Jupiter’s Great Red Spot is one of the most famous and spectacular sights in the Solar Systems. Wider than the diameter of the Earth, the spot is a giant vortex of winds up to 400 kilometers per hour. Its reddish color probably comes from complex organic molecules that form in its upper atmosphere, although nobody is quite sure.
The Spot may have first been seen by the Italian astronomer Giovanni Cassini in 1665 and then observed throughout his life until his death in 1712. But after that something strange happened: the spot disappeared and was not seen again for over a hundred years, despite observations by some of the leading astronomers of the time, such as Charles Messier and William Herschel. Then in 1831 it re-appeared and has since been studied in detail from Earth and from various spacecraft.
This hundred-year gap raises an interesting question. Some astronomers have suggested that Cassini didn’t see the Great Red Spot at all but some other atmospheric storm that dispersed soon after his death. The Spot we see today, they say, formed later. However, working out the truth of this conjecture has been hard.
