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Planetary Sciences American and Soviet Research/Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences (1991) / Chapter Skim
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15. The Role of Impacting Processes in the Chemical Evolution of the Atmosphere of Primordial Earth
Pages 203-217

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From page 203...
... Currently existing paleontological data offer clear evidence of the presence of life on Earth 3.5 billion years ago (Schopf and Packer 1987~. Furthermore, we can make the fundamental conclusion from the analysis of carbon isotope ratios i3C/~2C that an almost contemporary biogeochemical carbon cycle (Schidlowski 1988)
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From page 204...
... We shall consider possible scenarios for the formation of the Earth's early atmosphere and its initial chemical composition. EARTH'S INITIAL ATMOSPHERE A model for Earth's early atmosphere, formed from solar composition gas captured gravitationally during the final stages of accretion, was explored in Hayashi's studies (1981, 1985~.
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From page 205...
... Nevertheless, one should note that the accuracy of estimates of the time scales for a significant portion of cosmogonic processes at the early stages of protosolar nebula evolution is not reliable enough to rule out, with absolute certainty, the possibility that an appreciable initial atmosphere existed on Earth. According to this scenario, the initial atmosphere encounters considerable additional difficulties stemming from its chemical composition.
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From page 206...
... Under the currently adopted planetary accumulation models with an approximate 108 year time scale, such a hypothesis would only be justified in the case of a gigantic impact (Kipp and Melosh 1986; Wetherill 1986~. There is no question that a considerable portion of the planet would have melted as a result of a gigantic impact which would have released a huge quantity of gases.
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From page 207...
... These dimensions fit with estimates of the diameters of impacting bodies which formed the largest craters on the terrestrial planets and their satellites (O'Keef and Ahrens 1977~. The velocity of the collision of a random body of a preplanetary swarm with the embryo of primordial Earth is no less than the escape velocity.
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From page 208...
... 1974~. In these conditions, thermodynamic equilibrium in a gaseous phase is reached over a time scale tchem < 1O-5 seconds (Gerasimov et al.
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From page 209...
... This is the point where the reaction products undergo chemical hardening, since the time scale for chemical reactions becomes greater than the characteristic time scale for cloud expansion as the cloud expands further. Gases with a chemical composition corresponding to the moment of hardening are mixed with atmospheric gases, interacting with both these gases and bedrock from the uppermost layers of the planet (regolith)
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From page 210...
... We can estimate the chemical composition of the gas mixtures at high temperatures, using the standard methods of thermodynamical equilibrium (given the presupposition that the time scale for the breakdown of a given molecule is less than the time required to cool the elementary gas exchange. The most detailed computations of these processes were made in the studies of Fegley e!
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From page 211...
... considered several possible chemical compositions for unperturbed atmospheres, and they calculated the commensurate alterations in the composition owing to the passage of large bodies through the atmosphere. I would like to make the following comment regarding their work The authors used computation methods employed for purely gaseous reactions.
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From page 213...
... However, the question of shock differentiation has yet to be theoretically analyzed in-depth and requires additional investigation. The shock-degassing source was operative virtually throughout the planets' entire accumulation process.
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From page 214...
... Achievements made in recent years in investigating the impact source allow us to assess its strength and the chemical composition of the gases that were released. Nevertheless, the question of the evolution of the composition and density of the impact-generated atmosphere continues to remain an open one, primarily due to ambiguities in estimating gas flux rates from the atmosphere during accumulation.
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From page 215...
... 1984. Studies of the chemical composition of gaseous , , —r ~ 0 - phase released from laser pulse evaporated rocks and meteorites materials (abstract)
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From page 216...
... 1987. A kinetic study of the hydrous alteration of amorphous MgSiO smokes: Implications for cometary particles and chondrite matrix.
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From page 217...
... 1986. Accumulation of the terrestrial planets and implications concerning lunar origin.
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Key Terms

  • initial atmosphere
  • time scale
  • terrestrial planets
  • continuous degassing
  • protoplanetary nebula

  • atmospheric gases
  • dense atmosphere
  • primordial atmosphere
  • solar wind
  • initial chemical

  • noble gases
  • flux rates
  • terrestrial rocks
  • gaseous phase
  • prebiological evolution

  • active gases
  • gravitational capture
  • atmosphere gas
  • atmosphere existed
  • biogeochemical carbon

  • additional difficulties
  • rapid formation
  • observed data
  • chemically active
  • carbon cycle

  • gas dissipation
  • hypervelocity impact
  • final stages
  • accumulation process
  • nitrogen oxides


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