Thermochemistry of substellar atmospheres: Water, oxygen, sulfur, and phosphorus

Thermochemical equilibrium and kinetic calculations are used to investigate atmospheric chemistry in substellar objects: giant planets, extrasolar giant planets (EGPs), and brown dwarfs. These studies include an assessment of the water and total oxygen inventories in the interiors of Jupiter and Saturn, and detailed modeling of sulfur and phosphorus chemistry in the atmospheres of substellar objects.; In the first part of the dissertation, the water and total oxygen abundances in the deep atmospheres of Jupiter and Saturn are determined by considering the effects of H2O and O on the chemistry of CO, PH3, and SiH4. On Jupiter, the observed CO abundance indicates a water abundance of 0.4--1.4 times the protosolar H2O/H2 ratio (8.96 x 10-4). On Saturn, a combination of CO and PH 3 chemical constraints requires a water abundance of 1.9--6.1 times the protosolar abundance. Combining these results with Si mass balance considerations gives a total oxygen abundance of 0.7--1.7 and 3.2--6.4 times the protosolar O/H2 ratio (1.16 x 10-3) on Jupiter and Saturn, respectively. In both planets, oxygen is less enriched than other heavy elements (such as carbon) relative to hydrogen and the solar system composition. These results provide important constraints for giant planet formation mechanisms and models of tropospheric chemistry.; The second part of the dissertation is a detailed study of sulfur and phosphorus chemistry in substellar atmospheres. The chemical behavior of individual S- and P-bearing gases and condensates is determined as a function of temperature, total pressure, and metallicity. Aside from minor amounts of sulfur removed by metal sulfide cloud formation, H2S is approximately representative of the sulfur inventory throughout substellar atmospheres. Silicon sulfide (SiS) is a potential tracer of weather in EGPs and L dwarfs. Phosphorus chemistry is considerably more complex than that of sulfur. Disequilibrium abundances of PH3 approximately representative of the total phosphorus inventory are expected to be mixed upward into the observable atmospheres of giant planets and T dwarfs. In hotter objects, several P-bearing gases (P2, PH 3, PH2, PH, HCP) become increasingly important at high temperatures. Chemical kinetics are used to demonstrate the establishment of thermochemical equilibrium in the deep atmospheres of substellar objects.