The Origin of Water Vapor and Carbon Dioxide in Jupiter's Stratosphere

Abstract

Observations of H ₂O rotational lines from the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite (SWAS) and of the CO ₂ ν ₂ band by ISO are analyzed jointly to determine the origin of water vapor and carbon dioxide in Jupiter's stratosphere. Simultaneous modelling of ISO/LWS and ISO/SWS observations acquired in 1997 indicates that most of the stratospheric jovian water is restricted to pressures less than 0.5±0.2 mbar, with a disk-averaged column density of (2.0±0.5)×10 ¹⁵ cm ^-2. Disk-resolved observations of CO ₂ by ISO/SWS reveal latitudinal variations of the CO ₂ abundance, with a decrease of at least a factor of 7 from mid-southern to mid-northern latitudes, and a disk-center column density of (3.4±0.7)×10 ¹⁴ cm ^-2. These results strongly suggest that the observed H ₂O and CO ₂ primarily result from the production, at midsouthern latitudes, of oxygenated material in the form of CO and H ₂O by the Shoemaker-Levy 9 (SL9) impacts in July 1994 and subsequent chemical and transport evolution, rather than from a permanent interplanetary dust particle or satellite source. This conclusion is supported by quantitative evolution model calculations. Given the characteristic vertical mixing times in Jupiter's stratosphere, material deposited at ∼0.1 mbar by the SL9 impacts is indeed expected to diffuse down to the ∼0.5 mbar level after 3 years. A coupled chemical-horizontal transport model indicates that the stability of water vapor against photolysis and conversion to CO ₂ is maintained over typically ∼50 years by the decrease of the local CO mixing ratio associated with horizontal spreading. A model with an initial (i.e., SL9-produced) H ₂O/CO mass mixing ratio of 0.07, not inconsistent with immediate post-impact observations, matches the observed H ₂O abunda nce and CO ₂ horizontal distribution 3 years after the impacts. In contrast, the production of CO ₂ from SL9-produced CO and a water component deriving from an interplanetary dust component is insufficient to account for the observed CO ₂ amounts. The observations can further be used to place a stringent upper limit (8×10 ⁴ cm ^-2 s ^-1) on the permanent water influx into Jupiter. This may indicate that the much larger flux observed at Saturn derives dominantly from a ring source, or that the ablation of micrometeoroids leads dominantly to different species at Saturn (H ₂O) and Jupiter (CO). Finally, the SWAS H ₂O spectra do not appear fully consistent with the ISO data and should be confirmed by future ODIN and Herschel observations.