The composition of Saturn's atmosphere at northern temperate latitudes from Voyager IRIS spectra - NH3, PH3, C2H2, C2H6, CH3D, CH4, and the Saturnian D/H isotopic ratio

The vertical distributions and mixing ratios of minor constituents in the northern hemisphere of Saturn are investigated. Results are obtained for NH3, PH3, C2H2, C2H6, CH3D, and CH4; the D/H ratio is obtained from the CH4 and CH3D abundances. The NH3 mixing ratio in the upper atmosphere is found to be compatible with the saturated partial pressure. The inferred PH3/H2 ratio of 1.4 + or - 0.8 x 10 to the -6th is higher than the value derived from the solar P/H ratio. The stratospheric C2H2/H2 and C2H6/H2 ratios are, respectively, 2.1 + or - 1.4 x 10 to the -7th and 3.0 + or - 1.1 x 10 to the -6th; the latter decreases sharply below the 20-50 mbar level. The results for CH3D/H2 and CH4/H2 imply an enrichment of Saturn's upper atmosphere in carbon by a factor of at least three over the solar abundance. The interpretation of two NH3 lines in the five-micron window suggests a NH3/H2 ratio at the two bar level below the solar value.