SUBMILLIMETER LINE SEARCH IN JUPITER AND SATURN

A Fourier transform spectrometer was used to search for the rotational transitions of a number of molecular species in the atmospheres of Ju piter and Saturn in all available submillimeter atmospheric windows be tween 300 and 1000 GHz (1000 and 300 mu m). The molecules which were s earched for include the saturated molecules PH3 and H2S, the hydrogen halides HCl and HBr, and the alkali hydrides LiH and NaH, as well as H CN and HCP. A strong absorption feature at 800.5 GHz corresponding to the J = 3-2 transition of PH3 was detected in both Jupiter and Saturn, but the detailed analysis of these results will be presented elsewher e. A feature coincident with the J = 1-0 transition of HCl was also te ntatively observed in Saturn. No other molecules were detected in eith er planet, setting stringent limits on most of the species listed abov e. Our measurements set an upper limit of 0.3 ppb on the HCN mole frac tion in Jupiter if HCN is distributed with a constant mixing ratio. Th is concentration is significantly lower than the purported infrared de tection of Tokunaga et al. (1981). A conservative upper limit of 0.4 p pb is obtained for Saturn, although the molecule may actually be prese nt in Saturn at this abundance. If condensation of HCN is included, th e deep tropospheric HCN limits are increased to 2 ppb for Jupiter and similar to 15 ppb for Saturn. We set a tropospheric HCl upper limit of 5 ppb in Jupiter, and have tentatively detected this molecule at a mo le fraction of 1.1 ppb in Saturn. We set HBr upper limits of 7 ppb in Jupiter and 1.1 ppb in Saturn. To our knowledge, these are the first s pectroscopic limits on the halides in any jovian planet. They indicate that Cl is strongly depleted relative to the solar abundance in the u pper tropospheres of Jupiter and Saturn, but the tentative detection o f HCl in Saturn suggests that vertical transport is rapid compared to the chemical lifetime of HCl. We also obtain upper limits of 29 ppb (J upiter) and 16 ppb (Saturn) for H2S (the latter is an order of magnitu de improvement over existing measurements), and 22 ppb (Jupiter) and 5 ppb (Saturn) for HCP. For Saturn, this is nearly a factor of 2 smalle r than the value suggested by photochemical models of Kaye and Strobel (1984). Finally, we obtain upper limits of 11 ppt (Jupiter) and 12 pp t (Saturn) for LIH, and 8 ppt (Jupiter) and 5 ppt (Saturn) for NaH. Th ese are the first tropospheric upper limits to be placed on alkali met als in the troposphere of the Jovian planets, and indicate that both L i and Na are strongly depleted relative to meteoritic abundances. All limits are 3 sigma values. (C) 1996 Academic Press, Inc.