What are Asteroids Made of: The Meteorite Connection

Astronomers discover asteroids by detecting the sunlight they reflect. Just by observing the light from an asteroid over time, we can calculate its orbit, size, and shape. But how can we tell what asteroids are made of? One way would be to send a spacecraft to an asteroid, pick up a piece of it, and bring it back to Earth. One spacecraft mission is doing just that: Japan’s Hayabusa spacecraft visited the tiny asteroid Itokawa in 2005 and will return a piece of it to Earth in 2010. Scientists at NASA and the European Space Agency are planning similar missions for the near future. Those missions will supply samples from only a few asteroids. What about the thousands of others?

As turns out, we may already have samples of asteroids here on Earth already: the meteorites! Scientists recognized that meteorites came from outer space only about 200 years ago. But where in space did they come from? In the 1950s and 1960s, networks of cameras recorded the paths of infalling meteorites on film for the first time. The pre-impact orbits of these meteorites were calculated. Their orbits led back to the asteroid belt itself!

Pre-impact orbits of four asteroids

Computed pre-impact orbits of four meteorites shown in green.

So studying meteorites will tell us about the compositions and geologic processes that occurred on asteroids in general. However, determining which meteorites came from which asteroids requires additional measurements (visit the sections on Asteroid Spectra and Asteroid Colors). Now we get to know the different types of meteorites: what they look like and what they are made of.

There are many different types of meteorites, just like there are many different types of terrestrial rocks, and at first, meteorite names will seem just as strange as the names of terrestrial rocks. Meteorites are usually divided into three major groups: stones, stony-irons, and irons, according to their main composition. The stones are sub-divided into chondrites and achondrites, depending on whether or not they contain small rounded grains called chondrules. Here are brief descriptions of the major meteorite classes. Click on the names to see what they look like. Table 1 is a summary of the physical properties and compositions of the different classes.

  1. Stones are meteorites that look like ordinary earth rocks. And like there are different types of earth rocks, there are different types of meteorite stones.
    1. There are the coal-like Carbonaceous Chondrites that contain water and carbon compounds along with all the other elements — except hydrogen and helium — in the same abundances as we find in the Sun. They are essentially compacted clods of “solar dust.” They crumble easily and some feel greasy. They contain many chondrules (from the Greek word for “grain”). Chondrules were formed as molten or partially molten droplets before becoming part of the meteorite. All meteorites containing chondrules are called “chondrites.” There are four sub-classes of carbonaceous chondrites (C1 through C4) divided according to the amount of carbon and water they contain (see Table 1 below).
    2. Ordinary Chondrites look like typical earth rocks except that they contain many chondrules. They are similar to carbonaceous chondrites in composition except they lack most of the water, carbon, and some volatile (low melting point) elements found in the carbonaceous chondrites. There are several sub-classes of ordinary chondrites, often named after the most abundant mineral they contain (see Table 1).
    3. Achondrites also look like earth rocks. As indicated by the name, they lack chondrules. They were once completely molten and thus contain no water or other volatiles. There are several different sub-classes of achondrites (see Table 1). Some have compositions and appearances similar to terrestrial basalts (dense lavas) and some have compositions similar to rocks from Earth’s mantle.
  2. Stony-Irons are meteorites that grade in composition from droplets of olivine imbedded in pure nickel-iron to droplets of nickel-iron in pure olivine.
  3. Irons: consists of iron with small amounts of nickel and sulfur. These meteorites were once completely molten and, as indicated by sizes as large as several meters for some individual crystal, cooled slowly over millions of years.

Like earth rocks, meteorites have experienced different amounts of heating, metamorphosis, and melting. They are the result of geologic processes similar to the processes making different kinds of rocks on Earth. Since they come from the asteroids, an understanding of the processes that made the meteorites will tell us what geologic processes operated on the asteroids.

Table 1: Meteorite Compositions and Densities

Class and Name ρ gm/cm3 Composition
I. Stones
A. Carbonaceous Chondrites
1. C1 2.2-2.3 18-23% H2O + Carbon and Serpentine
2. C2 2.6-2.9 6-16% H2O + Carbon and Serpentine
3. C3, C4 3.3-3.6 2-4% H2O + Carbon + 70% Olivine
B. Ordinary Chondrites
1. H (Bronzite) 3.6-3.8 25-40% Ol + 15-19% Fe + Bronzite
2. L (Hypersth.) 3.5-3.6 35-60% Ol + 4-9% Fe + Hypersthene
3. LL 3.4-3.5 50-60% Ol + 1-3% Fe + Hy + Feldspar
4. E (Enstatite) 3.5-3.8 45-55% Enstatite + 20% Fe + S + Feldspar
C. Calcium-rich (basalt-like) Achondrites
1. Howardites 3.2-3.3 40-80% Hypersthene + Feldspar
2. Eucrites 3.1-3.2 40-80% Pigeonite + Feldspar
D. Calcium-poor (mantle-like) Achondrites
1. Diogenites 3.3-3.4 ~95% Hypersthene
2. Ureilites 3.3 ~85% Ol + 0.3-6% Fe
3. Aubrites 3.2 ~97% Enstatite + 1% Fe
II. Stony-Irons
1. Pallasites 4.3-5.8 28-88% Fe + Olivine
2. Mesosiderites ~5 40-80% Hypersthene + Fe
III. Irons
1. Octahedeites 7.8-8.0 Fe + 5-15% Ni + 7-10% S
2. Hexahedrites 7.8-8.0 Fe + 4-6% Ni + 7-10% S
Abbreviations: Ol = Olivine, Fe = Iron, Ni = Nickel, S = Sulfur