The red and blue have been greatly exaggerated in this frame to bring out fine detail in the cloud structure. [P-21431C]
The most visible cloud interactions take place in the region of the Great Red Spot. Material within the Red Spot rotates about once every six days. Infrared measurements show that the Red Spot is a region of atmospheric upwelling, which extends to very high altitudes; however, the divergent flow suggested by this upwelling seems to be very small—one bright feature was observed to circle the Red Spot for sixty days without appreciably changing its distance from the spot’s center. During the Voyager 1 flyby, spots were seen to move toward the Red Spot from the east, flow along its northern border, then either flow on to the west past the Red Spot or into the outer regions of its vortex. A spot caught on the outer edge of the Red Spot flow might break in two as it reached the eastern edge of the spot, with one piece remaining in the vortex and the other moving off to the east. Alternatively, a spot floating toward the Red Spot from the east might be pushed northward to join the eastward current flowing north of the giant red oval.
By the time of the second Voyager encounter, a ribbon of white clouds curled around the northern border of the Red Spot, blocking the motion of small spots that might otherwise have been caught up in the vortex. Spots approaching the Red Spot from the east just turned around and headed back in the direction from which they had come.
Voyager 2 captured the Red Spot region four months after Voyager 1, when some changes had taken place in the cloud circulation pattern around it. This is a mosaic of Voyager 2 frames, taken on July 6. The white oval to the south is not the same one that was present in a similar location during the Voyager 1 flyby, because of differential rotation at the two latitudes. [260-606]
In the northern hemisphere, small brown anticyclonic features speed around the planet, often colliding with one another. On collision, the spots may combine and roll around together for a while. Ultimately, part of the mass of the combined spots is ejected as a streamer, and the remaining material continues on its eastward path.
Order out of Chaos
Despite all the turbulence in Jupiter’s atmosphere—this ever-changing chaotic mixture of cyclonic and anticyclonic flows, of ovals and filaments, of reds, browns, and whites—a pattern may be emerging: There is an underlying order to the seemingly random mixing of patterns we see in the Jovian atmosphere.
First, the changing weather patterns are in some sense cyclic. The fact that Jupiter may be reverting to the appearance it presented at the time of Pioneers 10 and 11 is not surprising. From Earth-based studies, astronomers have found that the face of Jupiter often goes through a major change every few years. The transition is very rapid, but the planet maintains its new “look” for some time—until the next major transition. At the time of the Pioneer flybys, “The Red Spot was prominent, but it was surrounded by intense cloud. There was no visible structure at all in the south tropical zone—it was totally bland. You couldn’t see the turbulent area to the west,” explained Garry Hunt. “And, I believe that the buildup of cloud that we’re seeing to the east of the Red Spot is the beginning of the transition that will produce the Pioneer look.”