Non-detection of hydrogen cyanide on Jupiter

High spectral resolution observations of Jupiter performed at the NASA/Infrared Telescope Facility near 13.5 mu m are presented, Two spectral ranges including the R(7) and R(11) lines of the v(2) band of HCN were observed at high signal-to-noise. No evidence for HCN absorption in excess of 2% of the continuum was found, in contrast to the report by Tokunaga et al. (1981, Icarus 48, 283-289). An upper limit on the tropospheric HCN mixing ratio of 1 x 10(-9) was derived, assuming a uniform value up to the tropopause. When HCN condensation in the upper troposphere is taken into account, only a looser upper limit around 1.2 x 10(-8) can be set on the deep mixing ratio. The lack of emission cores at the position of the HCN lines provides an upper limit of 2 x 10(-4) cm-amagat on the HCN column density in the stratosphere (corresponding to a maximum average mixing ratio of 8 x 10(-10) above condensation level). A critical reanalysis of Tokunaga et al.'s observations is finally presented, leading to the conclusion that the previously reported detection of HCN is questionable. (C) 1996 Academic Press, Inc.