Chemical Constraints on the Water and Total Oxygen Abundances in the Deep Atmosphere of Saturn

Thermochemical equilibrium and kinetic calculations for the trace gases CO, PH3, and SiH4 give three independent constraints on the water and total oxygen abundances of Saturn’s deep atmosphere. A lower limit to the water abundance of H2O/H2≥ (1.7 −0.4)×10 is given by CO chemistry while an upper limit of H2O/H2 ≤ (5.5 −2.5)×10 is given by PH3 chemistry. A combination of the CO and PH3 constraints indicates a water enrichment on Saturn of 1.9 to 6.1 times the solar system abundance (H2O/H2 = 8.96×10−4). The total oxygen abundance must be at least 1.7 times the solar system abundance (O/H2 = 1.16×10−3) in order for SiH4 to remain below a detection limit of SiH4/H2 <2×10−10. A combination of the CO, PH3, and SiH4 constraints suggests that the total oxygen abundance on Saturn is 3.2 to 6.4 times the solar system abundance. Our results indicate that oxygen on Saturn is less enriched than other heavy elements (such as C and P) relative to a solar system composition.