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asteroids are smaller, apparently because of porous internal structures created by impact fragmentation and reassembly of these bodies since their formation. The densities of stony meteorites, small fragments from the asteroid belt, are p — 3000 kg m^3.

All these bodies appear to have formed by "binary accretion," the step-by-step growth occurring when two bodies collide and stick, starting from tiny dust particles in the original nebula about the Sun and reaching up to the sizes of the Earth and Venus. Indeed, the N-body models that are used to study the dynamics and growth of bodies in the outer Solar system have been honed to their highest levels of perfection in the study of terrestrial planet growth. Still, new data continue to surprise and unnerve us. For example, N-body accretion models show that Earth grew to its final mass on a timescale — 100-200My [18, 129], and this long timescale has remained more or less unchanged for the past several decades, since detailed estimates were first made by G. Wetherill. It stands in contrast to new isotopic data from the Hafnium-Tungsten (Hf-W) decay [67]. Hafnium decays to Tungsten, 182Hf ^ 182 W, with a 9-Myr half life. The quantity of 182 W in the Earth's mantle (relative to the core) provides a measure of the amount of the unstable Hf isotope at the epoch of core formation, and so sets the timescale for Earth's differentiation. The W-Hf data show that the Earth accreted the bulk of its mass within 30 Myr, whereas major asteroids such as Vesta formed in an even shorter 3 Myr [67]. This is a half to one order of magnitude discrepancy with the N-body models and remains unexplained.

The relevance to us is that models can give very plausible but wholly incorrect solutions. Without the benefit of independent constraints from the isotopes, we would remain completely unaware that the N-body terrestrial planet growth models are too slow. In the outer Solar system (where independent constraints on the models from isotopes or other sources are unavailable), it is easy to see that we are skating on very, very thin ice.

The Domain of the Giant Planets Gas Giants

Jupiter and Saturn (Figs. 4 and 5), in addition to being two orders of magnitude more massive than the Terrestrial planets (see Table 2), have very differ-

Fig. 4. Gas giant Jupiter from the Galileo spacecraft, showing its banded cloud structure and the Great Red Spot. Image from NASA

ent, much more volatile-rich compositions. Jupiter and Saturn are mass-wise dominated by hydrogen (H2) and helium (He) and are known as "gas giants."

The formation of the giant planets is imperfectly understood. Prevailing ideas suggest that, in the Solar system, the gas giant planets formed by a process of nucleated instability, a bit like a rain drop forming by condensation of water molecules on a refractory aerosol. The model was developed by Mizuno and others [111,123]. Briefly, solid bodies collide and grow by binary accretion in the protoplanetary disk, much as they did in the domain of the Terrestrial planets. Upon reaching a critical mass, generally estimated to be ~10 M®, the core precipitates the infall of surrounding nebular gas, producing a hydrody-

Fig. 5. Gas giant Saturn from the Cassini spacecraft. Courtesy NASA
Table 2. Giant Planets

Object

Mass/M®

Radius/fi®

P [kg m 3]

a [AU]

e

i [deg]

Jupiter

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