INTERPRETATION OF GALILEO PROBE DATA AND IMPLICATIONS FOR JUPITER DRYDOWNDRAFTS

The Galileo probe found the jovian abundance of H2S to be 30% solar at the 8 bar level, while the abundance of water was less than 3% solar at 12 bars. From 8 to 20 bars, H2S increased to three times solar, and water apparently increased as well. Since H2S and water condense at 2 and 5 bars, respectively, the probe probably entered a dry downdraft, wherein dry air above 2 bars is advected to 12 bars or deeper (Owen e t al. 1996, Eos (Spring Suppl.) 77, S171). This is consistent with the fact that the probe entered the south edge of a 5-mu m hot spot, a lo cal region of Jupiter's atmosphere known from spectral modeling to be unusually low in cloud abundance (Orton et al. 1996, Science 272, 839) . We use basic physical constraints to address three problems raised b y Galileo probe data. First, it is unclear how the hypothesized downdr aft remains dry, since simple models of convection preclude dessicatio n below the 2- and 5-bar condensation levels. We suggest that to suppr ess moist plumes from below, the downdraft must be of low density belo w 5 bars and hence thermally indirect, requiring mechanical forcing fr om other parts of the atmosphere. Second, if geostrophic balance holds , the Galileo probe winds imply that the hot spot (north of the probe site) contains a stable layer from 1 to 5 bars; this is inconsistent w ith a downwelling, since downwellings should be adiabatic below 2 bars due to the low radiative flux divergence. We show that when the centr ipetal acceleration of curving parcel trajectories is included in the force balance, however, a variety of density profiles is possible with in the hot spot (depending on the radius of curvature of the winds). T he most plausible profile implies that the hot spot is nearly dry adia batic and that the equatorial zone south of the probe site is stable f rom 2 to 6 bars, suggesting moist adiabatic upwellings with a water ab undance of 1-2 times solar. This is consistent with Galileo and Voyage r images suggesting upwelling at the equator. The profile further impl ies that from 1 to 5 bars the hot spot is denser than the equatorial z one south of the probe site. Third, probe data indicate that NH3 incre ased with depth below 1 bar and became constant by 8 bars, H2S began i ncreasing below 8 bars and leveled off by 16 bars, while water only be gan increasing below 12 bars and was still increasing with depth at 20 bars. We propose that lateral mixing along isopycnals (surfaces of co nstant potential density) could produce the observed pattern; alternat ively, the downwelling might consist of column stretching, so that the NH3, NH4SH, and water lifting condensation levels were pushed to 8, 1 6, and >20 bars, respectively. In either case, the simplest form of th is model requires the downdraft to be less dense than the surroundings from 0.5 to 20 bars. In its simplest form, this model is therefore in compatible with our favored interpretation of the winds; more detailed studies will be necessary to resolve the problem. (C) 1998 Academic P ress.