The first atmosphere was probably blasted away once the sun entered its so- called T-Tauri phase which is characterized by powerful winds, millions of times denser than what we see in the solar system today. The second atmosphere came onto the scene after the T-Tauri phase ended which usually seems to take about 10 million years or so based on observations, today, of T-Tauri stars similar in mass to the sun. Originally, it was thought that volcanic processes outgased the secondary atmosphere with a composition that was determined by the kinds of compounds that existed in chemical equilibrium at the earth's diatance from the infant sun. This would have led to a different mixture of gases than we now see coming out of volcanoes because the interior of the earth did not have quite enough time to fully differentiate and chemically remove the volitile 'hydrogenic' compounds. Newer ideas, based on studies of distant young stars with orbiting dust and gas disks, now suggest very strongly that comets were a major component of volatile compounds in such infant gas disks, so that perhaps all of the earth's secondary atmosphere, and oceans, may have been deposited by comet impacts!
All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY project.