What is it about?
The wind racing around Jupiter's Great Red Spot takes about 6 days to make a complete circuit, and a detailed analysis of how this wind behaves at different points along the journey has revealed the presence of deep jet streams beneath the giant storm. This has been accomplished by observing how fast the tops of vertical columns of air are spinning. When a column is on the poleward side of the Great Red Spot, it is observed to spin twice as fast as when it is on the equatorward side. Like a skater pulling in her arms, faster spin means the column has contracted horizontally. But such a contraction signals the column has elongated vertically, and thus has entered a deeper part of the atmosphere. Conversely, when it enters a shallower part of the atmosphere, the column expands horizontally and its spin slows down. That fact that the Great Red Spot is twice as deep on its poleward side as its equatorward side reveals a strong north-south pressure difference beneath the storm, which by Coriolis action means the Great Red Spot is riding on top of strong east-west jet streams.
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Why is it important?
The presence of jet streams below the Great Red Spot was established previously by Dowling and Ingersoll (1988, 1989), but their speed was not pinned down, because the depth of Jupiter's troposphere was not known, just the relative (factor-of-two) change across the Great Red Spot. This last piece of the puzzle, the depth, was furnished when the Comet Shoemaker-Levy 9 comet fragments impacted Jupiter, creating waves, like when a rock is thrown into a pond. The speed of these waves revealed the depth of Jupiter's troposphere, which combined with the previously determined depth variation to yield the wind speeds of the deep jet streams.
Perspectives
This study provided the first empirical evidence that Jupiter's deep jet streams are strong, months before the Galileo probe and a quarter century before the Juno gravity mission each came to the same conclusion. This was the only study to predict Jupiter's eastward jets get stronger with depth below the clouds. A few months after this prediction was published, the Galileo entry probe sampled an eastward jet at latitude 6.5deg, and found that its speed increased from 100 m/s at the cloud-tops to 170 m/s below them, which matched this paper's prediction. Gravity observations from Juno have confirmed that Jupiter's jet streams have roots that extend about as deep as the jets are wide, and Cassini observations have produced a similar result for Saturn.
Professor Timothy E. Dowling
University of Louisville
Read the Original
This page is a summary of: Estimate of Jupiter's Deep Zonal-Wind Profile from Shoemaker-Levy 9 Data and Arnol'd's Second Stability Criterion, Icarus, October 1995, Elsevier,
DOI: 10.1006/icar.1995.1169.
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