Equatorial Zonal Jets and Jupiter's Gravity

Abstract:

The equatorial region of Jupiter is dynamically special because its rapid rotation exerts the strongest control on the fluid motion there through the largest boundary curvature. Magnetic braking and stable stratification have negligible effects on the equatorial zonal winds that would extend unimpeded through Jupiter in the direction parallel to the rotation axis. The Jovian gravitational field is also highly sensitive to the equatorial region because of its location in the non-spherical Jupiter. Based on an oblate spheroidal-shell model of a polytropic Jupiter with index unity, we calculate, via a perturbation approach using a fully three-dimensional finite element method, the gravitational signature produced by the Jovian equatorial zonal winds that are equatorially symmetric and confined within a small equatorial region that contains only 0.18 percent of the total Jovian mass. It is found that the equatorial winds alone largely determine the high-order gravitational coefficients whose values are thought to reflect the penetration depth of the zonal winds. In other words, the gravitational signature of the Jovian zonal winds is dominated by the equatorial zonal winds and nearly independent of the depth of the non-equatorial winds. This result has an important implication for the ongoing Juno mission which aims to resolve this long-term scientific puzzle about the depth of the Jovian zonal winds through its high-precision gravitational measurements.