Near Earth Asteroid / NEO Geology
This section is for those who wish understand what near Earth asteroids and NEOs are made up of, their geologies. It's more technical than most, and can be skipped. Of course, the author finds this material fascinating, and I hope the reader will likewise!
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 with a significant gravitational field. 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 (and very hot down there).
For example, the Earth's crust has a maximum thickness of 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 homogeneous 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 common light brown dirt and rocks under your feet may be largely "feldspar" (the name comes from "field spar" in German), and are such things as calcium-aluminum-silicon-oxides (CaAl2Si2O8) and various related minerals. Redder dirt is richer in iron silicates and oxides.
The two processes, gravitational differentiation and chemical fractionation, 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 mainly 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 thought to be rare or practically nonexistent in Earth's core. Most of the gold, platinum group metals, cobalt, and so on sank 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:
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 or both bodies in the collision. The degree of dispersal depends mostly upon the relative sizes of the two colliding bodies and their relative velocity.
Volatile elements tend to escape in vacuum and boil away on the surfaces of small bodies close to the Sun. At distances as far as the asteroid belt, asteroids can retain some volatiles in their subsurface material, though the surface material will tend to be devoid of most volatiles.
Near Earth Comets
Comets come from the extremely cold outer reaches of the solar system, where water and other substances which are liquid on Earth are hard rocks out there. For example, quartz crystal on Earth is silicon dioxide (SiO2) and very hard. On a comet, "water" would be the rock di-hydrogen oxide (H2O), and a very hard rock. Likewise for some other "volatiles" -- they're not "volatiles" out there, they're rocks.
They may be made up of a majority of these volatile rocks, quite the opposite of the inner solar system asteroids.
Comets are bodies which fall in towards the Sun after a gravitational perturbation or collision. At their great distance, where the Sun's gravity is weak, they are moving slowly and it doesn't take a lot to get one to drop down.
The tail of a comet is a display of the volatiles blowing off of a comet.
Many near Earth objects (NEOs) are believed to be what's left after a comet has been captured by the inner solar system by gravitational interaction with planets. It is expected that the outside volatiles would boil away, but the interiors would still retain their volatiles.
Probe observations of comets has revealed powdery surfaces and jets in some places.
Some flyby probe observations of asteroids have revealed very dark, apparently carbon rich surfaces.
It is expected that these objects would be very fluffy and potentially easy to mine without the need for rock breaking and crushing machinery.