Accretion, planetary

After planetary accretion was complete there remained two groups of surviving planetesimals, the comets and asteroids. These populations still exist and play an important role in the Earth s history. Asteroids from the belt between Mars and Jupiter and comets from reservoirs beyond the outer planets are stochastically perturbed into Earth-crossing orbits and they have collided with Earth throughout its entire history. The impact rate for 1 km diameter bodies is approximately three per million years and impacts of 10 km size bodies occur on a... 

Tyburczy, J. A., Frisch, B., Ahrens, T. J. (1986) Shock-induced volatile loss from a carbonaceous chondrite Implications for planetary accretion. Earth Planet. Sci. Lett., 80, 201-7. 

What happened next depends sensitively on the vertical thickness of this solid-rich layer and on the relative velocities of the solid particles. Pioneering studies of planetary accretion showed that if the solid-rich layer was very thin, portions of it would become gravitationally unstable, collapsing to form solid bodies —1 km in diameter called planetesimals (Safranov, 1969 Goldreich and Ward, 1973). Note that this process is different from the large-scale disk instabilities that may have formed Jupiter-mass bodies. 

Analytic estimates suggest that lOM cores were unlikely to accrete in a minimum mass nebula on a timescale comparable to the lifetime of circumstellar disks. However, such cores could have formed if the surface density of solids at 5 AU was 5-10 times that of a minimum mass nebula (Lissauer, 1987). Numerical models of planetary accretion support this conclusion... 

Chambers J. E. and Cassen P. (2002) Planetary accretion in the inner solar system dependence on nebula surface density profile and giant planet eccentricities. Lunar Planet. Sci., abstract 1049. 

Lissauer J. J. (1987) Timescales for planetary accretion and the structure of the protoplanetary disk. Icarus 69, 249-265. 

The presence of FeO on Mercury and the notion that planetary accretion involved mixing throughout the inner solar system motivated models involving mixing of refractory and volatile materials. Goettel (1988) made broad estimates of Mercury s composition by combining his calculated refractory and volatile end-members (Table 2, column 3). Morgan and Anders (1980) used an elaborate seven-component model to calculate the composition of Mercury (Table 2,... 

These models produced a zoned Earth with an early metallic core surrounded by silicate, without the need for a separate later stage of core formation. The application of condensation theory to the striking variations in the densities and compositions of the terrestrial planets, and how metal and silicate form in distinct reservoirs has been seen as problematic for some time. Heterogeneous accretion models require fast accretion and core formation if these processes reflect condensation in the nebula and such timescales can be tested with isotopic systems. The time-scales for planetary accretion now are known to be far too long for an origin by partial condensation from a hot nebular gas. Nevertheless, heterogeneous accretion models have become embedded in the textbooks in Earth sciences (e.g.. Brown and Mussett, 1981) and astronomy (e.g.. Seeds, 1996). 

Having made all of these cautionary statements, one still can state something useful about the overall accretion timescales. All recent combined accretion/continuous core formation models (Halliday, 2000 Halliday et al., 2000 Yin et al., 2002) are in agreement that the timescales are in the range 10 -10 yr, as predicted by Wetherill (1986). Therefore, we can specihcally evaluate the models of planetary accretion proposed earlier as follows. 

Lissauer J. J. (1987) Time-scales for planetary accretion and the structure of the protoplanetry disk. Icarus 69, 249-265. Lissauer J. J. (1993) Planet formation. Ann. Rev. Astron. Astrophys. 31, 129-174. 

Models of planetary evolution assume that at the time of planetary formation the solar system had a single universal and well-mixed composition from which aU parts of the solar system were derived (see Podosek, 1978). Information as to the elemental and isotopic characteristics of this primordial composition is presently available from the Sun, meteorites, and the atmospheres of the giant planets (Wider, 2002). In the case of the Sun, distinction is usually made between the present-day composition, which is available via spectral analysis of the solar atmosphere and capture of the solar wind, either directly in space or by using metallic foU targets, and the proto-Sun (the composition at the time of planetary accretion) whereby the lunar regolith and/or meteorites are utilized as archives of ancient solar wind. As discussed below, the distinction is only really important for helium due to production of He by deuterium burning. 

TyburczyJ. A., Frisch B., and Ahrens T.J. (1986) Shock-induced volatile loss from a carbonaceous chondrite implications for planetary accretion. Earth Planet. Sci. Lett. 80, 201-207. Valbracht P. J., Staudacher T., Malahoff A., and Allegre C. J. (1997) Noble gas systematics of deep rift zone glasses from Loihi Seamount, Hawaii. Earth Planet. Sci. Lett. 150, 399-411. 

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