Jupiter's Banding and Jets May Be Caused by Tides

Abstract

The stratification parameters for Jupiter's outer 3000 km shell are calculated using a density profile recently derived from observations of the NASA Juno spacecraft currently in Jovian orbit. Using these parameters, the equations of classical tidal theory for a stratified, nonhydrostatic, compressible fluid are numerically solved for sectoral tidal forcing by Io. The results support a long-standing though little discussed proposal that the banding/jets (and possibly the unexplained endogenic heat) are caused by the tides. First, general arguments from eigenmode analyses expect resonantly forced tidal modes and the scattering of the tidal response to higher spatial degrees by Jupiter's fast rotation, with time-averaged tidal effects appearing in bands between critical latitudes (±50° for forcing by Io). Second, resonant tides and banding are specifically demonstrated in the tidal model configured with the Juno-derived stratification. While banding in the time-averaged tidal features is a robust expectation (from the well-prescribed forcing and rotation parameters) and is independent of the internal parameters, the details of the banding (e.g., number, width) are highly dependent. Hence, comparison of the tidal model with observations provides a test of the tidal hypothesis of the bands as well as assumptions of interior parameters and processes. Here, dissipation parameterized as a simple pressure relaxation term in the vertical balance equation shows a time-mean banded structure between the critical latitudes that can drive geostrophic jets matching the major observed features, including strong prograde flow at the equator. By contrast, alternate stratification/dissipation assumptions produce banded structures that do not agree with observations.