This paper presents the first detailed analysis of acetylene absorption features observed longward of 190.0 nm in a jovian spectrum by
the Faint Object Spectrograph on board the Hubble Space Telescope. The presence of two features located near 207.0 nm can only be
explained by a substantial abundance of acetylene in the upper troposphere. Using a Rayleigh–Raman radiative transfer model, it was
determined that the acetylene vertical profile is characterized by a decrease in the mole fraction with increasing pressure in the upper
stratosphere, a minimum around 14 to 29 mbar, followed by an increase to about 1.5 107 in the upper troposphere. Longward of 220
nm, the relatively high contrast of Raman features to the continuum precludes the existence of an optically significant amount of clouds from
150 to 500 mbar unless they are highly reflective. Instead, the reflectivity at these long wavelengths is determined by stratospheric, not
tropospheric, scatterers and absorbers. Analysis of the data also suggests that ammonia is extremely undersaturated at pressures below 700
mbar. However, no firm conclusions can be reached because of the uncertainties surrounding its cross section longward of 217.0 nm, which
are due to vibrationally excited states.
© 2003 Elsevier Science (USA). All rights reserved.