Great Red Spot. Image credit: NASA/JPL
Great Red Spot. Image credit: NASA/JPL

Astronomy, Guide to Space

Jupiter’s Great Red Spot

18 Jun , 2008 by

Jupiter’s Great Red Spot(GRS) is an atmospheric storm that has been raging in Jupiter’s southern Hemisphere for at least 400 years. Many believe that Giovanni Cassini was the first to observe it in the late 1600s, but scientists doubt that it had just formed at that time. About 100 years ago, the storm covered over 40,000 km of the surface. It is currently about one half of that size and seems to be shrinking. At the present rate that it is shrinking it could become circular by 2040. Scientists doubt that this will happen because of the distortion effect of neighboring jet streams. It is not known how long the spot will last, or whether the changes in size are a result of normal fluctuations

The GRS rotates counter-clockwise(anti-cyclonic) and makes a full rotation every six Earth days. The rotation period of the spot has decreased. Some believe that it is a result of its shrinking. Winds at the very edges of the storm gust to 432 km/h, but inside the storm winds seem to be somewhat stagnant with no inflow nor outflow. The spot is large enough to engulf three Earths. Infrared data indicates that the GRS is colder and at a higher altitude than most of the other clouds on Jupiter. The cloudtops of the GRS are about 8 km above the surrounding clouds. The spot is prevented from moving north and south across the planet by an eastward jetstream to its south and a very strong westward jetstream to its north. Its position moving east and west changes quite frequently. The spot has “lapped” the planet at least 10 times since the early 19th century. Its drift rate has changed dramatically over the years and has been linked to the brightness of the South Equatorial Belt, and the presence or absence of a South Tropical Disturbance.

It is not known exactly what causes the Great Red Spot’s reddish color. The most popular theory, which is supported by laboratory experiments, holds that the color may be caused by complex organic molecules, red phosphorus, or other sulfur compounds. The GRS varies greatly in hue, from almost brick-red to pale salmon, and white. The reddest central region is 4 K warmer than the surroundings, which is thought to be evidence that the color is affected by environmental factors. At times, the spot disappears from the visible spectrum and can only be seen as the Red Spot Hollow, which is area it occupies in the South Equatorial Belt(SEB). The visibility of the GRS seems to coupled to the appearance of the SEB. When the SEB is bright white, the GRS is dark and when the SEB is dark, the spot is usually light. The periods when the spot is dark or light occur at unpredictable intervals.

As you can see, Jupiter’s Great Red Spot is a subject of a great deal of study. Scientist hope that by fully understanding it, they will be able to better understand our giant neighbor.

Here’s an article on Universe Today about Red Spot Jr, and the possibility that Jupiter’s spots are disappearing.

Astronomy Picture of the Day has a great image and write up of the Great Red Spot, and here’s an image from NASA/

We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter, and Episode 57: Jupiter’s Moons.


Jerry Coffey is a Registered Nurse and father of 5. He enjoys skydiving, astronomy, and time with his children.

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