Predictions of thermal and gravitational signals of Jupiter's deep zonal winds

Abstract

NASA's Juno spacecraft will make microwave and gravity measurements of Jupiter. These can reveal information about the composition of Jupiter's atmosphere and about the temperature and density structure below the visible clouds, which is in balance with the structure of the zonal winds. Here we show that there exist strong physical constraints on the structure of the off-equatorial deep zonal winds, and that these imply dynamical constraints on the thermal and gravitational signals Juno will measure. The constraints derive from the facts that Jupiter is rapidly rotating, has nearly inviscid flow, and has strong intrinsic heat fluxes emanating from the deep interior. Because of the strong intrinsic heat fluxes, Jupiter's interior is convecting, but the rapid rotation and weak viscosity constrain the convective motions away from the equator to occur primarily along cylinders parallel to the planet's spin axis. As a consequence, convection is expected to approximately homogenize entropy along the spin axis, thereby adjusting the interior to a convectively and inertially nearly neutral state. In this state, entropy gradients perpendicular to the spin axis are constant but generally not zero on cylinders concentric with the spin axis. Additionally, thermal wind balance relates entropy gradients perpendicular to the spin axis to the zonal wind shear between the observed cloud-level winds and winds in the deep interior (pressures of order 10^6 bar), which must be much weaker because otherwise the Ohmic energy dissipation produced by the interaction of the zonal winds with the planetary magnetic field would exceed the planetary luminosity. Combining these physical constraints with thermal and electrical properties of the atmosphere, we obtain that zonal winds away from the equator likely extend deeply into Jupiter (to a depth between about 0.84R_J and 0.94R_J with Jupiter radius R_J) but have strengths similar to cloud level winds only within the outer few percent of Jupiter's radius. Meridional equator-to-pole temperature contrasts in thermal wind balance with the zonal winds increase with depth and reach ∼1–2 K at 50 bar; they would reach O(10 K) if the winds were shallowly confined, as has been proposed previously. Such temperature contrasts will be detectable by Juno's microwave instrument and are expected to be much larger than those associated with variations in water vapor abundance. The associated gravitational signals of the zonal winds will also be detectable by Juno, but they will be more difficult to distinguish from those implied by other flow models with deep zonal flows. The combination of Juno's gravity and microwave instruments should be able to distinguish deep flows (detectable gravitational signals) from shallow flows (detectable thermal signals), providing strong constraints on the penetration depth of substantial zonal winds.

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