Non-Detection of Hydrogen Cyanide on Jupiter

Abstract

High spectral resolution observations of Jupiter performed at the NASA/Infrared Telescope Facility near 13.5 μm are presented. Two spectral ranges including the R(7) and R(11) lines of the v2 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 × 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 × 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 × 10-4 cm-amagat on the HCN column density in the stratosphere (corresponding to a maximum average mixing ratio of 8 × 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.