Why Gas Giants Have Bands and What it Means

There is hardly any need to describe the surface appearance of Jupiter and Saturn in our solar system. Everyone has already see fly-by pictures of them. They are both distinguished, not only by their huge size compared to Earth, but by the bands of color that stretch across them in longitude. The bands are fairly well confined by latitude, but there are countless large and small cyclones scattered around the planets.

Where these bands come from is fairly easy to understand. Earth has retained heat from its collapse, and it slowly seeps to the surface. We see it in geothermal power stations, and watch it in volcanos. These two gas giants are so much heavier than Earth that the heat of collapse of them would be much larger than on Earth, and because distances are longer from the core to the surface, where the heat can be radiated into space, cooling is relatively slower. That does not mean that the amount of heat seeping up from the surface is smaller than on Earth, just the opposite to orders of magnitude. How it manifests itself is quite different, spectacularly so.

The core is hotter than the surface, so heat rises up. Most of a gas giant is fluid, so the heat is convected. But it is physically impossible for a whole spherical layer to rise uniformly, just because there is nowhere for the gas above the layer to go, but mostly because this is unstable, and specifically a Raleigh-Taylor instability. This means that somewhere hotter gas is being convected upwards, and cooler gas is moving down to compensate for the mass movement. Now we note that Jupiter is spinning rapidly and Saturn as well. This means there will be Coriolis forces, and since Coriolis force is in the direction of the cross product of the rotational vector and the velocity, any vertical velocity is going to result in a longitudinal force, opposite in the two hemispheres. So, gas rises from heat, and gets pushed longitudinally, leading to a longitudinal flow. But the Coriolis force on a longitudinal flow is latitudinal, and therefore gas which goes upwards gets pushed longitudinally and then latitudinally. The latitudinal flow leads to a downward flow. It is fairly easy to see that the overall effect will be gas flowing in a toroid, confined to some range of latitudes, and moving relative to the core rotation rate.

How deep does the flow go? Heat is convecting gas up from deep in the atmosphere, even from what might be called the mantle, where hydrogen solidifies under intense pressure. Gas rising deep in the atmosphere is subject to the same Coriolis forces, so the toroids should be quite deep, and certainly not surface features. Coriolis force depends on latitude, so the toroids would be moving with different rotation rates, leading to shear forces between them, where downgoing gas from one toroid meets upwelling gas from another. Shear forces lead to cyclones.  Cyclones are fed by heat convection and Coriolis forces as well.

Why are the bands differently colored? If they were all the same color, they would be much harder to detect and measure, and wouldn't look so spectacular in telescopic pictures. Just like the Earth, there is a mixture of different gases in the atmosphere of the gas giants, and just like the Earth, gravity separates them out, with heavier elements and molecules settling out at lower altitudes than lighter ones. We would expect to see hydrogen in the exosphere of Earth, and we do. This process, by the way, is how small planets like Venus and Mars lose components of their atmospheres. Jupiter and Saturn are large enough to be able to retain their hydrogen for very long times, compared to the age of the solar system. The composition difference at different depths means that as one layer of gas is getting mightily convected, en masse, different colored materials are being brought up for us to see. Coriolis force varies with latitude, so the depth to which the toroids extend should be different as well, meaning different compositions are at the lower edges of toroids manifesting as different bands, and therefore, different colors are at the upper surface.

What does this mean to us and to an alien civilization? It seems that gas giants act to stabilize planetary orbits, so an alien civilization on an Earth-like planet probably has a couple of gas giants in their heavens. We on Earth are hardly traveling at all through the solar system, just getting started in a small way to look closer at planets and satellites, but an alien civilization, centuries or millennia older than ours, might be traveling quite a bit in interplanetary space. What would they be doing with gas giants?

Energy is what keeps everything going. We have multiple sources, mostly now from carbon deposits, but also from uranium and solar photons. Wind and tides also play a role. Geothermal energy plays a role as well. An advanced alien civilization is going to be looking for other sources of energy, and a gas giant, rotating fast like Jupiter, has a tremendous amount of energy in its winds. Could even a millionth part of that be extracted and used for various purposes beyond the home planet?

It is almost impossible to even conceptualize something being done on a planet like Jupiter, which has storms many times the size of our entire planet, and lasting centuries. The scales of size and mass are so out-of-proportion that is is hard for an Earth person, and presumably an alien from an Earth-like planet, to think of. Even the scale of time is grandiose compared to our time scales. Maybe it takes some colony of aliens living out near their largest gas giant to be able to imagine how to do something with all that energy. Yet far out, solar power is much weaker, so a source of energy from Jupiter and Saturn, or their alien equivalents, would allow the exploration of the solar system to go much faster and be much more extensive.

Thus, we cannot visualize it now, but it might be that if energy can be harvested from a gas giant, large colonies might be possible out in the farther reaches of an alien solar system. This would change how we imagine the future of an alien civilization. It is the difference between a single-planet civilization that exploits some material resources beyond its own planet, and an interplanetary culture, with civilizations in multiple places, and where traveling for interplanetary distances becomes commonplace. The civilization that masters gas giant energy is much further and much closer to interstellar travel than one almost imprisoned on its home planet.