Why do the terrestrial planets have many of the same gases in their atmospheres? While the gases are present in very different abundances, and the atmospheres have very different pressures, Earth, Mars and Venus all have a similar cast of gases: nitrogen, argon, CO2, others in trace amounts. 
I've asked an earth scientist friend and his short answer was "volcanoes." I'm afraid I don't know enough about solid earth processes or geochemistry to figure out where to find out more...where could I start?
 A: I would guess the answer is the rather boring one that there are only a small number of common gaseous elements and because all the planets formed from the same dust cloud they all have a similar set of elements.
There are obvious differences that have straightforward explanations. For example small planets can't hold on to hydrogen and helium while the larger planets can. Most planets don't have significant oxygen levels because it's too reactive. The relative concentrations also depend on the distance from the Sun.
Volcanos play an important role in recycling gaseous elements. For example carbon dioxide is removed from the atmosphere by forming carbonates. The carbonate rocks are eventually subducted and volcanos bring the carbon dioxide back into the atmosphere. This is known as the geological carbon cycle.
A: Star outgassing and supernovae determine concentrations of light elements in the universe.  Comet accretion and planetary outgassing determine what gets added.  Temperature and pressure plus energy inputs (solar UV) drive the chemistry.  Geochemistry modulates it.  Granite is rather reactive over time.
