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

✍ Scribed by Thierry Fouchet; Emmanuel Lellouch; Bruno Bézard; Thérèse Encrenaz; Pierre Drossart; Helmut Feuchtgruber; Thijs de Graauw

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 326 KB
• Volume: 143
• Category: Article
• ISSN: 0019-1035
• DOI: 10.1006/icar.1999.6255

✦ Synopsis

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 15 NH 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 15 N at this pressure level [( 15 N/ 14 N) = (1.9 +0.9 -1.0 ) × 10 -3 , while ( 15 N/ 14 N) ⊕ = 3.68 × 10 -3 ]. It is not clear whether this depletion reveals the global jovian 15 N/ 14 N 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 15 N/ 14 N ratio.