(hence the 'V'), and even now only a handful have been added, all of them very small, and in orbits that suggest they are collisional fragments of Vesta. Classes R and Q also contain only a handful of members.

About 80% of classified asteroids fall into the S class, and about 15% into the C class. Members of the C class have geometrical albedos in the approximate range 2-7%, so they are very dark. Ceres, the largest asteroid, is C class, with an albedo of 7.3%. □ From Figure 3.7, how would you characterise the colour of S and C class asteroids? At visible wavelengths the reflectances of C class asteroids do not vary much with wavelength, so they are rather grey. The slightly greater reflectances at longer wavelengths gives them a hint of red. S class asteroids are distinctly red, and they have higher albedos, about 7-20%. In Figure 3.6, Ida is S class (as is Gaspra, Plate 10), whereas Mathilde is C class. Eros is also S class, as are most of the NEAs, including Itokawa. The surface composition of Itokawa has been obtained by the IR spectrometer on Hayabusa. It found it to be mainly the silicates olivine and pyroxene (Table 2.3), plus possibly some plagioclase (another silicate) and iron.

By comparing reflectance spectra with laboratory spectra of various substances, and with the aid of albedo and other observational data on asteroids, including radar reflectance, likely surface materials can be identified. The outcome is that class M asteroids match alloys of iron with a few per cent nickel, mixed with little or no silicates. Class S match mixtures of similar iron-nickel alloys with appreciable proportions of silicates. Class C match a type of meteorite called the carbonaceous chondrite, of which more later, but which consist of silicates mixed with hydrated minerals, plus small quantities of iron-nickel alloy, carbon, and organic compounds. These are compounds of carbon and hydrogen, often with other elements. Carbon and organic compounds are collectively called carbonaceous materials, and they are mainly responsible for the low albedos of class C. Classes P and D are broadly like class C, but correspond to material that is richer in carbonaceous materials. Class V match some subclasses of a type of meteorite called the achondrite, silicate meteorites of which, again, more later.

Note that these mineral matches are to the surface of the asteroid - it could be very different in its interior. Note also that there could be (rare) cases where the matches are merely coincidences, and that the surface composition of the asteroid is quite different from that of the corresponding type of meteorite.

Asteroid classes across the asteroid belt, and asteroid differentiation

Figure 3.8 shows the distribution with heliocentric distance of the five most populous classes. Fractions are shown, such that at each distance the fractions of all asteroids (not just those shown) sum to one. Some idea of the actual numbers can be obtained by comparing Figure 3.8 with Figure 3.1, though it must be noted that Figure 3.1 shows observed asteroids, whereas in Figure 3.8 an attempt has been made to correct for various observational biases: for example, that a greater proportion of high-albedo asteroids must have been discovered than of low-albedo asteroids.

It is clear that the distributions differ from one class to another. If the mineralogical interpretations outlined above are correct, then the broadest trend is that mixtures of silicates and iron-nickel predominate in the inner belt (class S), and that carbonaceous materials and hydrated minerals become increasingly predominant as heliocentric distance increases (classes C, P, D).

An explanation of these trends is that the materials now in the outer belt formed there, where the cooler conditions allowed condensation of the more volatile substances, such as carbonaceous materials and hydrated minerals. In the warmer inner belt this was not possible, so we get

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