, thus it is substantially easier to hoist large payloads off of its surface and into orbit. This could allow the construction of interplanetary spaceship components on the surface of the moon, where construction would be much easier than if attempted in orbit. These components could then be orbited and assembled in a modular form (in a similar fashion to the ISS), and then sent to their final destination.
At present, none of the above reasons has sufficient public interest or support to actually drive the construction of lunar bases. The present high cost of even launching material into Earth orbit makes the costs of a lunar base prohibitive. It is likely that it will be a very long time before we get back to the moon.
The Nature of the Moon
Even to the naked eye, the moon reveals some of its secrets: its surface has both bright and dark areas. To the ancients it was believed that the dark regions contained water, equivalent to the oceans and seas of the Earth. These were given the name of "Mare", Latin for sea.
With the invention of the telescope, it was found that in general, the lunar mare were smooth, while the brighter regions of the moon were rough, containing high mountains, and deep craters.
(Images of mare: Mare near the crater Posidonius, Mare Imbrium).
(Images of craters: Tsiolkovsky,
King, and Eratosthenes.)
The origins of both the mare and craters were under debate at the time of the Apollo missions. The leading idea was that both were due to impacts of the surface by large bodies (e.g., asteroids). In the case of the mare, huge impacts would melt large portions of the surface, and lava would flood the surface creating a smooth appearance. The individual craters would just be smaller impact events.
The alternative theory was that both the craters and the mare were due to normal geological processes, the craters were extinct volcanoes, and the mare were simply large lava flows.
Eventually the impact hypothesis gained support. It was found that the depth of a crater was related to its size-this does not happen in volcanoes (see Figure 2 on page 271). In other cases, material appears to have been "sprayed" out of the craters forming long, light-colored "rays" on the lunar surface that no volcano could create. Once laboratory investigations showed that most of the visible features of craters could be duplicated in the lab, the impact scenario for crater origin gained strength-but until we could actually get material from the surface, we could never be 100% certain.
With the realization that the craters resulted from impact events, we now knew that the early history of the solar system was violent. There was a considerable amount of debris left over from the formation of the planets throughout the solar system. This debris was swept-up by the planets (and their moons). Eventually, most of this material was removed (as the solar system is now fairly "clean"). These ideas meant that the catering rate is a function of the age of a surface. Regions with few craters are obviously younger than those heavily pock-marked regions. That the mare could be due to impacts, however, still bothered quite a few researchers due to the enormous size of the impacting bodies needed to create such features.
With these ideas in hand, we could now begin to trace the history of the moon, as well as that of the inner solar system. If we could get lunar material back to earth from all of the different regions on the moon, we could "date" it using geological techniques, and thus provide an accurate chronology. This was the science driver for the Apollo missions visiting as many sites with differing geologies as was possible.
What have we learned about the formation of the moon from the Apollo missions? Before we went to the moon, there were three main ideas about its formation:
1) The "capture" hypothesis: the moon formed elsewhere in the solar system and was captured by the Earth after a close approach.
2) The "fission" hypothesis: Early in its history the Earth was rotating very rapidly, and that the moon split off (fissioned) from the Earth due to centripetal forces.
3) The "co-formation or accretion" hypothesis: the moon simply formed in place from processes like those that created the Earth.
If we could do a chemical analysis of the moon, we could figure out which of these ideas was correct. For example, both #2 and #3 would suggest that the overall composition of the moon would be similar to that of the Earth. The first hypothesis, however, would suggest that there should be very little similarity in composition to the Earth, since it formed somewhere else in the solar system.
We already knew that the density of the moon was much lower than the Earth (3.3 vs 5 grams per cubic cm). This suggested it formed out beyond the orbit of Mars (where objects of similar density are found today). This is a strong argument, but was countered by the unique circumstances that allowed the moon to be captured-models suggest that it is very difficult to capture a body as large as the moon if it was on an independent orbit in the solar system.
The fission theory gained considerable clout because it explained the moon's density naturally: the moon originated as low-density crustal material. It was very difficult, however, to envisage a rotation rate sufficiently short (a 2.5 hr day is needed) to cause this fissioning. In addition, there are many processes to quickly slow the earth's rotation rate to the point that no fissioning would occur.
The co-formation theory was attractive in its simplicity, but could not explain the density of the moon. If both the Earth and the Moon formed from the same original cloud of gas and dust, they should be very similar in density.
With lunar samples, we could now determine which of these ideas was correct. The data led to a surprise.
If the fission theory was correct, than the composition of the moon should be very similar to the Earth's crust and upper mantle, but the abundances of elements in the lunar rocks suggested that this was not so. The lunar rocks were "under abundant" in potassium, sodium and iron compared to the Earth as a whole, and overabundant in titanium, chromium, aluminum and calcium, and deficient in potassium and sodium when compared to the Earth's mantle. Neither the fission nor co-formation theories were viable.
Yet the rather close agreement in the relative abundances of different elements in lunar rocks did follow the overall trend seen in Earth material-the capture hypothesis was instantly ruled out.
All of the leading formation theories were in trouble!
Out of these ashes would arise the current model for the moon's formation: "the giant impact model". In this theory, a large Mars-sized object has a glancing collision with the young Earth. The impacting object is destroyed, but succeeds in blasting a sufficient quantity of material off of the Earth's surface that coalesces into the Moon.
This model successfully explains all of the data. The moon's lower density is the result that the majority of the material from which it formed came from the low-density crust and mantle of the Earth. Models show that any iron originally in the "impactor", or removed from the Earth, would generally fall back to Earth (most of the iron on earth is in the molten core, as it would it be in a Mars-sized body). Thus, the moon wouldn't acquire much iron (in fact the moon has less iron than any solid object in the solar system).
How do we explain the differences between the moon and the Earth's crust/mantle? The impact of such a large body would generate an enormous amount of heat. This heat would vaporize the rocks, and those elements that are easily turned to vapor ("volatile") would be in a gaseous form during the blast phase. Those elements that need a higher temperature to be vaporized ("refractory" elements) would not be turned to gas. The volatile elements would then escape into space in their gaseous form due to their high temperature. Two such volatile elements are sodium and potassium-this can explain their deficit on the moon relative to the Earth's mantle.
In addition to explaining all of the data, one additional feature is predicted: a high rotation speed for the Earth. Such an oblique collision would speed-up the rotation rate of the Earth. Note that Earth's cousin, the planet Venus, rotates very slowly: one Venus day is the equivalent to 243 Earth days. Venus has no satellites. Mercury also has a slow rotation rate (58.6 days) and no satellites. Mars, on the other hand, has a rotation period similar in length to that of the Earth (24.5 hrs vs 24 hrs), and has two small moons.
These findings have revolutionized our ideas about the early solar system: it was littered with bodies of all different sizes that collided with the planets when they were still young. There were plenty of objects of sufficient size available to create the lunar mare. Thus, many previously unexplained phenomena were finally understood, and this is the true legacy of the Apollo missions.
A (relatively slow) site that has an excellent (somewhat technical) discussion of the origin of the moon can be found here.
