Reconciling Galileo probe data and ground-based radio observations of ammonia on Jupiter

Prior to the Galileo probe entry in Jupiter's atmosphere, the ammonia abund ance in the planet's deep atmosphere as deduced from microwave observations was thought to be close to the solar N value at P > 3 bar and subsolar at P < 2 bar, Analysis of the attenuation of the probe radio signal during its descent in Jupiter's atmosphere suggested NH3 to be 3.6 +/- 0.5 times sola r N at P > 8 bar (Folkner er al 1998). Assuming this high value is globally representative of the NH3 abundance in Jupiter's deep atmosphere, we show in this article that to match Jupiter's microwave spectrum the ammonia abun dance must, globally, decrease at pressures P less than or similar to4 bar, and reach subsolar (less than or similar to0.5) values at P less than or s imilar to2 bar, We confirm earlier analysis of the 1.3-cm wavelength region indicating that the disk-averaged relative humidity must be of the order o f 10% at P < 0.55 bar. We discuss various ways in which NH3 could decrease globally at altitudes well below the level where the NH3-ice clouds form, W e also present radio images of Jupiter taken with the VLA at 2, 3.6, and 6 cm wavelength in November/December 1995 and January 1996. The Galileo probe entered Jupiter's atmosphere on 7 December 1995, at a latitude of 6.5<degr ees>N, i.e., at the southern edge of the north equatorial belt (NEE), Simul ations of our data suggest that the longitude-averaged NH3 abundance in the NEB at the time of the Galileo probe entry is of the order of 50-70% of th e value in the equatorial zone (EZ), while the NH3 abundance in the EZ is a bout less than or similar to0.5 x solar N, This low ammonia abundance in th e NEB must exten down to the similar to4- to 6-bar level. (C) 2000 Academic Press..