Expert: Philip Stahl - 5/11/2009

Question
I was wondering why there are clear bands of different colored clouds on Jupiter and Saturn instead of a continual gradation of color throughout the surfaces of the globes. Uranus and Neptune don't seem to have much of that. Why don't they have bands? So i am asking why two have bands and why two don't.
Thanks for your time.

Answer
Hello,

The standard view on the cloud bands for Jupiter, Saturn are that they arise from convective processes in their atmospheres. Uranus and Neptune don't have such (at least to the same extent) bcause there is no similar heat "driver" (e.g. at the core of the planets) to keep the convection cycles going.

In this view the lighter bands (called "zones") have lower temperatures than the dark regions (called "belts") What this means is that the lighter zones are at higher altitude than the darker belts, and as we know (even on Earth) the temperature decreases with higher altitude.

In a convective view, the zones display the rising (upwelling) high pressure regions while the belts denote  descending regions associated with lower pressure. (Think here of the convection cycle in a saucepan and how the bubbles rise as another set - which has lost heat- descends).

Possibly in future a more refined explanation may come about which further examines more details in Jupiter and Saturn. But for now, the convection hypothesis appears to have the most support.