Jupiter's Downdrafts as Revealed by the Galileo Probe

A.P. Showman, A.P. Ingersoll (Caltech)

The Galileo probe found the jovian abundance of H tex2html_wrap_inline14 S and water to be 20-30% solar above the 10 bar level, a puzzling result. Since H tex2html_wrap_inline14 S and water condense at 2 and 6 bars respectively, the probe probably entered a dry downdraft wherein dry air is advected from above cloud top to 10 bars or deeper. This is consistent with the fact that the probe fell into the south edge of a hot spot, a region known from spectral modelling to be unusually low in cloud abundance.

We synthesize available observations and physical constraints into a set of self-consistent scenarios for the characteristics of Jupiter's hot spot circulations. Our primary focus is on the Galileo Probe Doppler-tracked winds. Assuming ``gradient wind'' balance (a balance between pressure-gradient, Coriolis, and centripetal accelerations due to curving flow trajectories), we calculate latitudinal gradients of density from the measured winds; extrapolating these gradients with latitude tells us the density difference between the hot spot and the region to the south. The gradients depend on the radius of curvature of the flow trajectory, which is unknown. Certain values of the radius yield gradients which cannot be reconciled with simple notions of convection on Jupiter, while others yield gradients which match such notions. The most likely scenario implies that (1) the region south of the hot spot is stable above 6 bars, consistent with moist convective upwellings containing a water abundance of 1-2 times solar, and (2) the hot spot is denser than the surroundings from 2-5 bars. The wind data also suggest that the hot spot is less dense below 5 bars, a curious situation requiring mechanical forcing to push the dry air downward. However, when interpreted as updraft-downdraft differences, our inferred densities imply that convection in the 2-5 bar layer releases enough energy to supply the required forcing at deeper levels.