Why is the Great Red Spot red?

What is the Great Red Spot?

It can already be seen with a small hobby telescope: the great red spot in the atmosphere of the planet Jupiter. With a length of 24,000 kilometers and a width of 13,000 kilometers, it is larger than the earth. Its color changes over the years - sometimes it is deep red, sometimes only slightly reddish, at times it even loses its color completely and is only visible as a bright spot in the surrounding band of clouds.

Jupiter with a large red spot

Close-ups from the American space probes Voyager and Galileo show that the Great Red Spot is a gigantic cyclone. It rotates counterclockwise with a period of six earth days. The measurements show that it is an anticyclone: ​​The Great Red Spot is connected to a permanent high pressure area in Jupiter's atmosphere.

Decades of cyclones

Hundreds of cyclones can be observed constantly in the dense atmosphere of Jupiter. While cyclones and anticyclones occur roughly equally on Earth, anticyclones dominate on Jupiter: around 90 percent of all cyclones larger than 2,000 kilometers there are associated with high pressure areas. The mean lifespan of cyclones in Jupiter's atmosphere is between one and three years. But occasionally, large stains can persist for decades.

This is also the case with the Great Red Spot: the German astronomer Samuel Heinrich Schwabe drew the first detailed pictures of this phenomenon as early as 1831. This makes the Great Red Spot the most consistent weather phenomenon in the entire solar system. However, it is disputed whether a spot in the atmosphere of Jupiter observed by the Italian-French researcher Giovanni Domenico Cassini between 1665 and 1691 is identical to today's Great Red Spot - then this cyclone would even have been raging for almost 350 years! A spot discovered in 1664 by the English physicist and mathematician Robert Hooke, on the other hand, is probably not identical with the Great Red Spot - and not even with Cassini's spot - because it was in a different position.

The visibility of the Great Red Spot seems to be linked to the appearance of the southern equatorial band, a strip of cloud into which the spot protrudes: If the band is light, the spot is dark, if the band is dark, the spot is light. So far neither the cause for this connection nor the striking red color of the stain is known. The color could come from sulfur compounds, red phosphorus, or complex organic compounds.

The Great Red Spot gets company

Three red spots on Jupiter

Between 1998 and 2000, three white cyclones observed since the 1930s merged into a large “white oval”. Images from the Hubble Space Telescope in 2006 showed that this oval was slowly turning reddish. Since then, this cyclone has mostly been referred to as the Little Red Spot or Red Spot Junior. In May 2008, a third red spot appeared, again emerging from a previously bright cyclone oval. But this new red spot was not granted a long life: it met the Great Red Spot in July 2008 and was devoured by it.

How long the Great Red Spot will last cannot be predicted. Since the first observations of the cyclone, its length has decreased from 40,000 to 24,000 kilometers. If this trend continues at the same pace, the stain could be round by 2040 - it is possible that it will then dissolve completely.