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The Origin and Composition of the Moon

It is important to first understand what the Moon is made of, and how it compares and differs from Earth and asteroids. To more fully understand this, it helps to know how the Moon came into existence and the general processes that occurred in its history.

Astrogeology is also discussed in the section on asteroids, but this section does not require that you read the other astrogeology section, though it may be helpful to read the latter.

Mystery solved -- where the Moon came from

The mystery of the origin of the Moon has been solved by modern science. The first clue regarding the origin of the Moon is the fact that the Moon is getting further and further away from the Earth with each orbit. It's a very slow rate of escape and it will be billions of years before Earth will lose the Moon. It's a well understood and measured phenomenon.

In fact, if you go backwards in time to see how far the Moon was from the Earth in the early days of the solar system, starting 4.6 billion years ago, you find that the Moon would be extremely close to the Earth if not part of the Earth at that time.

The best way we can theorize this happening is if the Moon were made of material blown off of the Earth by giant asteroid impact(s), and that all the material floating around in low Earth orbits created a big cloud or ring around Earth (much like Saturn and Jupiter's rings). Eventually, the material stuck together by gravitational force, making the Moon.

This is called the "Collision-Condensation Theory" of the Moon, and is the generally accepted modern theory.

(Notably, other moons in the solar system orbit around their planet's equator, just like the planets all orbit in the plane around the sun's equator. However, Earth's Moon is unique because it does not orbit around Earth's equator. It orbits in the plane of the Sun's planets, as you would expect from material blown off of a planet by asteroid bombardment coming from the plane of the other planets. Of course, this is fortuitous because it puts the Moon in the plane of present day asteroids in the solar system, thereby making it easy for the Moon to give gravity assists to asteroid payloads coming in from interplanetary space, as discussed in chapter 3.)

The Apollo and Luna samples have further supported this theory.

The structure of the Moon

The origin of the Moon has several fundamental effects on lunar geology.

First, the Moon is made of lighterweight material blown off of the Earth's surface, and is poor in materials from the Earth's mantle and core. We see this in the aluminum-rich lunar highland geologies. We also know by measuring the mass and density of the Moon by Apollo and other scientific instruments. Overall, the Moon is not very dense. The Moon does not have a large metal core, unlike Earth, as we know from seismic studies on the Moon, though later studies suggest it does have a small heavy, metal-rich core (if not pure metal). (In terms of percent, there's much more metal in asteroids on average than on the Moon.) The materials available from the Moon for building things in space are generally the lightweight silicate and metal oxide minerals of the lunar crust.

Second, the Moon is extremely poor in volatiles of all kinds, with the exception of permanently shadowed lunar polar craters. The Apollo soil and rock samples and various other scientific studies show that the Moon is deficient not only in water but also very deficient in compounds containing carbon, potassium, sodium and chlorine. That would be expected from a planet that formed late, after the Sun had already gotten big and started shining, and the clouds of interplanetary dust had already been gobbled up by planets so that the sun shined through. The infant sun would have caused the dust rings around Earth to lose much of their volatiles before they had a chance to accrete again to form the Moon.

Third, this lack of volatiles not only means deficient lunar supplies, but also means that certain ore forming processes that occurred on Earth could not have occurred as commonly if at all on the Moon. Certain volatile gases helped in the formation of ores on Earth, e.g., by dissolving and depositing certain minerals, and in making molten magmas more fluid.

Fourth, the Moon is small, the Earth being 81 TIMES as massive as the Moon. Hence, plate tectonic forces were not as strong on the Moon. Also, smaller bodies cool off quicker (in effect having less insulation). The Moon was molten when it first formed, but it cooled off quicker and underwent less metamorphosis than Earth. There's less chance for a diversity of deep layers to have been exposed on the surface. The lack of weather means no sedimentary ores as on Earth, and no exposed ores due to weathering (though cratering can expose surface strata, too, albeit not nearly as deep as folding mountain geologies).

The Moon has practically no folding mountains or volcanoes, and the landform geologic activity still active today on Earth's surface died out on the Moon billions of years ago. Many of the long, slow geologic processes that created many of the ores on Earth today did not occur on the Moon's surface.

Lunar mountains were caused by material splashed up by giant asteroid impacts and the accretion phase.

Distribution and densities of ores etc

In general, much less diversity of ores is expected on the Moon, according to the opinions of many geologists.

On the other hand, it should also be noted that some of Earth's richest ores are from its oldest geologies, all lopped together under the name Pre-Cambrian, which means older than 0.5 billion years.

