§ 1.3 Meteorites -- Samples of Asteroids

§ 1.3.1 Origins of meteorites

The meteorites we study are only those that survive passage through the Earth's atmosphere. Meteorites hit the atmosphere at typical speeds of 15 kilometers per second. They are subject to tremendous pressure forces and heating. Most break apart, especially those which have a former comet as their parent body. However, a large number of meteorites have fallen to Earth and been identified as nonterrestrial bodies. Nickel-iron meteorites and stony meteorites predominate in this mix.

A large class of meteorites are nickel-iron blobs which come from parent bodies which were gravitationally differentiated. By scientifically analyzing the pure nickel-iron asteroids, we can tell how long ago their parent body was broken up, the magnitude of the impact that broke up the parent planetoid, and the rough size of the parent planetoid. (The time of collision is determined by cosmic ray nuclear dating methods -- how long fragents have been exposed to cosmic rays after their crystalline formation. The magnitude of the collision is shown in the crystalline strain patterns, as well as by jammed together incompatible mineral grains in stony meteorites. The diameters of the parent bodies is determined by the crystalline structure of the metal alloy which reveals temperature and cooling rates. There are other methods as well which reveal, for example, how long stony meteorites have been solid.)

Interestingly, about 40% of of the pure metal alloy meteorites came from only two collisions, one at approximately 650 million years ago, and the other at about 400 million years ago, as sorted by cosmic ray dating methods. (When sorted by collision pressures and other catastrophic processes, these samples again sorted pretty neatly into the same two piles, reflecting the differences in magnitude between the two collisions.)

All of the numerous 650 million year old metal fragments show shock induced pressures of more than two million pounds per square inch, i.e., 250,000 tons per square foot, an immense collision indeed. Several nonmetal meteorites, which were probably near the surface of this impact, show that they experienced shock pressures of seven and a half million pounds per square inch. Samples commonly show shock heating of more than 1000 C (1800 F).

Metal meteorites have generally indicated parent bodies at least several hundred kilometers wide.

Many stony meteorites have generally reflected that surfaces of their parent bodies solidified around 4.6 billion years ago, about the same age as the early solar system and the oldest rocks from the Moon. (The Earth's crust has gone through continuous metamorphosis and mixing due to plate tectonics and weathering, so that there are few rocks near that age.)

One of the most authoritative asteroid researchers, Tom Gehrels, noting several areas of evidence, writes: "Chemical studies show that most meteorites come from as few as 4 to 30 parent bodies. Properties of iron meteorites indicate that their parent bodies were at least several hundred kilometers across."

In the early solar system, material accreted from the gravitational attraction of tiny bodies to each other. Big bodies attracted the most material. However, if two big bodies struck each other at high enough velocity, they risked breaking each other up into smaller bodies.

Our catalogs of asteroids' orbits and light spectra show several "families" of asteroids which have similar orbits (indeed, some orbit each other, too) and similar compositions which deviate marketly fromother asteroids in that orbital zone around the Sun. For example, the populous and well defined Themis family consists of a "core" of large asteroids surrounded by a "cloud" of smaller objects, which could be concluded to be the remnants of a planetoid at least 300 km in diameter. The Eos family, composed of two objects of about 90 km diameter, plus a few of around 50 km and many smaller chunks, was larger, around 550 km diameter by current estimates.

The dynamics of an asteroid collision could produce a variety of results, ranging from complete dispersal to formation of mixed bodies. For example, simulations have shown that collisions of 5 km/sec between bodies of 50 km and 200 km diameter may produce a cloud of shattered material most of which eventually falls back in on itself through gravitational attraction and collisions over time to produce a couple of bodies of shattered material around 150 km in diameter.

However, most collisions between asteroids in the solar system today would result in dispersal of the vast majority of material, since escape velocities are so low for most asteroids.

Much more frequent than big asteroids hitting each other are small impacts which serve to chip away at the outside surface of a large asteroid, with the crater ejecta escaping the asteroid's feeble gravitational field. In fact, telescopic spectroscopy reveals a few large asteroids apparently with large quantities of free metal at the surface, possibly the result of the stony exterior being chipped away over the eons to expose the mantle and core.

Asteroids are likely to be crumbly material due to all the shock waves of impacts. Whatever material was not blown off is likely to be pulverized.

