Why Is Jupiter's Great Red Spot Shrinking?

The mystery behind the solar system’s most famous storm

The most recognizable storm in the solar system used to be so big that it could fit three whole Earths. Now, it has room for only one.

Jupiter’s Great Red Spot is shrinking, and has been for decades.

The earliest observations of a massive, red spot on the face of Jupiter date back as far as the 1600s. Astronomers don’t know whether this spot was the Great Red Spot we know today, but it’s likely. Routine telescope observations of the oval-shaped storm, where winds can reach 400 miles an hour, began in the late 1870s. For the Great Red Spot, these were the “years of its glory,” according to John H. Rogers, who plotted the history of the storm’s dimensions in The Giant Planet Jupiter. Not long after, perhaps around 1920, astronomers noticed the storm was getting smaller. In 2012, amateur astronomers using backyard telescopes to observe Jupiter saw that the rate of shrinking had actually accelerated.

The storm’s glory days are behind it, and scientists are trying to figure out why. They don’t have an answer yet.

The Great Red Spot formed like most storms do on Jupiter, as elongated bands of wind that swirled into shape over time. “We think what happens is they hit a stable size, and that’s when it should stop and just kind of stay that size, unless something breaks it apart,” says Amy Simon, a senior scientist at NASA’s Goddard Space Flight Center who studies Jupiter.

But the Great Red Spot is a weird one. According to computer simulations based on our understanding of Jupiter’s weather, it’s rare for storms to last as long as the Great Red Spot has. And yet, here it is—shrinking, certainly, but persisting, quite stubbornly, for centuries. “We’re still figuring out what it all means,” Simon says.

Simon and her colleagues recently tracked the history of the Great Red Spot by combing through a number of sources, including the earliest telescope observations and data from the Voyager spacecraft and Hubble Space Telescope. Their analysis provided another confirmation that the storm has been steadily decreasing in size, since about 1878. They also found that the storm is growing taller as its base shrinks, like a chunk of clay being shaped into a vase on a potter’s wheel.

Despite years of observations by telescopes and spacecraft, much about Jupiter remains a mystery, including the mechanisms that drive the Great Red Spot. The storm maintains its latitude because it spins between two jet streams moving in opposite directions. Jupiter’s jet streams are bands of powerful winds that can descend as deep as 3,000 kilometers, or about 1,800 miles, beneath the planet’s cloud tops. The Great Red Spot feeds on a steady parade of small storms that emerge to its northwest, march all the way around the planet for a few months, and then return.

“The Great Red Spot is a lot like a big, beautiful mountain lake, with a small inlet and a small outlet,” says Timothy Dowling, a scientist at the University of Louisville who studies planetary atmospheres. “Any slight change to the inlet or outlet of such a lake eventually changes the lake’s appearance dramatically, and it is just the same with the Great Red Spot and its diet of small storms.”

Computer models show that changes to Jupiter’s jet streams, either in their speed or location, would result in changes to the inlets and outlets, and consequently to how the Great Red Spot absorbs and releases small storms. This, in turn, would lead to changes in its size and shape. Scientists don’t know for sure, but these jet streams likely play a role in the storm’s gradual shrinking. The jet streams, of course, are among the properties of Jupiter they’re still trying to really understand.

Simon’s analysis of the Great Red Spot over the years also revealed that the storm’s shades of orange and red are becoming deeper, particularly since 2014. The cause is—yep, you guessed it—another mystery. In fact, scientists don’t know why the Great Red Spot is red in the first place. Scientists say the oranges and reds come from the chemical composition of Jupiter’s atmosphere. One theory suggests the colors come from the presence of sulfur and phosphorus, while another says they’re a product of chemicals being broken down by sunlight.

Either way, the saturation of the storm’s colors may have to do with altitude. Scientists have seen that the deepest reds occur at higher altitudes. In the 1930s, three white, oval-shaped storms emerged elsewhere on Jupiter and eventually merged into a single storm. “Soon after, this storm turned from white to red,” Dowling says. “Apparently, once a high-pressure storm gets high enough, its middle is exposed to enough ultraviolet light from the sun for photochemistry to produce the red.” If this is indeed the case, the Great Red Spot will become the Great Redder Spot as it grows taller.

Scientists are hesitant to predict what will happen to the Great Red Spot in the coming years. The storm may continue to contract. It could also rebound and swell. Simon says the next decade will be particularly interesting because we may see the storm transform from an oval into a circle, a shape that would be more difficult to maintain.

If and when the Great Red Spot shrinks into oblivion, its winds absorbed into Jupiter’s swirling landscape, humanity—if it hasn’t shrunk into its oblivion itself—will have plenty of photographs to remember it by. Amateur image processors have spent nearly a year transforming raw data from NASA’s Juno spacecraft into stunning, high-resolution photos of the planet’s stormy landscape. Last July, the spacecraft flew directly above the Great Red Spot, producing wildly detailed images of the storm. Juno came within 5,600 miles of the cloud tops, marking humanity’s closest approach to the storm. It’s likely the closest humans will ever get, but at least we got there before it was too late.