Chemical constraints on the water and total oxygen abundances in the deep atmosphere of Saturn
ASTROPHYSICAL JOURNAL, 623 (2):
1221--1227 (2005)Abstract
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/H-2 >= (1.7(-0.4)(+0.7)
) x 10(-3) is given by CO chemistry, whereas an upper limit of H2O/H-2
<= (5.5(-2.5)(+0.8) ) x 10(-3) is given by PH3 chemistry. A combination
of the CO and PH3 constraints indicates a water enrichment on Saturn
of 1.96.1 times the solar system abundance (H2O/H-2 = 8.96 x 10(-4)).
The total oxygen abundance must be at least 1.7 times the solar system
abundance (O/H-2 = 1.16 x 10(-3)) in order for SiH4 to remain below
the detection limit of SiH4/H-2 < 2 x 10(-10). A combination of the
CO, PH3, and SiH4 constraints suggests that the total oxygen abundance
on Saturn is 3.2-6.4 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.