The Secrets of Jupiter's Great Red Spot

Jupiter's satellites and red spot
Jupiter, the largest planet in our solar system, with two of its satellites, Io on the left (above Jupiter's Great Red Spot) and Europa on the right, March 1979. The image was taken by the Voyager 1 spacecraft. Hulton Archive / Getty Images

Imagine a storm larger than Earth, raging through the atmosphere of a gas giant planet. It sounds like science fiction, but such an atmospheric disturbance actually exists on planet Jupiter. It's called the Great Red Spot, and planetary scientists think it has been whirling around in Jupiter's cloud decks since at least the mid-1600s. People have observed the current "version" of the spot since 1830, using telescopes and spacecraft to see it up close. NASA's Juno spacecraft has looped very close to the spot while orbiting Jupiter and returned some of the highest-resolution images of the planet and its storm ever produced. They're giving scientists a fresh, new look at one of the oldest known storms in the solar system.

What Is the Great Red Spot?

Jupiter, Great Red Spot, Earth
The Great Red Spot on Jupiter, shown with to scale. This gives an idea of the size of this massive storm on the largest planet in the solar system. NASA

In technical terms, the Great Red Spot is an anticyclonic storm lying in a high-pressure zone high in Jupiter's clouds. It rotates counter-clockwise and takes about six Earth days to make one complete trip around the planet. It has clouds embedded within it, which often tower many kilometers above the surrounding cloud decks. Jet streams to its north and south help keep the spot at the same latitude as it circulates.

The Great Red Spot is, indeed, red, although the chemistry of the clouds and atmosphere cause its color to vary, making it more pinkish-orange than red at times. Jupiter's atmosphere is largely molecular hydrogen and helium, but there are also other chemical compounds there that are familiar to us: water, hydrogen sulfide, ammonia, and methane. Those same chemicals are found in the clouds of the Great Red Spot.

No one is quite sure exactly why the colors of the Great Red Spot change over time. Planetary scientists suspect that solar radiation causes the chemicals in the spot to darken or lighten, depending on the intensity of the solar wind. Jupiter's cloud belts and zones are rich in these chemicals, and also are home to many smaller storms, including some white ovals and brownish spots floating among the swirling clouds.

Studies of the Great Red Spot

Jupiter Pictures Gallery - Ancient Storm
When 17th-century astronomers first turned their telescopes to Jupiter, they noted a conspicuous reddish spot on the giant planet. This Great Red Spot is still present in Jupiter's atmosphere, more than 300 years later. Amy Simon (Cornell), Reta Beebe (NMSU), Heidi Hammel (MIT), Hubble Heritage Team

Observers have studied the gas giant planet Jupiter since antiquity. However, they've only been able to observe such a giant spot for a few centuries since it was first discovered. Ground-based observations allowed scientists to chart the motions of the spot, but a true understanding was only made possible by spacecraft flybys. The Voyager 1 spacecraft raced by in 1979 and sent back the first close-up image of the spot. Voyager 2, Galileo, and Juno also provided images.

From all those studies, scientists have learned more about the spot's rotation, its motions through the atmosphere, and its evolution. Some suspect that its shape will continue to change until it is nearly circular, perhaps in the next 20 years. That change in size is significant; for many years, the spot was larger than two Earth-widths across. When the Voyager spacecraft visited starting in the 1970s, it had shrunk to just two Earths across. Now it's at 1.3 and shrinking.

Why is this happening? Nobody's quite sure. Yet.

Juno Checks Out Jupiter's Largest Storm

Great Red Spot from Juno
The highest-resolution close-up of the Great Red Spot was taken by the Juno spacecraft in 2017. Its image revealed details in the clouds swirling around in this giant anticyclone, and the spacecraft also measured the temperatures near the spot as well as its depth. NASA/Juno

The most exciting images of the spot have come from NASA's Juno spacecraft. It was launched in 2015 and began orbiting Jupiter in 2016. It has swooped low and close to the planet, coming in as low as 3,400 kilometers above the clouds. That has allowed it to show some incredible detail in the Great Red Spot.

Scientists have been able to measure the spot's depth using specialized instruments on the Juno spacecraft. It appears to be some 300 kilometers deep. That's much deeper than any of Earth's oceans, the deepest of which is just over 10 kilometers. Interestingly, the "roots" of the Great Red Spot are warmer at the bottom (or the base) than at the top. This warmth feeds the incredibly strong and fast winds at the top of the spot, which can blow more than 430 kilometers per hour. Warm winds feeding a strong storm is a well-understood phenomenon on Earth, particularly in massive hurricanes. Above the cloud, temperatures rise again, and scientists are working to understand why this is happening. In that sense, then, the Great Red Spot is a Jupiter-style hurricane.

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Petersen, Carolyn Collins. "The Secrets of Jupiter's Great Red Spot." ThoughtCo, Oct. 29, 2020, Petersen, Carolyn Collins. (2020, October 29). The Secrets of Jupiter's Great Red Spot. Retrieved from Petersen, Carolyn Collins. "The Secrets of Jupiter's Great Red Spot." ThoughtCo. (accessed January 23, 2021).