International Geologiical Congress - Oslo 2008

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PIP-04 From gas and dust to planets

 

Two stage mechanism of planet formation

 

Vsevolod Anfilogov, Institute of Mineralogy (Russian Federation)
Yuriy Khachay, Institute of geophisic (Russian Federation)
 

 

The modern planet formation models are presume that the protoplanrtary material had gone through two consistent sages: 1 - the stage of condensation of the solid particles from the gaseous state, which has gone until all gaseous phase transforms to solid one. 2 - the stage of of solid particles agglomeration, which brought to planet formation. The two stage mechanism of the planet formation has been proposed by us in 2005 . It is based on the assumption that the solid particle agglomeration begins at once when they appear in protoplanetary cloud as the result of their condensation from gas. In this case, the central part of a planet embryo will be composed from the highest temperature fraction of condensation. To a first approximation this fraction is the same as CAI - white inclusions in Allende meteorite. As the embryos grow and protoplanetary cloud cool itself the condensed fractions composition changes and iron and iron-silicate composition as pallasite and enstatite chondtite will be precipitated on embryo surface after CAI-composition. The size of these primary embryos is 200 ? 300 kilometers.
Thermal evolution of the embryos is caused by the emission of the heat under the decay of short-lived isotopes, the main of which is 26Al. The center of embryos is enriched by Al2O3 and ratio of 26Al/27Al in it is equal 5?10-5 .Temperature in the center of embryo, which radius is 200 km may be 1830-2200 K at this ratio. It is sufficiently for partial melting of material in the central part of embryo and full melting of the Fe-Ni mixture in its middle part. The outer part of embryo remains solid.
The second stage of the planet formation follows in the next way. As it is shown by Safronov embryos, which are formed on the initial stage of condensed fraction agglomeration are to be in great number and they are often to collide with each other. The impact of two embryos having the same size, partially molten alumosilicate core, molten Fe-Ni middle part and solid outer silicate envelope terminates in their failure. The molten middle Fe-Ni envelopes will merge together and forms a new embryo, which is composed from Fe-Ni alloy. The material of alumosilicate core will be ejected from the center of the impacted embryos and partially throw out beyond new embryo. Outer solid envelopes will be destroyed and their fragments throw out beyond growing planet too. The duration of the first stage of the planet formation is no more than 10 m.y. It is conformed to the chemical reservoirs of core and mantle separation time.
The integration of Fe-Ni envelopes after two embryos impacted gave birth to the new planet embryo, which has Fe-Ni core. The Fe-Ni core of a new embryo is able to grow by impact with other ones. Two stage mechanism of planet formation allows solve two the most complicated problems: the formation of the Fe-Ni core on the early stage of planet growth and the possibility of the Earth to gain the magnetic field.

 

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