I. Dynamics at the Galileo probe site on Jupiter and II. Orbital and thermal evolution of Ganymede.

摘要

The low abundances of water and H2S measured by the Galileo probe at Jupiter suggest that the probe entered a downdraft where dry air from above cloud top advects to 10 bars or deeper. I use basic physical constraints to extract dynamical information from three aspects of Galileo probe data. First, I suggest that to remain dry, the downdraft must be underlain by a stable layer which inhibits mixing of volatiles from below; this requires the downdraft to be mechanically forced. Second, on rapidly rotating planets, the Coriolis and centripetal forces caused by winds usually balance the horizontal pressure-gradient force (which gives information about horizontal density differences). Therefore, I use the known winds versus depth at the probe site to infer horizontal density differences between the probe site and its surroundings. Under reasonable assumptions, these densities are consistent with a downdraft at the probe site. Third, I use ideas of horizontal mixing and column stretching to explain how the observed vertical profiles of ammonia, H2S, and water vapor can be produced in a downdraft. Finally, I discuss some apparent inconsistencies between these simple models. I also describe preliminary efforts to explore the origin, stability, and evolution of the downdrafts using the shallow water equations.;The differences between Ganymede and Callisto have led to speculation that Ganymede's history was shaped by tidal heating from an orbital resonance. Using the numerical model developed by Malhotra (1991, Icarus 94, 399), I demonstrate that Io, Europa, and Ganymede could have passed through either of two (previously unexplored) Laplace-like resonances en route to the current Laplace resonance. Under reasonable conditions, these resonances produce great enough tidal heating in Ganymede to be geophysically significant. I also coupled the orbital evolution to an internal model of Ganymede to explore the effect of the resonance on Ganymede's interior. If Ganymede's tidal Q decreases strongly with temperature, the coupling can lead to massive, short-lived heating episodes which can melt much of the icy interior. Such heat pulses require the initial ice temperature to be extremely cold (200 K), however, so they may be unlikely.