Earth accretion history

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... 

Abe Y, Matsui T (1986) Early evolution of the Earth Accretion, atmosphere formation, and thermal history. J Geophys Res 91(B13) E291-E302... 

Carr M. H. and Wanke H. (1992) Earth and Mars water inventories as clues to accretional histories. Icarus 98,61-71. 

Wanke H., Dreibus G., and Jagoutz E. (1984) Mantle chemistry and accretion history of the Earth. In Archean Geochemistry (ed. A. Kroner). Springer, Berlin, pp. 1-24. 

Morgan J. W. (1986)Ultramafic xenohths clues to Earth s late accretional history. J. Geophys. Res. 91, 12375-12387. 

Venus is similar in size to the Earth and might be expected to have differentiated to a similar extent. However, while the early accretion history might have been similar (with the exception of the absence of a moon-forming event), silicate differentiation did not proceed according to the familiar plate tectonic mechanisms. There is, of course, no data on the interior of Venus, and so planetary degassing characteristics must be deduced from limited atmospheric data and observations of volcanic activity at the surface. 

Distributions in the solar system. More data on volatiles throughout the solar system are clearly required to confidently describe the volatile acquisition history of the terrestrial planets in the proper context. There are several unknown values for the solar composition, including the nitrogen-and carbon-isotope compositions. The compositions of comets from different orbital distances are needed to assess the extent of radial transport of volatiles late in accretion history. In addition, the causes of carbon- and nitrogen-isotope variations in chondrites must be better understood. While it is clear that the Earth cannot be constructed simply by mixing of different meteorite classes, it is not yet possible to unambiguously extrapolate to the volatile compositions of protoplanetary materials. 

Farley K. A. (2001) Extraterrestrial helium in seafloor sediments identification, characteristics and accretion rates over geologic time. In Accretion of Extraterrestrial Matter throughout Earth s History (eds. B. Peucker-Ehrenbrinck and B. Schmitz). Kluwer, New York, pp. 179-204. 

Formation of a terrestrial magma ocean. Extensive melting as a result of giant impacts would lead to the formation of a terrestrial magma ocean during the Earth s late accretion history. 

Evidence of such a deep Earth abundance pattern coupled with solar isotopic compositions would be a strong indicator that adsorption could have played a role early in the Earth s accretional history (Pepin 1991, 1998). Although there is evidence that solar-like light noble gas isotopic compositions exist, the abundance pattern of trapped noble gases in the Earth cannot be easily constrained, since there is the possibility that subduction of heavy noble gases has since altered the pattern. With a Mars-size terrestrial embryo and the high value of 2 x 10 for the Ne Henry s constant assumed in Pepin s... 

Flynn GJ (2001) Atmospheric entiy heating of interplanetary dust. In Accretion of extraterrestrial matter throughout Earth s history. Peucker-Ehrenbrink B, Schmitz (eds) Kluwer, New York, p 107-127 Galer SJG, Mezger K (1998) Metamorphism, denudation and sea level in the Archean and cooling of the Earth. Precamb Res 92 389-412... 

Farley KA (2001) Extraterrestrial helium in seafloor sediments Identification, characteristics, and accretion rate over geological time. In Accretion of extraterrestrial matter throughout Earth s history. B Peucker-Ehrenbrink, B Schmitz (eds) Kluwer Academic/Plenum Pubhsheis, p 179-204 Farley KA, Patterson DB (1995) A 100-kyr periodicity in the flux of extraterrestrial e to the sea floor. Nature 378 600-603... 

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