Asteroids, Meteorites, and Geologic Processes
Detailed chemical and isotopic studies of meteorites show that they all come from the same original “pot” of material, but that different meteorites have experienced different amounts of heating and melting. Here is a simplified description of how the meteorites are related to each other and what geologic processes have affected them:
Our Sun formed at the center of a collapsing cloud of gas called the solar nebula. As the Sun shrank to its final size, the surrounding cloud formed a hot disk with the Sun at its center (see upper left panel in Figure 1 below). Initially, the Sun and the disk had the same composition—mostly hydrogen and helium, with a few percent of all other elements—that they inherited from the giant interstellar cloud in which they formed. As the disk cooled, different compounds condensed from the gas into solid grains. Grains forming near the Sun contained only iron and rocky minerals. Grains forming beyond the orbit of Mars included carbon compounds and water. Some of the grains melted by an (as yet) unknown process to form the drop-like chondrules. The grains rich in carbon and water contained the same elemental abundance as the Sun, and so can be considered “sun stuff” minus hydrogen and helium that mostly remained as gases in the nebula.
Over time, gravity caused the grains and chondrules to clump together into primitive asteroids. The “sun stuff” materials in these original asteroids are now called carbonaceous chondrites (classes C1, C2, etc., asteroid at upper right in Figure 1 ). These original asteroids formed in a range of sizes, from tiny rocks to mini-planets hundreds of kilometers in diameter. Different asteroids were heated by different amounts. Asteroids that were hardly heated at all stayed cold—far below freezing. These kept their compounds—including carbon and water—almost unchanged. These are the “parents” of the C1-type meteorites. Other asteroids were heated slightly, so that some of their water and carbon were driven out of the grains. The water and carbon were lost to space because of the low gravity on these small bodies. These asteroids became the parents of the C2 meteorites. Still other asteroids were heated enough to drive out most of the carbon and water, and even cause serpentine (a mineral containing chemically bound water) to metamorphose into the mineral olivine. These asteroids became the sources of C3 and C4 meteorites. We can still see tiny fissures in some carbonaceous meteorites through which the ancient water flowed. The fissures are now filled with soluable minerals that were left behind as the asteroid dried out.
Many originally carbonaceous asteroids were heated to interior temperatures of several hundred degrees Centigrade, sufficient to boil off all the water and carbon. The remaining rocky materials did not melt, but were heated sufficiently to cause the original minerals metamorphose into different minerals depending on the amount of heating. In spheroidal bodies like the asteroids, the center reached a higher temperature than the surface layer, so different minerals formed at different depths. These asteroids became the sources of the various types of ordinary chondrite meteorites (asteroid at middle-left in figure 1).
Finally, some asteroids were heated so much that they melted. If an ordinary chondrite asteroid melts, all the original structures like chondrules disappear and the melted materials separate by density (a process called “differentiation”) to form a three-layered body (asteroid at lower right in Figure 1). The dense iron and nickel sink to the center, surrounded by a “mantle” of less-dense olivine and pyroxene. There is some mixing of the iron and olivine at the core-mantle interface. The least-dense minerals rise to the surface and flow out in massive volcanic eruptions to form a basaltic crust. The iron core, insulated by many kilometers of rock, cools slowly and solidifies over periods of tens of millions of years. These asteroids become the sources of several different types of meteorites: iron meteorites come from the iron cores, stony-irons come from the core-mantle boundary, mantle-like achondrites come from the mantle, and the basalt-like achondrites come from the crust.
If an asteroid were heated long enough and hot enough, then granite-like rocks would have formed, as on Earth (asteroid at lower left in figure 1). However, no granite-like mineral has yet been found in any meteorite, so no asteroid reached this last stage of modification.
After the formation of these different types of asteroids, impacts kicked pieces of them into space. Some of the pieces ended up in orbits that intersected Earth, and those became the meteorites in our collections. The different compositions and structures of meteorites show that many of the geologic processes familiar on Earth operated on the asteroids as well. Carbonaceous chondrites are sedimentary rocks, some of them showing the effects of minerals being dissolved, transported and redeposited by flowing groundwater. Ordinary chondrites are metamorphic rocks changed by heat and pressure. Irons, stony irons, and achondrites are volcanic rocks formed out of large-scale differentiation.