Mining in Manitoba

Asteroid Mining


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Asteroid Composition

There are estimated to be about 200,000 sizeable Apollo, Amor and Aten asteroids, and millions of Main Belt asteroids.

The vast majority of Near Earth asteroids are thought to have originated in two ways:

  • - asteroids from the Main Belt (between Mars and Jupiter) which dropped down to lower orbits due to gravitational perturbations by Jupiter followed by Mars or inner planets, or occasionally due to a collision with another asteroid in the Main Belt.
  • - comets from the outer reaches of the solar system that were captured when they passed close to a planet or planets in the inner solar system. These would probably be volatile rich.

It is generally estimated at this time that more than about 25% of the near Earth asteroids are captured comets, and the rest came from the asteroid belt.

Astrophysicists' calculations have led to a consensus that over the next 100 million years, most near-Earth asteroids will have been thrown back out by close gravitational encounters with the inner planets or will collide with the inner planets. Conversely, a new supply will be constantly generated from the Main Belt and incoming comets will replace these losses.

The Minor Planet Center at the Smithsonian Astrophysical Observatory is responsible for the collection and dissemination of astrometric observations and orbits for minor planets and comets on behalf of the International Astronomical Union.

Asteroid Composition

One and often two processes occured to practically every major body in the solar system in its early years: "gravitational differentiation" and "chemical fractionation" (chemical metamorphosis). Both occurred early in the solar system 4.6 billion years ago when most large bodies were molten or very hot, due to such sources as early radioactivity, the heat of impacts and accretion, and a unique electromagnetic field which existed early in solar system formation.

Gravitational differentiation occurs only in large bodies. Simply put, the heavier elements sink to the center core, whereas the lightweight elements rise to form the crust. The core is made up of free metal, predominantly iron and nickel. The crust consists of lightweight metal oxide silicates. In between these two extremes is the mantle, composed of heavier metal oxide silicates than crustal material.

Thus, the core is made up of much different material than the crust, and the mantle consists of significantly different material than either. Earth's inaccessible core thousands of kilometers down would be a miner's dream if it were accessible, but we have to settle for what is available in the crust, and the deepest mines are only a few kilometers deep.

For example, the Earth's crust has a maximum thickness of less than roughly 100 kilometers, whereas its mantle is 2,800 kilometers thick and its core is about 3,500 kilometers in radius.

Chemical fractionation is the formation of certain most stable minerals. A mineral is formed when atoms of a certain element (e.g., magnesium) tend to bond with atoms of certain other elements (e.g., oxygen, silicon) to form molecules (e.g., magnesium-silicon-oxygen molecules), and clumps of identical molecules stick together in sizes ranging from "grains" (smaller than a few millimeters) to big rocks. These clumps are called minerals. A mineral is a homogenous solid substance having a definite chemical composition, crystalline structure, color and hardness. An example is a greenish mineral called "olivine" which is common in meteorites and asteroids, consisting of magnesium, silicon and oxygen in ratios according to its chemical formula Mg2SiO4. (Olivine on Earth is mainly found not in the crust but in the mantle. The dirt and rocks under your feet are mostly "feldspar" (the name comes from "field spar" in German), and are such things as calcium-aluminum-silicon-oxides (CaAl2Si2O8) and various related minerals.)

The two processes can work in tandem -- lighterweight minerals with oxygen and silicon rise to the surface to form the crust and upper mantle, whereas the heavier minerals and the substances which do not bond with silicon or oxygen (such as gold, platinum, and others, including heavy radioactive heat sources) mostly sink to the core. Then chemical fractionation further occurs among the localized materials.

In hot or molten bodies, the elements can most easily migrate randomly and over time congregate with those other elements and minerals which they are most compatible with. Elements can also compete for oxygen and silicon, two elements which are liked by many other elements. Those elements which cannot compete as well for oxygen and silicon will often be found in heavier minerals or in pure form and sink to the mantle or core.

Similar minerals usually congregate over time in a stable body. However, they can mixed up with other minerals by planetary geologic processes if the planetoid is large enough (like plate tectonics, vulcanism, flood plains and weathering on Earth).

Thus, in planets (including Earth's Moon), some types of minerals and elements are found predominantly in a planetary crust, others in the mantle, and still others in the core.

With asteroids, we have access to collections of elements and minerals in forms quite different than what is available on Earth's or the Moon's crust. A large proportion of the asteroids are remnants of large bodies which broke up due to collisions, giving us a core and mantle.

