ISO-SWS observations of Jupiter: measurement of the ammonia tropospheric profile and of the 15N/14N isotopic ratio

We present the results of the Infrared Space Observatory Short Wavelength Spectrometer (ISO-SWS) observations of Jupiter related to ammonia. We focus on two spectral regions; the first one (the 10-μm region), ranging from 9.5 to 11.5 μm, probes atmospheric levels between 1 and 0.2 bar, while the second one (the 5-μm window), ranging from 4.8 to 5.5 μm, sounds the atmosphere between 8 and 2 bars. The two spectral windows cannot be fitted with the same ammonia vertical distribution. From the 10-μm region we infer an ammonia distribution of about half the saturation profile above the 1-bar level, where the N/H ratio is roughly solar. A totally different picture is derived from the 5-μm window, where we determine an upper limit of 3.7 × 10−5 at 1 bar and find an increasing NH3 abundance at least down to 4 bar. This profile is similar to the one measured by the Galileo probe (Folkner et al. 1998). The discrepancy between the two spectral regions most likely arises from the spatial heterogeneity of Jupiter, the 5-μm window sounding dry areas unveiled by a locally thin cloud cover (the 5-μm hot spots), and the 10-μm region probing the mean jovian atmosphere above 1 bar. The NH3 mixing ratio is measured around 400 mbar from ν2 band absorptions in the 10-μm region. We find the atmosphere of Jupiter highly depleted in 15N at this pressure level ((N/N)J = (1.9 −1.0)× 10−3, while (N/N)⊕ = 3.68 × 10−3). It is not clear whether this depletion reveals the global jovian 15N/14N ratio. Instead an isotopic fractionation process, taking place during the ammonia cloud condensation, is indicated as a possible mechanism. A fractionation coefficient α higher than 1.08 would explain the observed isotopic ratio, but the lack of laboratory data does not allow us to