Why do Jupiter and Saturn have atmospheric bands, and why don't Earth and Venus?

[arun_mid]

hi, could anyone explain why Jupiter and Saturn have those seemingly perfect bands across its atmosphere(except for a few 'small' storms in between), and why we don't find such things on Earth or venus? Also, about Saturn's rings, if they were formed out of some debris of a broken moon, then why was a ring formed, when the debris could simply rotate around the planet in any orientation? why do all (or if not all, most of) the debris have to rotate in that particular plane?

 

[Mr. dude]

Well, I'm going to attempt to anwer the first Q. I think that it might be similar to our Coriolus Effect, except for the fact of course that its on Saturn or Jupiter. It may be so defined because of different gases in the air. Our winds change at rather exact points. Though we can't really see our winds and the Coriolus Effect, it would be much easier to see on Jupiter or Saturn. They are after all gas giants. I'm not sure. Just a possibility. For the second Q I'm quite in the dark.

 

[Janus]

As to Saturn's rings: The answer is conservation of angular momentum. If they were formed from a moon torn apart by tidal forces, the pieces would tend to orbit in the same plane as the original moon. To cause them to to do otherwise would take a considerable effort. (like trying to tilt the axis of a gyroscope.) Tidal forces would not act in a away to cause this, so you end up with the remains orbiting in the same plane and forming a ring.

 

[Mr. dude]

Makes sense.

 

[Phobos]

good link for the question about Saturn's rings...
http://www.astronomycafe.net/qadir/q58.html

 

[russ_watters]

Also, while the bands may look "seemingly perfect" at a magnification of, say, 100x, you're only able to see them on a scale of about the size of the earth. In a shameless display of own-horn-blowing, attached is a picture I took of Jupiter at around 500x magnification, where you can clearly see that those bands are anything but perfect, even at a resolution only as fine as the size of our moon.

 

[Tropo]

The bands are due to a combination of heat transport from the equator to the poles and the planet's rotation. If a planet with an atmosphere does not rotate, then this heat transfer will occur by having winds establish themselves that blow from the equator to both poles at high altitudes, they dump their heat at the poles, and return to the equator by blowing back that way at low altitudes.

Now, think about a small, slowly rotating planet like Earth. You end up with about four different interlocking subcells (Hadley cells) due to an interaction with the coriolis force. These subcells have winds that blow towards the poles and towards the equator at different altitudes but, due to the coriolis force, they also have east and west components, too. These subcells mesh just like gears, and that's why we have some lattitudes with easterly winds and some with westerlies at the surface. The trade winds are a good example. Sailors knew to go N or S to find winds that go E or W.

Now, think about a big planet that rotates very fast, like Jupiter. These subcells grow in number and shrink in latitude and become narrow bands. The bands on the gas giants are the same as our wind patterns, only there are more of them due to the higher rotation rates and angular momentum. The colors of the bands are due to trace amounts of CH4, P compounds and other colored substrances.

Hard to type a visual description. Hope that helps!