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8.15 Composition of Asteroids

Even before a spacecraft went into orbit around an asteroid, astronomers had a good idea of what asteroids are made of. They could use telescopes to measure the colors, reflectivity, and spectra of asteroids. Combined with radar data, this allowed them to estimate their mineral properties. Using such techniques, astronomers in the 1970s and 1980s discovered that asteroids fall into several groups that differ in composition. The three broad classes of asteroids are stony (S), metal-rich (M), and carbonaceous (C). These are called “spectral types,” because they were originally based on the asteroids’ different spectra. Scientists can match these spectra with those of meteorites, to get indirect measurements of asteroids’ actual compositions.

The main belt of asteroids runs from about 2.1 AU to about 3.4 AU. Within the main belt, the three spectral classes of asteroids are concentrated at different distances from the Sun. Stony and metal-rich asteroids dominate the inner half of the main belt. These asteroids are fairly light-colored, reflecting 10 to 20% of the light that strikes them, like many familiar rocks on Earth. A few additional minor classes of asteroids have also been found in this region. 

In the middle of the asteroid belt, at about 2.7 A.U., lies a “soot line,” beyond which black carbon-rich minerals dominate most asteroids and comets. In other words, most interplanetary bodies beyond 2.7 A.U. are black in color, reflecting only about 4% of the light that strikes them. The sooty carbonaceous materials mixed into these bodies cause their low reflectivity. The two groups of Trojan asteroids clustered in Jupiter’s orbit also have this dark color, as do comet nuclei associated with the Kuiper Belt and Oort Cloud, even farther from the Sun. Some of these outer Solar System objects are actually more dark brown colored than black, probably due to coloration from organic compounds.

Somewhere between 3 and 4 AU is another important dividing line that does not show up as clearly in asteroid appearance. This is called the “frost line,” where frozen water is stable. Asteroids beyond this distance probably contain substantial amounts of ice as well as black soot. In other words, they are really comet nuclei, even though they might not be active. They might become active comets if they were deflected into new orbits that took them into the inner Solar System, where the Sun’s heat would warm up the ices and release gas and dust. In fact, most interplanetary bodies beyond 5 A.U., even if cataloged as “asteroids,” are probably comet nuclei – or potential comet nuclei, with large amounts of ice. (Indeed, several distant objects that were initially classified as asteroids, such as 2060 Chiron at 10 A.U., later gave off telltale puffs of gas, proving that they really are comets. But the official “asteroid” designation stays in place until actual comet emissions have been observed.)

The composition of asteroids depends on their distance from the Sun. This fact shows that the asteroids have not been strongly mixed up; they lie at roughly the distance where they originally formed. The Sun’s radiation varies with the inverse square of distance, so the inner edge of the belt receives (3.4 / 2.1)2 = 2.6 times as much radiation as the outer edge. Some of the bodies closest to the Sun got hot enough to melt, differentiate, and develop iron cores. Asteroids farther from the Sun originally contained sooty carbonaceous material, water molecules, and even ice. They didn’t get warm enough to melt the rock, but we know from mineral studies of carbonaceous meteorites that the ice in some of them melted and percolated through the rock fractures as liquid water. Still farther from the Sun, ice remained solid and produced comet nuclei. From this point of view, comets are merely “icy asteroids.” They formed so far from the Sun, in such cold regions, that they retained much of their original ice, mixed with carbonaceous material.

Ceres, the largest asteroid in the asteroid belt. Click here for original source URL.

Asteroid Ida with its tiny moonlet Dactyl. Click here for original source URL.