Figure 3.8 The distribution in the asteroid belt of the five most populous classes of asteroid.

only silicates and iron-nickel alloy. This distinction could have been enhanced during the T Tauri phase of the proto-Sun, when the solar wind would have heated the asteroids by magnetic induction, i.e. by the heating from electric currents induced in the asteroids by the action of the magnetic fields entrained in the wind. The heating decreased with heliocentric distance, so asteroids in the inner belt would have been heated more than those in the outer belt. This explanation requires that there has been only limited migration of the different classes across the asteroid belt, and that differences across the belt never became obscured by migration into the belt of planetesimals that formed elsewhere in the Solar System. An alternative explanation places more weight on the loss of volatile materials from the inner belt throughout Solar System history, in which case substantial inward migration of C class asteroids with subsequent modification is a possibility.

Class M asteroids, which have surfaces that are largely or entirely iron-nickel, are presumably iron-nickel throughout - there is no reasonable way of getting such an iron-rich surface and an iron-poor interior. There is no plausible scheme of condensation and accretion in the solar nebula that would give such a silicate-free composition throughout, and it is therefore necessary to assume that internal temperatures in some asteroids rose to the point where the interiors became partially or wholly molten. This allowed a process called differentiation to occur, whereby denser substances settled towards the centre of a body, and the less dense substances floated upwards to form a mantle, overlain in turn by a mineralogically distinct crust. The melting could have resulted from heat released by asteroid accretion and collisions, plus heat from the decay of short-lived unstable isotopes, notably the isotope of aluminium 26Al, nearly all of which decayed in a few million years. The asteroid interior would then have cooled, and became solid after a further interval of a few million years.

The temperature rise caused by isotope heating is greater, the larger the body. This is because the mass of the isotopes present is proportional to the volume of the body, whereas heat losses from the body are proportional to its surface area, and the ratio of volume to surface area is greater, the larger the body. The temperature rise from accretion, all other things being equal, is also greater, the larger the body. In a body consisting of mixtures of silicates and iron-nickel alloy (which thus excludes C class), differentiation would have occurred at sizes larger than about 200 km across, and would have resulted in a predominantly iron-nickel core overlain by a mantle and crust largely composed of silicates. There will also be a core-mantle interface consisting of a mixture of iron-nickel alloy and silicates. Collisions can break up these bodies, and fragments of the cores give us chunks of iron-nickel alloy, i.e. class M. Fragments of the mantle and the mantle-core interface could be an important source of S class. The surface properties of Vesta are consistent with the sort of silicates that would form the crust of a fully differentiated body.

The scarcity of M and S classes in the outer belt indicates that differentiation was uncommon there. One explanation is that supplementary heating by T Tauri magnetic induction was too weak at these greater heliocentric distances.

Further discussion of the composition of asteroids is in Section 3.3.4, in relation to meteorites.

Beyond the main belt

The Trojans and Centaurs cannot readily be placed into the asteroid classes outlined above. The Trojans are dark, with albedos in the range 0.03—0.13, similar to the small outer satellites of Jupiter and the other giant planets. Of the small proportion of Trojans that have been classified, most have been placed in D class, and the rest in either C or P. □ In what does this suggest that the surfaces are rich?

This suggests that the surfaces are rich in carbonaceous materials. Trojan spectra are similar to those of the nuclei of short-period comets, and to some of the Centaurs and some of the E-K objects.

There is no spectral evidence for water ice at the surface of any Trojan, though planetesimals at their distance from the Sun would have been icy-rocky, so this could be the typical internal composition. By contrast, there is such spectral evidence for some Centaurs, presumably because of their greater average distances from the Sun and the consequent preservation of water ice at their surfaces. The Centaurs otherwise resemble the Trojans, with albedos covering about the same range. The dark surfaces of the Trojans and Centaurs suggest that all the surfaces are rich in carbonaceous materials, further darkened by small impacts that produce dust at their surfaces, and by the bombardment of ions and electrons.

As has been noted, the Centaur Chiron shows evidence of cometary activity. This cannot be driven by the sublimation of water - Chiron is too cold - but it could be driven by CO, CO2, or NH3. Moreover, the Centaurs' spectra generally match those of EKOs. This fits with the view noted earlier that the Centaurs are a transitory population between the E-K belt and the short-period comets (except for those Centaurs that are flung out of the Solar System).

Question 3.4

The reflectance spectrum and albedo of the asteroid Eros are shown in Figure 3.9. Explain why it is placed in the S class. Hence deduce its likely surface composition. Where might it have originated?

Eros (medium albedo)

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