BAROCLINIC INSTABILITY IN THE INTERIORS OF THE GIANT PLANETS - A COOLING HISTORY OF URANUS

We propose a quasigeostrophic, baroclinic model for heat transport wit hin the interior of a stably stratified Jovian planet, based on motion in thin cylindrical annuli. Density decreases from the center outward and is zero at the surface of the planet. In the homogeneous case (no core), we find instability for the poles hotter than the equator, but not for the reverse. If the motion is bounded by an impenetrable core , instability occurs for both cases. Much of the behavior can be expla ined by analogy to conventional baroclinic instability theory. Motivat ed by our results, we explore a possible connection between the highly inclined rotation axis of Uranus and its anomalously low surface heat flux. We assume that the planets formed hot. Our conjecture is that h eat was efficiently convected outwards by baroclinic instability in Ur anus (with the poles hotter than the equator), but not in the other th ree Jovian planets. The surface temperature was higher for the stably stratified case (Uranus), leading to a higher rate of infrared emissio n and faster cooling. Therefore, we propose that Uranus lost its inter nal heat sooner than Neptune because baroclinic motions, permitted by its inclination to the sun, were able to extract its internal heat whi le the surface was still warm. (C) 1994 Academic Press, Inc.