Planetary Geoscience

Planetary volatiles occur in gas, liquid, and solid forms. In this chapter, we will see that the terrestrial planets have secondary atmospheres formed by outgassing of their interiors. The chemical compositions of the atmospheres of Venus and Mars are dominated by CO2, but the Earth’s atmosphere is distinct because CO2 is sequestered in the lithosphere and life has added O2 to the mix. Giant planet atmospheres are mostly hydrogen with some helium. Titan has an atmosphere of N2 and reducing gases, along with seas of hydrocarbons. Mars has briny groundwater and had lakes and possibly oceans in the distant past. Some moons of the giant planets have subsurface oceans. Noble gases and stable isotopes hold keys to the origin and evolution of volatiles. Differences in temperature and pressure cause atmospheric circulation, controlled by planetary rotation and energy transport. Frozen volatiles are common as polar deposits and sometimes permafrost, but they are especially abundant in the outer Solar System, where they may comprise the crusts of giant planet satellites. Volatile behaviors can be described in terms of geochemical cycles. Greenhouse warming has important implications for planetary climates.