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 Jupiter and Saturn in all available submillimeter atmospheric windows between 300 and 1000 GHz (1000 and 300 mu m). The molecules which were searched for include the saturated molecules PH3 and H2S, the hydrogen halides HCl and HBr, and the alkali hydrides LiH and NaH, as well as HCN 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 elsewhere. A feature coincident with the J = 1-0 transition of HCl was also tentatively observed in Saturn. No other molecules were detected in either planet, setting stringent limits on most of the species listed above. Our measurements set an upper limit of 0.3 ppb on the HCN mole fraction in Jupiter if HCN is distributed with a constant mixing ratio. This concentration is significantly lower than the purported infrared detection of Tokunaga et al. (1981). A conservative upper limit of 0.4 ppb is obtained for Saturn, although the molecule may actually be present in Saturn at this abundance. If condensation of HCN is included, the 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 mole 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 spectroscopic limits on the halides in any jovian planet. They indicate that Cl is strongly depleted relative to the solar abundance in the upper tropospheres of Jupiter and Saturn, but the tentative detection of HCl in Saturn suggests that vertical transport is rapid compared to the chemical lifetime of HCl. We also obtain upper limits of 29 ppb (Jupiter) and 16 ppb (Saturn) for H2S (the latter is an order of magnitude improvement over existing measurements), and 22 ppb (Jupiter) and 5 ppb (Saturn) for HCP. For Saturn, this is nearly a factor of 2 smaller than the value suggested by photochemical models of Kaye and Strobel (1984). Finally, we obtain upper limits of 11 ppt (Jupiter) and 12 ppt (Saturn) for LIH, and 8 ppt (Jupiter) and 5 ppt (Saturn) for NaH. These are the first tropospheric upper limits to be placed on alkali metals in the troposphere of the Jovian planets, and indicate that both Li and Na are strongly depleted relative to meteoritic abundances. All limits are 3 sigma values. (C) 1996 Academic Press, Inc.