ISO-SWS Observations of Jupiter: Measurement of the Ammonia Tropospheric Profile and of the 15N/ 14N Isotopic Ratio

Abstract

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 bar. 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 NH 3 abundance at least down to 4 bar. This profile is similar to that measured by the Galileo probe. 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 15NH 3 mixing ratio is measured around 400 mbar from ν 2 band absorptions in the 10-μm region. We find the atmosphere of Jupiter highly depleted in 15N at this pressure level [( 15N/ 14N) [formula]=1.9 +0.9 −1.0)×10 −3, while ( 15N/ 14N) ⊕=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 decide unambiguously on the origin of the observed low 15N/ 14N ratio.