What was the earth's pre-life atmosphere likely composed of

Question

What gasses were likely the most prominent in the earth's pre-life atmosphere?

Answer 1

The first atmosphere the Earth had was dominated by CO2, just like Venus & Mars. The massive volcanic reactions spewed CO2, CO, SO2, HCl, N2,  and H2O. gases reacted to form CH4, NH4, HCN. It was free oxygen that was rare in the first atmosphere. Water is volatile so much was lost to space as Earth formed however the comets and asteroids brought water in replacing and adding to what was lost..

Early Earth and the Evolution of the Atmosphere

http://www.globalchange.umich.edu/globalchange1/current/lectures/Perry_Samson_lectures/evolution_atm/index.html

http://eesc.columbia.edu/courses/ees/climate/lectures/earth.html