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Distance travelled ~ 606'376'800

As Hurricane Irene gathers strength it reminds me of the hurricane that has been raging on Jupiter for at least the last 350 years. Called rather imaginatively, the Great Red Spot (GRS) it was first observed in the 17th Century by Giovanni Cassini through the recently invented called the telescope. Its not just its longevity that brings it whirling into the record books though, its size is also impressive, measuring three times as big as the Earth, compare that to Irene which is about 400km in diameter and you realise quite how big a storm it is.

As Voyager 1 flew by Jupiter in 1979, it captured this photo of the Great Red Spot. Credit/Nasa

Jupiter, unlike Earth, is a planet made up almost entirely of gas with a fluid core but when we look at it through telescopes, its the tops of the dense atmosphere that we can see. Heating from the Sun and from internal sources, drives the convective activity in the atmosphere to produce the familiar high and low weather systems. Just like the Earth, its these high's and low's which effectively cause air to move and produce wind. Its here though that the similarities end. Simple observation from even modest telescopes will reveal strange belt structures in the atmosphere that seem symmetrical in both northern and southern hemispheres.

NASA/Freddy Willems, Amateur Astronomer, July 26 2011

In one of these belts, called the Southern Equatorial Belt, we can readily see the GRS which is an anticyclonic storm (a storm which rotates anti-clockwise) taking about 6 days to complete one revolution. Its elliptical shape seems to be due to the fast jet streams that neighbour it, blowing easterly on the south and westerly to the north. Infrared observations have shown that the temperature of the GRS is lower than that of the surrounding clouds suggesting a higher altitude, estimated to be towering over neighbouring jet streams by 8km. From such a monstrous storm you might expect astronomical wind speeds but surprisingly modest speeds of 430km per hour are measured, compared to Irene's more sedate maximum speeds of 160km per hour.

The colour of the spot isn't even stable, changing from pale pink to a deep red but what causes this colour remains a mystery. We do know that its affected by environmental factors though as the darker central region always appears slightly warmer than the paler, cooler surroundings. Its perhaps the presence of complex organic compounds such as red phosphorous that gives it its distinctive colour but for now, the GRS remains one of the beautiful yet enigmatic mysteries of the Solar System.

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