Accretion of the Planets

A collision with a Mars-sized object may have resulted in the formation of the Earth s moon. Our moon is by no means the largest satellite in the solar system, but it is unusual in that it and the moon of Pluto are the largest moons relative the mass of the planets they orbit. Geochemical studies of returned lunar samples have shown that close similarities exist between the bulk composition of the moon and the Earth s mantle. In particular, the abimdances of sidero- 

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 

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Although the basic chemical and material building blocks for the planets and their satellites were fairly uniform during the initial formation of the solar nebula from inter-stellar cloud materials, chemical differentiation, and segregation occurred over time during accretion of the planets, and their moons such that the volatile chemical components of the solar nebula ended up as present day near-surface ice on Earth, and ice plus solid CO2 on Mars, and as ice and other molecular solids and fluids (such as hydrocarbons and ammonia) on most of the moons of Jupiter and Saturn, and as water ices and increasingly volatile species such as nitrogen in the outermost solar system. 

Figure 6.2 Formation of the solar system (a) unstable molecular cloud possessing some angular momentum (b) angular moment conservation produces the disc shape under collapse (c) matter accretion forms the planets (d) the mature system of planets seen today evolves after 4 Myr...

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

The Earth s crust, mantle and core are strongly influenced by differentiation processes which could have resulted from gravitational separation ( smelting ) in an early molten phase of the planet, or from the sequence in which different chemical species condensed from the primitive solar nebula and were subsequently accreted. Seismology indicates that there is a liquid core (with a solid inner core) with radius 3500 km consisting mainly of iron (with some Ni and FeS) surrounded by a plastic (Fe, Mg silicate) mantle of thickness 2900 km. 

The origin of the components that were accreted to make up the planets is the subject of intense discussion. Chondrite-mixing models attempt to build the planets using known chondritic materials. These models are constrained by the mean densities, moments of inertia, and, to the extent that they are known, the bulk chemical and isotopic compositions of the planets. Mars and 4 Vesta can be modeled reasonably well by known types of chondritic material (Righter et al., 2006). However, the Earth seems to have formed, at least in part, from materials that are not represented in our collections of chondritic meteorites (see below). 

The earth was formed by a process of accretion about 4.6 billion years ago. Initially it was a molten mass lacking the gravitational pull to retain its gases at the prevalent elevated temperatures. And yet, within a mere 700 million years of the planet s birth, as calculated from the isotopic record of sediments, cellular life almost certainly existed. What raw materials were available to bring about this amazing turn of events What were the sources of energy used to drive the necessary reactions Where did the important reactions take place Was it in the atmosphere, in the oceans, on dry land, or all three ... 

The masses, sizes, and overall structure of protoplanetary disks are important to quantify as they set the total amount and the distribution of planet-forming materials. However, over time disks evolve and the dust contained within is transported, processed, and accreted into larger bodies. This evolution plays a critical role in determining both the physical and chemical properties of the dust, and by extension, of the planets that will eventually form. 

The recent detection of the [Nell] line emission at 12.81 pm from several disks by the Spitzer Space Telescope (e.g. Pascucci et al. 2007) has confirmed theoretical predictions that the disk atmosphere is heavily ionized and superheated, either by X-rays (Glassgold et al. 2007) or by extreme UV irradiation (Pascucci et al. 2007). However, X-rays and cosmic-ray particles (CRPs) may not be able to penetrate further toward the mid-plane of the planet-forming disk zone (r 3-20 AU), which makes the mid-plane essentially neutral and thus stable against accretion ( Dead Zone Gammie 1996 Dolginov Stepinski 1994). 

The rate at which the planetary cores accreted gas increased slowly until their masses reached 30M . After this, gas accretion was very rapid (Pollack et al., 1996). The growth timescale depends on the opacity of the planet s gas envelope, since this determines the rate at which the energy of accretion could be radiated away. Eor interstellar dust opacities, a lOM core would require 10 Myr to grow to Jupiter s mass (Pollack et al., 1996). However, growth would have been quicker if the opacity was lower due to coagulation of grains in the envelope (Ikoma et al., 2000). 

The giant planets ceased growing when the flow of gas onto their envelopes was cut off. This may have been the result of gap formation or because the nebula dispersed. The latter seems unlikely, since the timescale for gas accretion onto a Jupiter-size planet is small compared to the lifetime of the nebula. However, hydrodynamical simulations suggest that gas would continue to flow onto Jupiter after it cleared a gap in the disk (Lubow et al., 1999), so this explanation is problematical too. In addition, it has been suggested that some gas would remain at the same orbital distance as the planet after it cleared a gap if the disk viscosity was low (Rafikov, 2002), and this would also be accreted by the planet eventually. 

Scott E. R. D. and Taylor G. J. (2000) Composition and accretion of the terrestrial planets. The Lunar Planet. 

Quahtatively speaking, aU accretion involves several stages, although the relative importance must dilfer between planets and some mechanisms are only likely to work under certain conditions that currently are underconstrained. Although the exact mechanisms of accretion of the gas and ice giant planets are poorly understood (Boss, 2002), all such objects need to accrete very rapidly in order to trap large volumes of gas before dissipation of the solar nebula. 

Canup R. M. and Agnor C. (1998) Accretion of terrestrial planets and the earth-moon system. In Origin ofthe Earth and Moon, LPI Contribution No. 597 Lunar and Planetary Institute, Houston, pp. 4-7. 

Gafifey M. J. (1990) Thermal history of the asteroid belt implications for accretion of the terrestrial planets. In Origin of the Earth (eds. H. E. Newsom and J. H. Jones). Oxford University Press, Oxford, pp. 17-28. 

SNC meteorites (Harper et al., 1995 Borg et al., 1997). This isotopic anomaly requires early differentiation of mantle and crust. Because hafnium and mngsten fractionated into silicate and metal, respectively, the short-lived Hf- W system can be used to determine that martian core formation occurred within —13 milhon years of the planet s accretion (Kleine et al., 2002 Yin et al., 2002). Correlation between Nd and isotope anomalies, as well as the initial Os/ Os ratios for martian meteorites (Brandon et al., 2000), indicate synchronous differentiation of core, mantle, and cmst (Figure 14). On Earth, core formation took substantially longer, convection has stirred the mantle sufficiently to erase any evidence of early isotopic heterogeneity, and cmst formation continues throughout geologic history. 

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