Prior to the Galileo probe entry in Jupiter's atmosphere, the ammonia abundance 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 NH 3 to be 3.6 ± 0.5 times solar N at P > 8 bar (Folkner et al. 1998). Assuming this high value is globally representative of the NH 3 abundance in Jupiter's deep atmosphere, we show in this article that to match Jupiter's microwave spectrum the ammonia abundance must, globally, decrease at pressures P 4 bar, and reach subsolar ( 0.5) values at P 2 bar. We confirm earlier analysis of the 1.3-cm wavelength region indicating that the diskaveraged relative humidity must be of the order of 10% at P < 0.55 bar. We discuss various ways in which NH 3 could decrease globally at altitudes well below the level where the NH 3 -ice clouds form. We 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 • N, i.e., at the southern edge of the north equatorial belt (NEB). Simulations of our data suggest that the longitude-averaged NH 3 abundance in the NEB at the time of the Galileo probe entry is of the order of 50-70% of the value in the equatorial zone (EZ), while the NH 3 abundance in the EZ is about 0.5 × solar N. This low ammonia abundance in the NEB must extend down to the ∼4to 6-bar level. c 2001 Academic Press
2. INTRODUCING THE GALILEO ↔ GROUND-BASED MICROWAVE PARADOX
On December 7, 1995, the Galileo probe entered Jupiter's atmosphere at a latitude of 6.5 • N and at ∼3 • W longitude (Syst. III 1965.0 coordinates, which are based on the rotation period of Jupiter's magnetic field). During its descent, the probe measured the atmospheric structure (temperature, pressure, density), gas composition, and cloud properties down to a pressure level of ∼20 bar. This depth was determined by the atmospheric opacity at the frequency of the radio link (1.4 GHz or 21 cm) between the probe and the orbiter; the probe signal was progressively absorbed by NH 3 gas until the signal was lost. The decrease in signal strength was used to determine the NH 3 volume mixing ratio (or mole fraction) along the probe trajectory. The latter 66