Transcript: The composition of the atmosphere of any planet depends on three things. The first is the chemical composition of the material available to form an atmosphere. This is the original hydrogen and helium of the solar nebula plus trace elements combined with the amount of gas that outgases from the interior of the planet though it evolution. Second is the mass of the planet which dictates its surface gravity, and third is the temperature of the planet determined by its distance from the Sun which in turn dictates the speed of atmospheric particles. In these terms giant planets have relatively high escape velocities, above 20 kilometers per second, and can retain even the lightest gases, hydrogen and helium. Terrestrial planets like the Earth and Venus have escape velocities around 10 kilometers per second and cannot retain hydrogen and helium but can retain heavier gasses, and the smallest solar system planets and other objects like Mercury and Pluto have escape velocities of only a few kilometers per second and cannot even retain carbon dioxide, nitrogen, or other heavy gasses.
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Dr. Christopher D. Impey, Professor of Astronomy, University of Arizona
Dr. Christopher D. Impey, licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License