HST observation of the atmospheric composition of Jupiter’s equatorial region: evidence for tropospheric C2H2☆

Abstract

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 ϫ 10 Ϫ7 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.

On the abundance of non-cometary HCN on Jupiter

“…c d e f Fouchet et al 115 Edgington et al 104 Lara et al 126 Edgington et al 103 Carlson et al 136 Griffith et al 137 Irwin et al 138 de Pater et al 109 Folkner et al 118 Gibson et al 114 Burgdorf et al 139 Achterberg et al 140 Betremieux et al 133 Sromovsky et al 117 The larger optical depth leads to increased shielding of the NH 3 , with a corresponding slight reduction in the NH 2 production rate at the base of the NH 3 cloud and a very slight increase in the NH 3 mole fraction at the top of the cloud. This change in optical depth in the 300-700 mbar pressure region has almost no effect on species abundances, except for that of N 2 , whose production rate slightly drops.…”

“…The HST/FOS ultraviolet spectra are affected by scattering and absorption from many gas-phase and aerosol species, all of which have poorly constrained parameters. Both NH 3 and C 2 H 2 are clearly detected in the HST/FOS ultraviolet spectra, 132,133 but deriving mole fractions from the spectra may be problematic. The strongest argument against the 1.5 × 10 −7 tropospheric C 2 H 2 mole fraction derived by Bétrémieux et al 133 is that such a large amount would generate absorption wings around observed mid-infrared C 2 H 2 emission lines, which Bétrémieux et al recognize is inconsistent with their own ground-based infrared data, 133 as well as with other mid-infrared observations.…”

“…We therefore investigate a photochemical model for which the bottom boundary condition for C 2 H 2 has been changed to a mole fraction of 1.5 × 10 −7 . 132, 133 We also change the bottom boundary conditions for N 2 and CH 3 NH 2 to mole fractions of 3.52 × 10 −5 and 1.52 × 10 −12 , respectively, to reflect the deep-tropospheric quenched source from our nominal thermochemical kinetics and transport model (see Fig. 4 is not currently included in the model, will eventually limit the column abundance.…”

Jupiter in the ultraviolet: acetylene and ethane abundances in the stratosphere of Jupiter from Cassini observations between 0.15 and 0.19 $μ$m

“…A subset of the HST FOS data-set was re-visited by Bétrémieux et al (2003). Their modelling required an ethane abundance of 150 ppb at 120 to 300 mbar in the upper troposphere, disagreeing with previous measurements in the ultraviolet (see Table 1) and disagreeing with observations in the mid-infrared.…”

“…Here, we describe the development of our radiative transfer code to be able to model and retrieve atmospheric abundances from ultraviolet reflectance spectra. We then apply this model to observations of Jupiter by the Cassini Ultraviolet Imaging Spectrograph (UVIS), comparing them to results derived from mid-infrared observations acquired HST 10-30 Bétrémieux et al (2003) HST 40…”