Photographs of Mars' two tiny moons, which are themselves captured asteroids between 8 km and 28 km wide, show pulverized surface material, cracks, and steam tubes from volatiles that vaporized from the energy of impacts. One of Mars' moons, Deimos, is U-shaped when viewed from one angle.

As one can see, there are a great variety of sources for asteroidal material.

Next, we look at the composition of meteorites that fell to Earth's surface, since this is largely what we have to go on when we think of designing space industrialization around asteroidal materials. However, we may visit the asteroid with a probe before designing a specific mission to go there for materials retrieval.

§ 1.3.2 Meteorite classifications and compositions

Most meteorites fall into one of four categories. The first three categories apparently have their origins in parent bodies that were gravitationally differentiated, as opposed to the fourth category.

• "Iron meteorites", also called "irons", are usually just one big blob of iron-nickel (Fe-Ni) metal, as if it came from a industrial refinery without shaping. The alloy ranges from 5% to 62% nickel from meteorite to meteorite, with an average of 10% nickel. Cobalt averages about 0.5%, and other metals such as the platinum group metals, gallium, and germanium are dissolved in the Fe-Ni metal. (Fe is the chemical symbol for iron.) While most "irons" are pure or nearly pure metal, the technical definition of an "iron" includes metal meteorites with up to 30% mineral inclusions such as sulfides, metal oxides and silicates. The irons represent the cores of former planetoids.
• "Stony irons" consist of mixtures of Fe-Ni metal of between 30% and 70% along with mixtures of various silicates and other minerals. The Fe-Ni metal can be present as chunks, pebbles and granules. Stony irons resemble the outer cores or mantles of planetoids or else a mix of materials due to a collision.
• "Achondrites" are silicate rich meteorites apparently formed by crustal igneous (i.e., molten or volcanic) activity in their parent bodies, and consist of a broad range of minerals. Achondrites are the result of gravitational differentiation in relatively large bodies by melting and gravitational separation of mineral phases, and most resemble the Earth's crust. Different types of achondrites average between 0 and 4% free Fe-Ni granules.
• "Chondrites" probably came from parent bodies that were too small to undergo a large degree of gravitational differentiation, or are collision ejecta from less than catastrophic collisions of slightly differentiated bodies. Chondrites are named after the tiny pellets of rock called "chondrules" embedded in them, a result of a kind of chemical fractionation unique to small bodies. If you were walking around in a field and saw a chondrite, it would be much more recognizable as being of nonterrestrial origin than the above achondrites.

There are different subcategories of chondrites.

Chondrites generally show much cooler histories than other meteorites. Some chondrites appear to be from noncollisional origins, e.g., a small archaic accretion.

Chondrites are crumbly, composed primarily of various silicates, with an Fe-Ni free metal content between 0.3 and 35%. Chondrites are often classified according to their free metal content:

• "Enstatite" (E) chondrites are around 35% free Fe-Ni granules.
• "High iron" (H) chondrites average about 19% Fe-Ni.
• "Low-iron" (L) chondrites average 9% Fe-Ni.
• "Low iron, low metal" (LL) and "high iron, low metal" (HL) chondrites are a technical scale that reflects different abundances of free metal versus metal oxides, in the neighborhood of 5% Fe-Ni granules plus about 15% to 30% iron oxide in minerals (e.g., magnetite, silicates), due to the level of oxygen depletion in the silicate mix.

The nonmetal ingredients of meteorites consist predominantly of silicates, oxides and sulfur minerals, which can be typically broken down as follows: silica (SiO₂) typically between 35% and 40%, magnesia (MgO) between a whopping 20% to 25% (in contrast to Earth's surface), aluminium (Al₂O₃) between only 2% and 3% (in contrast to Earth and the Moon's crusts), and calcia (CaO) around 2%. Iron sulfide (FeS), also called "troilite" (and "fool's gold"), usually occurs as around 6% of these meteorites.

As regards the precious (and "strategic") metals such as the platinum group, cobalt, gold, gallium, germanium, and others, the lower the Fe-Ni metal content, the more enriched the Fe-Ni metal is in these rare and precious metals and elements. These elements readily dissolve into the metal that exists, and the less metal that exists, the less diluted they are. Many asteroids are richer in most of these precious metals than the richest Earth ores which we mine. Further, these metals all occur in one ore when it comes to asteroids, not in separate ores. As discussed later in this chapter, the exact same process used to extract and separate these precious metals from the world's largest nickel ore mine at the Sudbury Astrobleme in Canada is easily used in space, and is a simple process using only carbon, sulfur and oxygen, all of which can be derived from asteroids, too.

