The possibility of inferring the depth of Jupiter's Great Red Spot with the Juno gravity experiment

Abstract:

Our goal is to assess the possibility of inferring the depth of the Great Red spot by detecting its gravitational signature with the upcoming Juno gravity experiment, depending on the deep structure of the vortex. We estimate the strength of the gravity signal coming from the vortex using an idealized dynamical model for the atmosphere of the giant planet. The gravity anomaly is then compared to the expected accuracy in the retrieval of the surface gravity at the GRS location obtained with numerical simulations of the Doppler data inversion based on the expected trajectory of the spacecraft. Starting from observations of the planet's velocity vectors at the cloud level, we propagate the profiles along coaxial cylinders parallel to the spin axis and explore a wide range of decay scale heights in the radial direction. Assuming the large

scale vortex dynamics are geostrophic, and therefore thermal wind balance holds, the density anomaly distribution due to Jupiter's winds can be derived from the velocity maps. The novelty of this approach is in the integration of thermal wind relations over a three-dimensional grid, and in the inclusion of the observed meridional velocity as measured during the Cassini flyby of Jupiter. The introduction of longitudinal perturbations to the mean zonal flows gives rise to fluctuations in the tesseral spherical harmonics of Jupiter's gravitational potential. We show that the deeper the penetration of the jets, the stronger is the gravity signal coming from the GRS. On the other hand, the larger the mass involved in the planet's atmospheric motion, the higher are the degree and order of the spherical harmonic expansion of Jupiter's gravity field needed to fit the simulated Doppler data. We show that the mass anomaly associated with the Great Red Spot is detectable by the Juno gravity experiment if the winds penetrate at depth of 2,000 km below the cloud level of Jupiter and that the large mass involved with deep winds does not render much the ability to measure the feature.