ABSTRACT
In the previous chapter, we have seen how the atmospheres originated in planets andsatellites.Sincetheatmosphericmassinthelessmassivebodieschanges,to avariabledegree,inresponsetotheexternalforcingortotheinteractionwiththe surfaceandinteriorofthebodyvolatilereservoirs,wediscussthemeanatmospheric compositionoftheactual(present?)atmosphereshere.Theatmosphereiscomposed of a variety of gases that can be in atomic, molecular, or ionic form: some are active tosunlightandthermalradiation(absorbandemitradiation),othersareinertcompounds(asthenoblegases),otherschemicallyactive(reactingwithothersspecies), andsomearecondensablegasesthatsufferphasechanges(condensation,evaporation, or sublimation) depending on temperature and pressure.
