Speaking of stability, I just read that the Great Red Spot on Neptune, discovered by Voyager 2 in 1989, had disappeared by the time the Hubble Space Telescope looked at the planet in 1994. How could that be, if the reason the Great Red Spot on Jupiter has not disappeared for at least 300 years is that it would have to be disrupted by a larger storm, which does not exist (at least not yet) on Jupiter?
Speaking of stability, I just read that the Great Red Spot on Neptune, discovered by Voyager 2 in 1989, had disappeared by the time the Hubble Space Telescope looked at the planet in 1994. How could that be, if the reason the Great Red Spot on Jupiter has not disappeared for at least 300 years is that it would have to be disrupted by a larger storm, which does not exist (at least not yet) on Jupiter? Current wisdom is that Neptune's "Dark Spot" and Jupiter's "Red Spot" are different phenomena. I wouldn't place too much credence on "explanations" of either, though, because neither is yet well understood!
Neptune's Great Dark Spot of 1989
Unlike Jupiter's Great Red Spot, the Great Dark Spot of Neptune is thought to be a hole, similar to the hole in the ozone layer on Earth, in the methane cloud deck of Neptune. The white clouds shown in the picture are above the "hole". In many images of Neptune, the Great Dark Spot can be seen to change size and shape.
The Great Red Spot of Jupiter is thought to be a hurricane which has been raging on Jupiter for at least 400 years. The Great Dark Spot, seen here by Voyager in 1989, disappeared (either dissipating or being masked) in 1994, and was replaced very soon by a similar "Spot" in a similar place, but in the north instead of in the south.
This is an image of Neptune's Great Dark Spot of 1989.
Click on image for full size version (80K JPG)