When you think of Jupiter, you might think of its massive size, or colorful bands of gases stretching across its face. Or you might think of the iconic storm, that huge, churning red hurricane twice the size of Earth that's remained a signature of our solar system's largest planet since for more than a century. This is Jupiter's Great Red Spot and it has captivated humans for generations.
Jupiter's Great Red Spot was first observed in 1831 by amateur astronomer Samuel Heinrich Schwabe, so we know the storm has existed for at least 150 years. But it could be even older than that. Some astronomers speculate that, back in 1665, when astronomer Gian Domenico Cassini (the namesake for NASA's Cassini mission) wrote about a "Permanent Storm," he was referring to the Great Red Spot.
What is the Great Red Spot?
Jupiter's Great Red Spot is a gigantic storm that's about twice as wide as Earth, circling the planet in its southern hemisphere. At the storm’s center, winds are relatively calm, but on its edges, wind speeds reach 270-425 mph (430-680 km/h). That's more than twice the speed of even the strongest hurricanes on Earth, which can generate wind speeds of up to 175 mph (281 km/h).
The storm is contained by an eastward-moving atmospheric band to its north and a westward-moving band to its south. Those swirling bands are also what formed the storm in the first place and have kept the storm spinning for more than a century, Glenn Orton, a lead Juno mission team member and planetary scientist at NASA Jet Propulsion Laboratory, told Business Insider.
The Great Red Spot’s longevity is partially be explained by the fact that Jupiter doesn't have a solid surface. Jupiter's "sky" is 70 km (44 miles) deep, and consists of cloud layers made of ammonia ice, ammonium hydrosulfide or water ice and vapor. Scientists believe that beneath these layers exists an ocean of liquid hydrogen. And beneath that ocean is the planet's core — but scientists are not sure yet what Jupiter is made of. On Earth, hurricanes start to slow and break apart when they reach solid land, but with nowhere for the Great Red Spot to make landfall, the storm can rage on and on.
"We think what happens is [storms] hit a stable size, and that's when it should stop and just kind of stay that size, unless something breaks it apart," Amy Simon, a senior scientist at NASA's Goddard Space Flight Center, told The Atlantic.
And the Great Red Spot might actually be breaking apart. Since scientists started regularly observing the storm in 1850, they have noticed that the storm occasionally shrinks and grows, but is currently on a shrinking trend. What once was three times the size of Earth now only stretches to twice our planet's diameter.
Will the Great Red Spot disappear?
Since 1878, observers have kept a robust record of Great Red Spot observations. In a recent study, a team of scientists analyzed old observations and combined them with new ones from various modern spacecraft, like the Voyager missions and the Hubble Space Telescope.
"There is evidence in the archived observations that the Great Red Spot has grown and shrunk over time," Reta Beebe, an emeritus professor at New Mexico State University in Las Cruces, said in a statement from NASA.. "However, the storm is quite small now, and it's been a long time since it last grew."
As the storm contracts, it also grows taller and changes color, becoming a more intense orange. Scientists aren't sure yet why this is happening, but it could be due to chemical reactions as new material is brought up from below.
As of April 2017, the storm measured 10,159 miles (16,350 km) wide. That’s about a third the size observers noted in the 1800s, Orton told Business Insider. He said the storm could continue to shrink for the next 10 to 20 years, and may even disappear.