On a recent public radio broadcast, Philip Marcus, professor of fluid dynamics at the University of California Berkeley, explains the persistence of Jupiter’s Great Red Spot (see Section 7.1b, p. 170).
The Great Red Spot is huge – 36 times larger than the United States, and its winds, which clock in at 250 miles per hour, surpass those of the most violent hurricane. But, its most baffling property is its multi-century lifetime.
Scientists believe that the Red Spot should have lasted just a few years. It avoids being torn apart because it is sandwiched between layers of cold and hot air. But, those layers should have warmed and cooled their ways to oblivion in only 5 years or so and taken the Red Spot along with them.
So why is the Red Spot still here? One explanation is that the Great Red Spot merges with, and absorbs, smaller “spots”. It was thought that by cannibalizing smaller spots, the Red Spot could stay alive indefinitely, but now we know. That diet is too meager.
Dr. Marcus’s group recently found a new explanation for the Red Spot’s longevity – it has weak, vertical winds. Like storms on Earth, the vertical winds of the Great Red Spot appear to be negligible; they are hundreds of times smaller than the horizontal winds. Therefore, previous studies ignored them. To their surprise, when they accurately calculated the vertical winds using computer models, the Red Spot’s lifetime increased from 5 years to 800 years. The vertical winds escaped the Great Red Spot and then threaded through the atmosphere, where they harvested energy from the surrounding air. When the winds returned to the Red Spot, they brought their bounty, and that excess energy has sustained the Great Red Spot for centuries.
Links and source: WAMC’s Academic Minute.