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

Observations of H2O rotational lines from the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite (SWAS) and of the CO2 ν2 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)×1015 cm−2. Disk-resolved observations of CO2 by ISO/SWS reveal latitudinal variations of the CO2 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)×1014 cm−2. These results strongly suggest that the observed H2O and CO2 primarily result from the production, at midsouthern latitudes, of oxygenated material in the form of CO and H2O 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 CO2 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) H2O/CO mass mixing ratio of 0.07, not inconsistent with immediate post-impact observations, matches the observed H2O abunda nce and CO2 horizontal distribution 3 years after the impacts. In contrast, the production of CO2 from SL9-produced CO and a water component deriving from an interplanetary dust component is insufficient to account for the observed CO2 amounts. The observations can further be used to place a stringent upper limit (8×104 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 (H2O) and Jupiter (CO). Finally, the SWAS H2O spectra do not appear fully consistent with the ISO data and should be confirmed by future ODIN and Herschel observations. [Copyright &y& Elsevier]