In contrast, another large proportion of asteroids comes from bodies which accreted but never became big enough to differentiate gravitationally. These generally consist of free metal granules mixed up with various silicates and other minerals, including some interesting substances coming from some unusual geochemical processes.

Two of the most common solid elements in the universe are iron and silicon. Oxygen is also highly abundant and finds itself in solid form in bonds to silicon and metals. Nickel is also common. (Iron and nickel are the most stable nuclear elements, residing at the bottom of a nuclear energy curve.)

Almost all of the nickel and much of the iron on Earth reside in Earth's core and mantle. Iron exists on the surface bonded to oxygen, silicon and sulfur, but never in its free form. However, asteroids are rich in free iron and nickel metal, as well as in the platinum group metals which are rare and valuable in Earth's crust. In fact, about half the world's nickel comes from one mining area in Canada called the Sudbury Astrobleme where a giant asteroid impacted Earth in prehistoric times. The Sudbury Astrobleme also produces platinum group metals which are separated from the nickel.

The ratios of the elements and minerals vary markedly from asteroid to asteroid, due to the diversity of origins and histories of asteroids.

For example, in an asteroid that did not differentiate due to gravity, the oxygen was gobbled up by the elements silicon, magnesium, aluminum and calcium, leaving out most of the iron and other metals and elements which could not compete for the oxygen in terms of strength of molecular bonds. The result is granules of free metals and other substances mixed with traditional silicates.

Earth's crust averages 47% oxygen (bound in minerals, not including water or air), 28% silicon, 8% aluminum, 5% iron (mostly bound with oxygen though sometimes with sulfur to form "fool's gold"), 3% calcium, 2% magnesium, 5% sodium and potassium, and tiny fractions of other elements. All of these elements except iron are are thought to be rare or practically nonexistent in Earth's core. Most of the gold, platinum group metals, cobalt, and so on sunk to the Earth's core, along with the heavy radioactive elements such as uranium which help keep the core and mantle very hot. Some of the traits of gold and platinum with make them valuable as jewelry -- their bright shine -- are the same traits that make them rare -- they don't tarnish (i.e., oxidize) in a short time. In contrast, iron "rusts" in the presence of oxygen and water. Notably, Earth's mantle consists of two distinctly different layers, as does its core, but the boundary between the mantle and core is apparently due to conversion of the material to pure metal.

Geochemists have generally classified elements into four basic mineral categories:

  • "Lithophile" (Greek for "rock-loving") elements are chiefly found as oxide and silicate rocks and dirt, and include silicon, aluminium, iron (as oxide, not free iron), calcium, magnesium, sodium, potassium, and some trace elements. These are all abundant in planetary crusts. (Notably, earth's crust is called the "lithosphere".)
  • "Siderophile" (Greek: "iron-loving") elements tend to dissolve in free iron when it's available. These include nickel, the platinum group metals, cobalt, copper and gold. These are primarily found in the planet's core, with little left in the crust.
  • "Chalcophile" (Greek: "sulfur-loving") elements frequently bond to sulfur. Many siderophile elements are also found to be closet chalcophiles when given the chance. (There's more iron than sulfur in the Universe, so when the sulfur's all taken then many elements have to settle for iron.) Ores of copper, gold, lead and zinc are found either as sulfides or in sulfur rich areas in Earth's crust. Most chalcophiles reside in the Earth's mantle, since they are of medium density, but they bubble up to form "veins" of ore wherever a concentration of hot sulfuric compounds start to leak up thru cracks and crevasses. Sulfides average about seven percent of some classes of asteroids, whereas they are a negligible fraction of Earth's crust and are found only in unusual spots on Earth.
  • "Volatile" elements, or "volatiles", are found (or lost) predominantly in the vapor, frozen or liquid stage, and include hydrogen (e.g., bound with oxygen as H2O), carbon, sulfur, nitrogen, and chlorine. Due to their "volatility", these elements and molecules rarely survive the heating of a meteor passing thru Earth's atmosphere and their concentration and rapid heating often causes meteors to explode in the air. Volatiles are believed to exist both as ices and attached to minerals of many kinds by a loose chemical bond, easily liberated by heating.

Finally, many asteroids are probably not homogenous but consist of mixed materials from more than one asteroid, or from more than one part of a large asteroid. When two asteroids collide, much of the debris can fall back together to form an asteroid composed of rubble from the parent body. The degree of dispersal depends mostly upon the relative sizes of the two colliding bodies and their relative velocity.

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