Jupiter’s Shrinking Red Spot

Jupiter’s Great Red Spot is famous. A giant storm twice as large as Earth, it’s been active for hundreds of years. What you probably don’t know about the Great Red Spot (GRS) is that it is shrinking. Amateur astronomers studying Jupiter noticed it first; the Hubble Space Telescope was turned towards Jupiter to confirm. Sure enough, the GRS is now more oval than before, and is the smallest size since recordings of its size began.
Today’s article, “Dramatic change in Jupiter’s Great Red Spot from spacecraft observations” by Amy A. Simon, Michael H. Wong, John H. Rogers, Glenn S. Orton, Imke de Pater, Xylar Asay-Davis, Robert W. Carlson, and Philip S. Marcus, published in 2014 in The Astrophysical Journal Letters, describes the changes of Jupiter’s GRS as seen by Hubble observations.
A close-up of Jupiter's Great Red spot and swirling clouds, taken by Voyager 1. Image credit NASA/JPL.
A close-up of Jupiter’s Great Red spot and swirling clouds, taken by Voyager 1. Image credit NASA/JPL.
Usually, the GRS shrinks by about two-tenths of a degree every year in longitude (east-west). In 2014, it suddenly shrunk by 1.4 degrees–1760 km (1094 miles) in a span of just 21 months, or four times faster than normal.  There were no changes seen nearby, and the cause is still a mystery. The GRS also became redder than usual. Usually this happens when the South Equatorial Belt fades, becoming whiter, as part of Jupiter’s climate cycle. In 2014, however, the South Equatorial Belt did not whiten, leaving the question of why the GRS changed.
Color depends on the reflection and absorption of light. The GRS showed that it changed which amounts of which wavelengths of light were being reflected and absorbed, suggesting a change in the concentration or placement of aerosols within the atmosphere. These changes were not seen elsewhere on Jupiter.
Also changing was the spectral slope, or the change of reflectivity over a certain range of wavelengths (in this case, 500 to 630 nanometers). This shift may be related to increased UV radiation of chemical compounds that create the colors of the GRS. Unfortunately, it’s not entirely known which chemicals create the colors in Jupiter, though laboratory experiments give some indication that they may be ammonia and hydrocarbons (chemicals containing both hydrogen and carbon). Based on the lab results, the suspected chemicals become redder with longer exposure to UV radiation. Other chemicals may be a factor, but research remains to be done.
A true-color image of Jupiter taken by the Cassini spacecraft. Image credit Nasa/JPL/Space Science Institute.
A true-color image of Jupiter taken by the Cassini spacecraft. Image credit Nasa/JPL/Space Science Institute.
Circumstances that would redden the GRS all involve some sort of change, making the reddening part of a chain reaction. The authors of today’s paper looked at the wind speed and direction of and around the GRS. At times, these speeds can be as great as 150 meters per second (335 mph), and the results of the authors analysis puts the speeds in the same range. In other words, there was no notable change in wind speed, so this is not likely to be a major factor in the changes of the GRS.
In the latitudinal (north-south) direction, the GRS shrinks much slower than longitudinally, around 0.04 degrees every year. Between 2012 and 2014, it underwent a much faster change of 0.7 degrees. These are most likely due to the changes in the zonal jets to the north and south of the GRS. One goes to the east, the other to the west, creating a chaos of eddies. The change in width in the GRS is likely due to the changes in the north and south edges of these jets, which changes the environment around the GRS. These changes have placed the GRS away from the cusp of the eastward jet, so it no longer pulls in white clouds from this area. The process of pulling in white clouds and then pulling them apart is a process similar to what happens when the Southern Equatorial Belt fades, when the GRS usually reddens. If the GRS in not pulling in the white clouds, this may explain why it is reddening without the Southern Equatorial Belt fading.
We are still not sure how the Great Red Spot interacts with the zonal jets around it, or what compounds affect the cloud colors, or why they vary. For scientists, unanswered questions are a problem to be solved, so more research is always being done. Like the Great Red Spot that has been active for centuries, science always continues.

 

Bibliography
Candanosa, Roberto. “Jupiter’s Great Red Spot: A Swirling Mystery.” Text. NASA. N.p., 4 Aug. 2015. Web.
An article discussing the difficulties in trying to determine which chemicals are responsible for the colors seen on Jupiter.
National Aeronautics and Space Administration. “Jupiter: Overview.” Solar System Exploration. N.p., n.d. Web.
This webpage, and the content under the navigation tabs, was used as a resource for basic information on Jupiter and the Great Red Spot, as well as the images.
Simon, Amy A., et al. “Dramatic change in Jupiter’s Great Red Spot from spacecraft observations.” The Astrophysical Journal Letters 797.2 (2014): L31.
Today’s main paper.
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