The spectrum of Saturn from 1990 to 2230 cm −1: Abundances of AsH 3, CH 3D, CO, GeH 4, NH 3, and PH 3

Abstract

We have investigated the mole fractions of six gases (AsH 3, CH 3D, CO, GeH 4, NH 3, and PH 3) from extensive computer modeling of a high-resolution spectrum of Saturn from 1994 to 2229 cm −1 (4.5 to 5.0 μm). Five new AsH 3 spectral features and three previously observed absorptions were detected, leaving no doubt as to the presence of this gas in Saturn. The mole fraction of AsH 3 is 3 × 10 −9, in agreement with earlier work and 10 times higher than that found in Jupiter. Thirteen lines of CO were detected, including eight new lines. Our CO abundance is lower than previous estimates, which we attribute to an improved atmospheric model. We find that the CO mole fraction is 1 × 10 −9 if its distribution is uniform and 2.5 × 10 −8 if concentrated in the stratosphere at pressures P < 80 mbar. The detection of one high- J line of CO in Saturn's spectrum, P14, favors a tropospheric distribution but the evidence is not strong enough to make a firm conclusion regarding the vertical distribution of CO. The PH 3 abundance increases dramatically below the photochemically active portion of the atmosphere. For P < 400 mbar the PH 3 mole fraction was fixed at 1 × 10 −6, in agreement with previous work, which produced a good fit to that region of Saturn's 5-μm spectrum dominated by reflected sunlight. At P > 400 mbar, however, we find that the PH 3 mole fraction is 7 −2 +3 × 10 −6, which indicates a surprisingly high elemental abundance of P in Saturn. The mole fraction of CH 3D is (3.3 ± 1.5) × 10 −7, in general agreement with values determined at other wavelengths, which leads to a D/H ratio of (1.7 ± 1.1) × 10 −5. Our constraints to the abundances of GeH 4 and NH 3 are weak. We find that the mole fraction of GeH 4 is (4 ± 4) × 10 −10 and that of NH 3 is ≤ 3 × 10 −4. The observed elemental abundances of P and As are important as remnants of the solid component of the solar nebula. Their pattern of enrichment in Jupiter and Saturn appears to be different than that for C and N.