Earth rocks are typically 10 million to 0.5 billion years old. (The "age" of a rock is the time since it last solidified from molten liquid.) The very oldest Earth rocks are 3.5 billion years old, and are rare specimens. In contrast, practically no Moon rocks are younger than 3.1 billion years old. The Moon rocks brought back to Earth are ALL quite old, well preserved rocks, with highlands rocks having solidification dates as old as 4.48 billion years ago.

The slow cooling of early molten planetary material generally causes separation of different elements and minerals at different temperatures as the magma cools slowly over time. The magma cooled much slower (due to nuclear radiation) and in a less disturbed way on the Moon, compared with Earth volcanism. This could possibly have created some special ores. While the oldest crustal ores on Earth have mostly been eroded, dissipated or buried on Earth eons ago, they're well preserved on the Moon.

There's little physical evidence of an original Earth crust. Much more recent Pre-Cambrian rock layers have been exposed due to uplifts and erosion, and at some places at the continent's edge. However, these Pre-Cambrian rocks aren't as rich as the lunar highlands in aluminum and calcium, nor are the oldest known Earth rocks, on average.

Of course, it's possible that some unusual and unpredicted ores could be discovered on the surface of the Moon, though we can't depend on that.

Extraterrestrial deposits of ores on Earth

The Republic of South Africa, the richest country in mineral wealth (non-fuel minerals), is largely a Pre-Cambrian geology. Witwatersrand ("the Rand") in South Africa is by far the single richest gold producer in the world. South Africa has half the world's platinum group metal resources, and most of the world's chromium resources. Deep gold mines in Brazil and India, and the extraordinary deposits of Kalgoorlie in Western Australia, are of Pre-Cambrian origin, as is Siberian gold. The two most important deposits of uranium-bearing minerals are found in the Pre-Cambrian rocks of the Belgian Congo and northern Canada.

Of course, the Sudbury Astrobleme in Ontario, the geology that produces more than half of the world's nickel and yields cobalt and platinum-group metals, is a well preserved Pre-Cambrian asteroid impact crater of massive size. It's possible that the slowly cooling magma oceans produced by asteroid impacts on the Moon could have concentrated exotic asteroid elements. But we won't know until we go there to investigate.

The role of water in ore formation -- critical or optional?

Earth literature often suggests or implies that water is vital in ore-forming processes, even in igneous processes (those resulting from cooling of molten rock), because water dissolved in a magma increases fluidity (i.e., decreases viscosity) and encourages elements to move around and become concentrated. However, water is not necessarily vital for ore production. Besides asteroid crater deposits, the Moon has plenty of its own sulfur. Besides, atoms and minerals move around in a dry magma, albeit slowly, and an undisturbed magma that stayed molten for millions of years could produce some interesting results. Experiments still haven't been done on various "dry magma systems" to see what processes occur; such experiments haven't received support largely because they aren't seen as commercially relevant to Earth ores and magmas. But 3 billion year old preserved magma lakes exist only on the Moon. On Earth, they're long gone.

We should guard against being overly biased by Earth ores. Only a substantial post-Apollo survey will tell us for sure whether there are any special ores on the Moon. If aliens sent half a dozen small Apollo-style scouting expeditions to Earth, it would be extremely unlikely that they would find any of our great ores. A post-Apollo survey could possibly find fantastic ores that have no equivalent on Earth. Ore formation processes have been unveiled and understood usually by hindsight after discovery and analysis, not by prediction. Many ores are still not well understood, e.g., in Pre-Cambrian South Africa deposits. Indeed, the Apollo and Luna samples have given us many surprises, and lunar geology has turned into a very complex field with many mysteries.

However, we can't count on finding any special ores when it comes to investing to establish the initial space based infrastructure, building solar power satellites, etc..

Every major study into mining and processing lunar material assumes that we will use only what Apollo samples offer.

Nonetheless, the Apollo samples show that there are minerals abundant in the common lunar soil which are fairly easily processible to produce major quantities of fiberglass, ceramics, clear glasses, aluminum, calcium, iron, magnesium, titanium and chromium, as well as other materials -- the basic building blocks of space development.

For the basics of space development, we don't need anything exotic.

However, bulk supplies of volatile elements, e.g., hydrogen, carbon, sulfur, will probably need to come from asteroids. One exception is that hydrogen could come from the permanently shadowed lunar polar craters based on the discovery of ice by the Clementine and Lunar Prospector probes, as discussed later. > Lunar Resources (Mining The Moon) > Origin and Composition

Additional, children pages of this current parent page:

Origin and Composition :
  Geology, Highlands vs. Lowlands
  Apollo and Luna Samples
  Major Minerals
  Polar Water and Volatiles
  Non-Polar Volatiles
  Powdery Texture

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