Some chondrites are poor in volatiles, while others are rich in volatiles, such as water and carbon.

There are subcategories of chondrites, called "carbonaceous chondrites", which are further split up into five categories:

• "C1 carbonaceous chondrites" average about 10% water in a clay mineral matrix and as water of hydration (often in magnesium salts, 5% to 15%), 2% to 5% carbon in the form of graphite, hydrocarbons and organic compounds, several percent sulfur in elemental, iron sulfide and water soluble sulfate forms, some nitrogen and other volatiles, and 5% to 15% magnetite.
• "C2 carbonaceous chondrites" have very little magnetite, a little less water, carbon, and sulfur, and about 10% soluble sodium and magnesium salts, all in a mineral assemblage.
• "C3, C4 and C5 carbonaceous chondrites" are not really "carbonaceous" as their name implies but instead are very poor in water, carbon and other volatiles, but have other semblances to C1 and C2 carbonaceous chondrites.

It should be noted that there are meteorites that defy categorization in that they are significantly different from meteorites in any of the above classes. Some potentially new classes of meteorite are represented by only one specimen.

Two classes of unusual meteorites which have more than one unrelated specimen are worth noting:

• "Carbonados" have tiny black diamonds produced by the shock of astronomical impact on a carbon rich body.
• "Ureilite" achondrites typically contain about one percent industrial grade diamonds.

Likewise, there are sometimes major compositional differences within established classes of meteorites, and frequent overlap between classes in terms of the mineral distributions. For example, some achondrites have a few percent carbon. Some chondrites have veins of water soluble salts as if deposited by movement of water as in hydrothermal processes that occured on Earth. Enstatite meteorites (above, listed as a class) often contain large amounts of exotic and strange substances, such as titanium nitride, silicon oxynitride, and unusual metal sulfides.

Some readers from the mining industry are probably asking what mineral distributions exist in meteorites. Below are the approximate properties of four different typical asteroids which probably exist, based on four meteorites. (Chemical analysis in weight percent. Extracted from NASA SP-428, except for the iron meteorite.)

The big caveat to the above analysis is that meteorites vary dramatically in composition, and the above is only a sample meteorite from within just four categories.

§ 1.3.3 "Falls" vs. "Finds"

There is also a major caveat when extrapolating from meteorites to asteroids. Most meteors break apart, disperse or burn up in Earth's atmosphere. Volatile rich meteors usually explode due to heating during the fiery atmospheric entry. Soft and crumbly chondrites would be expected to break apart often, even if they were volatile poor. Irons and stony irons punch through intact much more commonly than other meteors.

Those which wind up in laboratories come in two categories: "finds" and "falls":

"Finds" are meteorites which were found on the ground unrelated to any sighting, due to the finder recognizing them to be clearly identifyable as being of nonterrestrial origin.

"Falls" are meteorites which were seen to fall from the sky and which were tracked down successfully. Meteorites found on top of the snow in Antarctica are also classified as finds. Some finds come from rooftops and the like. A few have come from cars and the street.

Because of all of these biases, analysts must also rely on studies performed by analyzing the light reflected off the surfaces of asteroids, and some future asteroid rendezvous missions. The only way to eliminate bias is to study the asteroids themselves.

Suggested reading

Further analyses of meteorite compositions are beyond the scope of this presentation. If you are interested in further information, you can try the following for starters:

The Lunar and Planetary Institute (LPI) has databases on meteorites.

The Cosmochemistry Group at the University of Arkansas performs laboratory studies of meteorites, which helps in developing theories on the chemical and physical processes which could have occurred on asteroids. This group also operates the WWW home page for the journal Meteoritics & Planetary Science, founded in 1953 and one of the leading journals in the field, published by The Meteoritical Society.

A couple of charming little sites are the Swiss Meteorite Lab and the Dutch Meteor Society (DMS).

Back to Asteroids Near Earth - Locations

Up to top

Forward to Asteroids Near Earth - Spectroscopy