What Is Jupiter's Great Red Spot? A Storm Older Than Any Nation

The Spot is a high-pressure storm, an anticyclone, in Jupiter's southern hemisphere, rotating anticlockwise once every four and a half days or so, and it sits between two of the jet streams that run around the planet in opposite directions, rolling between them like a ball bearing between two belts. It is about 16,000 kilometres wide at present and its cloud tops stand some eight kilometres above the surrounding deck, so that it is a raised plateau of cloud as well as a spinning one. The Juno spacecraft, flying over it in 2017 and probing it with microwaves and by measuring its gravity, found that its roots reach 300 to 500 kilometres down into the atmosphere, far deeper than the clouds that show it, though not to the bottom of the weather layer.

Storms on Earth die in days because they run over land, lose their supply of warm water and are torn apart by the winds around them. Jupiter has no land and no surface, its atmosphere is thousands of kilometres deep, and a storm there has nothing to hit. The Spot is also fed: it absorbs smaller storms that drift into it along the jet streams, taking their energy, and it draws heat from below, since Jupiter radiates nearly twice the energy it receives from the Sun from the slow contraction of its interior. The jets on either side hold it in its latitude, and simulations of a vortex between two such shear flows find that it can be stable for centuries, dissipating its energy slowly and topping it up as it goes. The rest of the planet's weather:

• •Bands: the pale zones are rising air with high ammonia clouds, the dark belts sinking air with the clouds lower and warmer
• •Jets: a dozen east-west streams alternating in direction between the bands, at up to 500 kilometres an hour
• •White ovals: smaller anticyclones, of which three merged between 1998 and 2000 into one that turned red in 2006, Oval BA, now called the Little Red Spot
• •Polar cyclones: Juno found the poles ringed by geometric arrays of storms, eight around the north pole and five around the south, each the size of a continent

Jupiter's clouds are ammonia, ammonium hydrosulphide and water, all of them white, and the colour of the Spot, which ranges from brick red to pale salmon and sometimes nearly vanishes, has no settled explanation. The leading idea, supported by laboratory experiments at NASA in 2014, is that the red is made at the top: ammonia and acetylene broken apart by ultraviolet sunlight recombine into a reddish compound, so that the Spot's height, which lifts its clouds into stronger sunlight, is why it is coloured and the deeper white ovals are not. The older idea was that the colour came from phosphorus or sulphur compounds dredged up from below. Whichever it is, the material is a trace, a thin veneer of tint on a white storm, and its variation from year to year is one of the clues to what is happening inside.

Giovanni Cassini recorded a permanent spot on Jupiter from 1665, and it was drawn intermittently until 1713, after which no one reported it for more than a century. The Spot that is watched today was first drawn clearly in 1831 and has been observed continuously since 1878, when it became large and vivid; whether it is the same storm as Cassini's, having faded or been hidden for a century, or a new one that formed in the same place, was argued for decades and was answered, provisionally, in 2024 by a study that reconstructed the old drawings and concluded that Cassini's spot was a different feature that vanished, and that the present one formed in the early nineteenth century. That would make it about 190 years old, which is still older than any storm anywhere else by a factor of thousands.

In the 1880s the Spot was about 40,000 kilometres across, three Earths side by side; by the Voyager flybys of 1979 it was 23,000; and it is now around 16,000 and rounder than it was, losing width at roughly a thousand kilometres a year, though the rate is not steady. Its winds have meanwhile sped up a little at the edge. Nobody knows whether it is dying. Some models of vortices between jets predict that a shrinking one becomes unstable below a certain size and breaks up within decades, and in 2019 the Spot was seen shedding flakes of red cloud tens of thousands of kilometres long, which looked like the beginning of the end and turned out to be surface disturbance that left the deep storm intact. If it does go, the Little Red Spot to its south, born in 2000, will inherit the title, and Jupiter's weather will have done in a human lifetime what it has been doing for four billion years.

The Great Red Spot is an anticyclone in Jupiter's southern hemisphere, wider than the Earth and rooted hundreds of kilometres deep, that has persisted for at least 190 years because Jupiter has no surface to break it on, the jet streams either side hold it in place, and it feeds on smaller storms and the planet's internal heat. Its red is probably sunlight-made compounds at its raised cloud tops, it has shrunk to less than half its nineteenth-century width, and whether that is a phase or an ending is unknown.