Why Is There a Great Red Spot on Jupiter?

October 14, 2015 | Sarah Tse

Size comparison of Earth and Jupiter showing the Great Red Spot
Photo credit: NASA

That angry red pimple marring Jupiter’s complexion stands out prominently in most images of the gas giant, but did you ever wonder what it is and why it’s been there for as long as we can remember?

Since Jupiter doesn’t have a solid surface, the mysterious Red Spot isn’t a geological feature. It’s actually a storm — a massive one that doesn’t ever have to worry about weakening over land.

Astronomers have observed massive elliptical storms on Jupiter’s surface ever since they first began gazing through telescopes in the 17th century. While we’re not certain that those ancient astronomers were looking at the same famous storm we see today, we have continuous records of this particular Great Red Spot dating back to 1878, making it at least a century old!

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Being located in the southern hemisphere should give its winds a clockwise spin, except the storm is “anticyclonic,” so it flows against the predictions of the Coriolis Effect. It takes six Earth-days for the storm’s winds to travel around the spot’s girth, which is vast enough to engulf two Earth-sized planets. These winds regularly whip up to speeds of 680 km/h — twice as fast as the most powerful hurricanes recorded on Earth.

Unlike terrestrial hurricanes, the storm rotates around a high pressure system and towers far above the surrounding clouds in Jupiter’s atmosphere. This keeps it at a lower temperature, although the storm has a swirling central region that’s a few Kelvins hotter than the rest. This central “eye” is also a deeper red, whereas the rest of the storm’s color can vary over time from scarlet to the barest hint of peach.

As such, astronomers suspect that temperature and other environmental factors may affect the Red Spot’s eponymous hue, a conundrum they have yet to definitively explain. Surprisingly, under the top layer of clouds, the particles flowing within the storm are mostly white. Like a bubbling cauldron, the spot’s circulating winds confine colorful chemicals called “chromophores,” allowing them to react in isolation and float to the top. Astronomers still aren’t sure about the exact identity of those chromophores, although they have guessed that ammonium hydrosulfide reacting with UV radiation may be the culprit.

Perhaps most mysteriously, the Spot has been shrinking since as far back as the 1930s, at a rate that has recently jumped much higher. Historic observations in the 19th century estimated the storm to stretch across 40,000 km at its widest point, but those measurements have steadily diminished. In a 2009 photo, it was measured at just 18,000 km. Since 2012, the Spot has begun shrinking by almost 1,000 km per year, which is rapidly warping its once elliptical shape into a circle.

Astronomers at NASA suspect that this accelerating reduction could be a result of changing dynamics and energy within the Spot as more and more eddies of smaller winds feed into the storm. The Juno mission in 2016 will bring a spacecraft closer to Jupiter than ever before, and hopefully solve the riddle about the solar system’s most powerful storm.

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