The Great Red Spot used to be bigger. Over the course of the twentieth century, it lost half of its size. Just between 1996 to 2006, its area diminished by 15%. If the shrink rate is constant, then the spot will vanish within 20 years. However, a computer simulation showed that Jovian weather is stable enough to sustain smaller red spots and the Hubble Space Telescope showed two smaller red spots besides the Great Red Spot, indicating that small red spots will persist even if the Great Red Spot vanishes. The reason for this shrinkage is unknown but scientists suggest it has something to do with the Jovian equivalent of global warming and climate change.
Unlike Earth, Jupiter does not have solid landmass so energy lost due to friction is out of question. This explains the long lasting nature of Jovian storms. The Great Red Spot is indeed long-lived, even by Jovian standards. Researchers are able to pinpoint that Jovian storms that are located in cloud bands with the same direction of rotation tend to be longer lasting. So, the shrinkage of this spot might indicate that the spot is drifting from the clouds bands.
The Great Red Spot is confined by an eastward jet to its north and a westward jet to its south, confining the storm to a constant latitude. However, it has undergone considerable changes in longitude over time, and recent evidence suggests that its rate of westward longitudinal motion is increasing. The bands have also undergone little change in latitude over the time during which they have been observed. So, researchers suggest that this dynamic change of the cloud bands and drifting of the storm might be the reason for the change in the shape of the Great Spot.
Although the reason has not been proved yet, researchers are trying to trace the evolution of the Great Red Spot, analyzing its size, shape, color, drift rate, and the storm’s internal wind speeds from the information available from spacecraft and hope they will provide an explanation in future.
- Hartston, W. The Things that Nobody Knows: 501 Mysteries of Life, the Universe, and Everything. Atlantic Books Ltd (2011).
- Lawrence, J.L. Celestial Calculations: A Gentle Introduction to Computational Astronomy. MIT Press (2019).
- Pierce, D. Jupiter's Great Red Sot is shrinking. Cosmos (2019).
- Imster, E. Jupiter's Great Red Spot gets taller as it shrinks. EarthSky (2018).
- Simon et al. Historical and contemporary trends in the size, drift, and color of Jupiter's Great Red Spot. The Astronomical Journal, Vol. 155, No. 4 (2018).