Jupiter’s atmosphere is doing some pretty funky stuff, and we don’t fully understand all of what’s going on. We know that the light stripes and dark stripes are made of slightly different gases, and that at the boundaries of these stripes are narrow regions of high wind, called jets, which push the nearby atmosphere around with it, and that the bands are relatively stable.
The light stripes seem to be made of cold gas which is coming up towards the surface of the atmosphere of Jupiter, and the dark stripes are doing the opposite (warmer gas, sinking down towards the centre of the planet). The light ones are light because there’s a lot of ammonia in the upper atmosphere of Jupiter, and as it cools, it forms pale clouds, like the clouds in our sky. If the gas warms up, the clouds will disappear, and what we’re seeing as dark bands are actually a deeper, darker layer of clouds. However, the sun also has upwellings of gas and regions where gas is sinking back, and the surface of the sun looks like boiling water - there’s no order there. So why does Jupiter have ordered bands and not just look like a roiling mess?
At a very basic level, it’s because those jets of wind running around the planet are there. These jets form a boundary for the gas. When gas faces a strong wind, it’s going to be redirected in the direction of the wind instead of continuing the way it was originally headed. Because the direction of the wind jets alternates as you go from the equator to the poles, the boundaries of the jet and the band forms eddies and whirlpools of gas, which helps to pull the gas along with the wind. As a result, the jets describe the edges of the different bands of coloured gas, and direct the motion of the gas within each stripe. The part that we don’t yet understand is why those jets exist in the first place.
Broadly, there are two main ideas - one is that this is turbulence at the surface level, like clouds in the upper atmosphere of the earth. Perhaps there was some turbulence - a little bit of bumpy gas, and it ran into another patch. As patches of turbulence catch up to each other, they can combine, and form one bigger piece of turbulence - called a cascade. If there’s a constant source of the little eddies of turbulence, then you can maintain bigger turbulence (like the wind jets) just by tossing the little things together. However, this method is bad at keeping the stable winds we see, so this isn’t a great description.
The second idea is that since Jupiter is rotating relatively quickly, and is basically all gas, the gas can form cylindrical shells of material that rotate a little differently as you go out away from the centre of the rotation. However, we have all noticed that planets are not cylinders, they are spheres. But if you start with a cylinder, and carve into it, you can create a sphere. So these bands appear as the surface of Jupiter cuts into different layers, as you get closer to the poles. (As another illustration, if you sharpen a wood pencil, you wind up with a stripe of pencil lead, a stripe of the wood that’s been cut into, and the outer layer of the pencil that you didn’t touch, if you look at it from the side.) The main problem with this method is that generally it doesn’t make enough bands.
Since neither of these ideas really sets out a comprehensive solution to the whys of Jupiter’s atmosphere, for the moment we have to be stuck with observing the banded clouds of Jupiter and the jets that drive them, in hopes that the details of their behaviour will hint towards one answer over another.
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