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.