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Temperature (ti)

Fig. 4.7 The condensation sequence for a gas of solar composition (After Grossman and Larimer, Rev. Geophys. Space Sci. 12, 71, 1974)

According to these investigations, the order in which the elements condensed is as follows: Al, Ti, Ca, Mg, Si, Fe, Na, and S. The abundances of the main elements, as with the stable phases around 140 K, are in extremely good agreement with the measured abundances in refractory inclusions in C3 chondrites (for example, in the Allende meteorite), which are the most ancient samples that we have at our disposal.

It should, however, be noted that the complex composition of meteorites, where components that condensed at high and low temperatures are in close association, suggests changes in heating that were very localized in both time and space, and possibly cold zones, outside the plane of the disk, that were subject to sudden surges in solar activity. The measurement of certain isotopic anomalies (such as those of oxygen in the Allende meteorite, and those of magnesium in other meteorites) appears to indicate the presence of presolar grains, formed in another nucleosynthetic environment, and which have survived without being vaporized when they were incorporated into the protosolar nebula.

It is possible to estimate the growth rate of grains through condensation, assuming that whenever there is a collision between a grain and a molecule, the molecule remains trapped on the grain. For a solid:gas ratio of 10~2 and a mean molecular mass of 20, calculations (carried out without allowing for turbulence) show that objects several centimetres across may form rapidly, and then collapse into the disk with characteristic times of 103 to 106 years, depending on whether the particles are some millimetres across or less than a few microns in size. Within the disk, increasing numbers of collisions allow bodies tens of kilometres across, the planetes-imals, to form. The chemical composition of these varies with heliocentric distance

Fig. 4.8 The distribution of minor-planet classes within the Main Belt. The asteroids with the highest densities (types E and S) occur close to the Sun, whereas primitive asteroids (type D), occur at the greatest heliocentric distances (After Bell, 1989)

Fig. 4.8 The distribution of minor-planet classes within the Main Belt. The asteroids with the highest densities (types E and S) occur close to the Sun, whereas primitive asteroids (type D), occur at the greatest heliocentric distances (After Bell, 1989)

A (AU)

(Fig. 4.8): close to the Sun, it is dominated by silicates and heavy metals, whereas beyond 5 AU, ices predominate. The planetesimals are more massive beyond the ice line, because there solid material is far more abundant.

4.3.2.3 The Burst of Accretion and the Formation of the Giant Planets

The growth of the protoplanets and small bodies, beyond a diameter of 10 km, is governed by gravitational attraction. To take account of the phenomena involved, numerical simulations use a statistical approach. According to the models, two bodies may agglomerate if they impact on one another with a collision velocity that is less than, or comparable to, their escape velocity. Two classes of solutions emerge from these calculations: (1) an orderly growth resulting in the formation of a few large bodies, with a power-law distribution for the mass of the smallest; (2) a burst of accretion, linked to the way in which a given body sweeps up surrounding material, and which ceases then the zone available for accretion becomes empty. This second mechanism allows rapid accretion of the giant planets, in particular Jupiter, in a time compatible with the model's other constraints (the existence of a minor-planet belt, and dispersal of the disk by the Sun's T-Tauri phase, about one to a few million years after the collapse of the nebula).

Jupiter's great mass, compared with that of the other giant planets, is very probably explained by its position relative to the ice line. Lying just outside the latter, it was able to sweep up the increase in solid material generated by the condensa

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