Still in its spot

By Richard A Lovett

Rumours that Jupiter’s Great Red Spot is dying may be premature.

The giant storm – larger than the Earth – has been raging on the planet for at least 350 years, scientists believe, but when amateur astronomers observed “flakes” or “blades” breaking off it back in May, and NASA confirmed this, stories started circulating that it was beginning to fall apart. 

But not so fast, says Philip Marcus, a professor of fluid dynamics at the University of California, Berkeley. These blades do not mean the Great Red Spot is in its death throes. 

More likely, he told a meeting of the American Physical Society’s Division of Fluid Dynamics, the spot had an encounter with another storm with a different type of rotation.

The Great Red Spot is an example of what atmospheric physicists call an anticyclone, meaning it rotates in the opposite direction to the planet. “It has a high-pressure centre, like an ‘H’ on the weather map,” Marcus says. {%recommended 9618%}

Not all of Jupiter’s storms are anticyclones. Jupiter also has cyclones, which rotate in the opposite direction and are akin to low-pressure zones, like Earth’s tropical storms.

For reasons that aren’t well understood, Marcus says, Jupiter’s anticyclones produce distinctive cloud colours that are easily seen. Cyclones are often invisible.

That’s important, he says, because the two types of storms interact differently if they get close to each other. 

Anti-cyclones, for example, will attract each other until eventually the big one eats the little one. Not that this process is instantaneous. 

“There’s a bulge around [the big one’s] boundary as it digests [the small one],” Marcus says. “It’s like a python eating a cow: you can watch the cow going through the body of the python.”

When an anticyclone encounters a cyclone, however, the opposite happens: rather than merging and cancelling each other out, they repel each other, with each going on its merry way… unless a cyclone approaches an anticyclone while the anticyclone is in the process of eating a smaller anticyclone. 

In that case, Marcus says, models show that the interaction will rip bits of the smaller anticyclone away from the bigger one. “Instead of being digested, it’s being thrown off as a blade [or flake],” he says. 

“These are normal, healthy activities for the Great Red Spot and its colleagues,” Marcus says, “so we do not believe the spot is dying. It should be around for a long time.”

Though, he laughs, “I probably just gave it the kiss of death and it will fall apart next week”.

Meanwhile, amateur astronomers will undoubtedly be on the look out for the next such occurrence. 

“They do a great service to our profession,” Marcus says. “They look at the Great Red Spot every single night.”