Photochemistry and diffusion in Jupiter's stratosphere: Constraints from ISO observations and comparisons with other giant planets

Abstract

[1] We have developed a one-dimensional, diurnally averaged, photochemical model for Jupiter's stratosphere that couples photodissociation, chemical kinetics, vertical diffusion, and radiative transport. The predictions regarding the abundances and vertical profiles of hydrocarbon compounds are compared with observations from the Infrared Space Observatory (ISO) to better constrain the atmospheric composition, to better define the eddy diffusion coefficient profile, and to better understand the chemical reaction schemes that produce and destroy the observed constituents. From model-data comparisons we determine that the C 2 H 6 mole fraction on Jupiter is (4.0 ± 1.0) Â 10 À6 at 3.5 mbar and (2.7 ± 0.7) Â 10 À6 at 7 mbar, and the C 2 H 2 mole fraction is (1.4 ± 0.8) Â 10 À6 at 0.25 mbar and (1.5 ± 0.4) Â 10 À7 at 2 mbar. The column densities of CH 3 C 2 H and C 6 H 6 are (1.5 ± 0.4) Â 10 15 cm À2 and (8.0 ± 2) Â 10 14 cm À2 , respectively, above 30 mbar. Using identical reaction lists, we also have developed photochemical models for Saturn, Uranus, and Neptune. Although the models provide good first-order predictions of hydrocarbon abundances on the giant planets, our current chemical reaction schemes do not reproduce the relative abundances of C 2 H x hydrocarbons. Unsaturated hydrocarbons like C 2 H 4 and C 2 H 2 appear to be converted to saturated hydrocarbons like C 2 H 6 more effectively on Jupiter than on the other giant planets, more effectively than is predicted by the models. Further progress in our understanding of photochemistry at low temperatures and low pressures in hydrogen-dominated atmospheres hinges on the acquisition of high-quality kinetics data.

Insights into the role of polycyclic aromatic hydrocarbon condensation in haze formation in Jupiter’s atmosphere

“…This would not account for the production of haze particles at mid-latitudes, however. Further progress in our understanding of photochemistry at low temperatures and low pressures in hydrogen-dominated atmospheres hinges on the acquisition of high-quality kinetics data (Moses et al 2005).…”

Neptune's atmospheric composition from AKARI infrared spectroscopy

“…As AKARI spectra offer no constraint on the atmospheric temperatures beneath the tropopause, we follow Meadows et al (2008) and use the Neptune T (p) structure from Moses et al (2005) as a starting point for fitting temperatures above the 1-mbar level. The initial profile was formed from a combination of results from Voyager radio science investigation in the troposphere (RSS, Lindal et al 1990), with updates in the stratosphere from the ISO (Bézard et al 1998;Feuchtgruber et al 1999;Fouchet et al 2003;Burgdorf et al 2003) and from 17-24 μm ground-based spectra (Orton et al 1987(Orton et al , 1990.…”

“…The reference temperature profile used in this analysis had been used as input for a one-dimensional model of Neptune's stratospheric photochemistry (Model A from Moses et al 2005) which provided the hydrocarbon abundance profiles for scaling analysis in this data. The deep CH 4 abundance was set to 2.2% following Baines & Hammel (1994) who determined this value using a combination of the Voyager/RSS temperature profile and ground-based measurements of H 2 quadrupole lines.…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…It is therefore no surprise that tentative evidence for comet impact signatures has been found in giant planets beyond Jupiter using observations of this species. The observability of cometary-derived species like CO is made plausible given the existing trade-space found between km-size comet impact rates at Jupiter, Saturn, Uranus, and Neptune (∼ 0.01-0.001 year −1 ; Zahnle et al 2003), and stratospheric meridional/vertical mixing timescales (∼ 100-1000 years; Moreno et al 2003;Moses et al 2005). This trade-space seems to ensure that comets of decent size can leave observable signatures on the long term in giant planet stratospheres.…”