Thermochemical equilibrium and kinetic calculations for the trace gases CO , PH _ { 3 } , and SiH _ { 4 } give three independent constraints on the water and total oxygen abundances of Saturn ’ s deep atmosphere .
A lower limit to the water abundance of H _ { 2 } O/H _ { 2 } \geq ( 1.7 _ { -0.4 } ^ { +0.7 } ) \times 10 ^ { -3 } is given by CO chemistry while an upper limit of H _ { 2 } O/H _ { 2 } \leq ( 5.5 _ { -2.5 } ^ { +0.8 } ) \times 10 ^ { -3 } is given by PH _ { 3 } chemistry .
A combination of the CO and PH _ { 3 } constraints indicates a water enrichment on Saturn of 1.9 to 6.1 times the solar system abundance ( H _ { 2 } O/H _ { 2 } = 8.96 \times 10 ^ { -4 } ) .
The total oxygen abundance must be at least 1.7 times the solar system abundance ( O/H _ { 2 } = 1.16 \times 10 ^ { -3 } ) in order for SiH _ { 4 } to remain below the detection limit of SiH _ { 4 } /H _ { 2 } < 2 \times 10 ^ { -10 } .
A combination of the CO , PH _ { 3 } , and SiH _ { 4 } 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 the solar system composition .