Interpretation of Galileo Probe Data and Implications for Jupiter's Dry Downdrafts

Abstract

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 (Owenet al.1996,Eos(Spring Suppl.) 77, S171). This is consistent with the fact that the probe entered the south edge of a 5-μm hot spot, a local region of Jupiter's atmosphere known from spectral modeling to be unusually low in cloud abundance (Ortonet al.1996,Science272, 839).We use basic physical constraints to address three problems raised by Galileo probe data. First, it is unclear how the hypothesized downdraft remains dry, since simple models of convection preclude dessication below the 2- and 5-bar condensation levels. We suggest that to suppress moist plumes from below, the downdraft must be of low density below 5 bars and hence thermally indirect, requiring mechanical forcing from 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 with a downwelling, since downwellings should be adiabatic below 2 bars due to the low radiative flux divergence. We show that when the centripetal acceleration of curving parcel trajectories is included in the force balance, however, a variety of density profiles is possible within the hot spot (depending on the radius of curvature of the winds). The most plausible profile implies that the hot spot is nearly dry adiabatic and that the equatorial zone south of the probe site is stable from 2 to 6 bars, suggesting moist adiabatic upwellings with a water abundance of 1–2 times solar. This is consistent with Galileo and Voyager images suggesting upwelling at the equator. The profile further implies that from 1 to 5 bars the hot spot is denser than the equatorial zone south of the probe site. Third, probe data indicate that NH3increased with depth below 1 bar and became constant by 8 bars, H2S began increasing below 8 bars and leveled off by 16 bars, while water only began increasing below 12 bars and was still increasing with depth at 20 bars. We propose that lateral mixing along isopycnals (surfaces of constant potential density) could produce the observed pattern; alternatively, the downwelling might consist of column stretching, so that the NH3, NH4SH, and water lifting condensation levels were pushed to 8, 16, and >20 bars, respectively. In either case, the simplest form of this 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 incompatible with our favored interpretation of the winds; more detailed studies will be necessary to resolve the problem.