A Second Look
This module serves as a good introduction to the concepts of atmospheric retention, but many simplifications were used. For example, all temperatures used were sub-solar temperatures – the theoretical temperature due to sunlight alone from a location on the surface where the sun is at the zenith. Thus our temperatures don’t take into account heat released from a body, greenhouse effects, or the fact that the escape of a gas would occur from the top of the atmosphere – so it’s the temperature there that is really important. Simplifications were also applied to escape velocity where we ignored air resistance, rotation of the body, and again it’s the escape velocity at the top of the atmosphere that is important which is less than that at the surface.
There are also many other factors involved in determining a body’s atmosphere. For example, the Earth’s primeval atmosphere consisted of hydrogen, helium, methane, and ammonia. Outgassing from volcanic activity later added nitrogen, carbon dioxide, and water vapor. Much later plants produced oxygen.
What happened to all of these gases? From this module we know that the hydrogen and helium quickly escaped into space. Before plants produced oxygen and the ozone layer that protects us from ultraviolet radiation, methane and ammonia were broken apart by ultraviolet radiation. As the Earth cooled, water vapor formed oceans which were able to absorb carbon dioxide which was ultimately incorporated into rocks like limestone at the bottom of the ocean. This leaves us with the nitrogen and oxygen rich atmosphere that we see today.