Presentation: jovian planets

Moons, rings, and belt-zones, oh my! (Video link: "Outer Space.")

Despite their fascinating moons and rings, in this presentation we will concentrate only on certain features of the jovian planets themselves, many of the details will be covered only in the textbook reading.

All of the jovian planets are composed primarily of hydrogen, but for the purposes of comparison we can group together Jupiter and Saturn as "gas giants," while Uranus and Neptune are grouped together as "ice giants," due to their weird warm slushy ice layers.

First, the gas giants: Jupiter and Saturn.

These are cylindrical maps of Jupiter and Saturn, if you unwrapped their exteriors and laid them flat by unrolling them. You are actually looking at the tops of their clouds, which are visibly more active and colorful on Jupiter than on Saturn. Let's investigate the reasons why weather on Jupiter is more active and bolder. (Video link: "PIA02863: Planetwide Color Movie.")

Jupiter is much more massive than Saturn, so the "turkey/cornish hen effect" discussed for terrestrial planets applies here as well--Jupiter retains much more core heat than Saturn.

The weather on jovian planets is driven by core heat, like these cups of coffee, one of which is steaming hot, while the other has been chilled in a refrigerator. Cream is poured into both cups, and the only stirring is due to convection currents (or lack thereof). What do you observe that lets you know which cup is hotter, and which is cooler? (Video link: "081126-1060756.")

Although core heat provides the energy for active weather patterns on Jupiter, sunlight is the energy for Jupiter's bolder cloud colors. Here are cross-sections of Jupiter's and Saturn's atmospheres (scale has been normalized for comparison), where the sun is shown at different sizes to represent the amount of energy each planet receives. The topmost clouds of both planets is identical in composition and color, but the clouds in Jupiter a warmed more by sunlight, and rise higher up than on Saturn, where clouds do not receive as much sunlight, and so sink lower in the atmosphere, where their colors are obscured.

So there are two distinct sources of energy that drive the weather in these gas giants--core heat (determined by mass), and sunlight (determined by distance from the sun) that make weather more active and colorful on Jupiter, and less active and hazy on Saturn.

Second, the ice giants: Uranus and Neptune.

Cylindrical maps of Uranus and Neptune, if you unwrapped their exteriors and laid them flat by unrolling them. (These approximate features visible to the naked eye, many images of Uranus and Neptune that show more features have been enhanced, or are other wavelengths such ultraviolet or infrared.)

Why does Neptune have more atmospheric circulation than Uranus?
(A) Neptune is closer to the sun.
(B) Neptune has more moons to exert tidal heating.
(C) Neptune is hotter.
(D) Neptune rotates faster.
(E) (Unsure/guessing/lost/help!)

This is interesting because Neptune is further from the sun than Uranus, so sunlight cannot be the energy source for Neptune's more active weather patterns, and they are approximately the same mass, so core heat does not seem to be the energy source that accounts for their differences in weather activity. What is markedly different is that Uranus' axis is drastically tilted over. Rotating on a tilted axis by itself should not be the cause for differences in weather activity, but it may stem from the cause of this tilted axis...

Consider a Cooper Cooler^TM, which spins a bottle in a circulating ice water bath. This demonstrably chills faster than keeping a bottle still in an unstirred ice water bath. It is not the sideways rotation axis that is important here, but the continuous forced circulation that accounts for the faster cooling rate. A large impact hypothesis may explain not only how Uranus' axis was tilted over from being vertical (like all other planets, and the sun from the formation of the solar system) to sideways, but the stirring up of Uranus' interior during this large impact would have forced it to cool off faster, resulting in much less weather patterns than Neptune, which has retained more of its core heat. These large impacts in the early stages of planet-forming may indeed be very common, as seen with similar hypotheses for the formation of the moon and the disproportionally large core of Mercury. (Video link: "081108-1060446.")