Question

Can you explain the accretion theory of the Earth’s early atmosphere?

The original theory regarding the Earth's atmosphere involved a process of accretion of material in the solar nebula, which consisted mostly of hydrogen, a reducing (electron donating) element. It is true that comets, rich in easily vaporized elements and compounds (volatiles) most likely contributed to these conditions. However, according to Donald Goldsmith and Tobias Owen, authors of the book The Search for Life in the Universe, a few adjustments to the accretion theory must be made in order to reconcile the fact that Earth's early atmosphere was not predominantly reducing. In the process of accretion, either from the solar nebula or comets, hydrogen, carbon, nitrogen, and oxygen may have joined the Earth last. The outer layers of the Earth then would have lost these volatiles because of the heat produced by the collisions of material in the atmosphere with the Earth's crust. Hydrogen atoms that were bound to heavier molecules such as water vapor would have stayed on Earth due to gravity. Small, free hydrogen atoms and molecules, of which there were not many according to the authors, would have been "boiled away"--their volatility would have overcome the force of gravity. Therefore, the result would have been a mildly reducing atmosphere in the very beginning, but a continued oxidation due to loss of hydrogen. Thus, the eventual atmosphere was largely oxidizing, with an ozone buildup and oxygen molecules predominating. For more information, see: http://nai.arc.nasa.gov/index.cfm?page=comet_water
May 1, 2002