By Adrienne LaFrance
The Great Red Spot wasn’t always red.
Early observers said the oval swirl, Jupiter’s most puzzling and distinctive marking, was more of a salmon pink—or even pale violet—before darkening to a brick red in the early 1880s. After that, it seemed on the brink of vanishing for a time, before it swelled and again deepened in hue. Today it is again shrinking, and it’s turning orange.
No one is sure why, but we may soon find out. NASA’s Juno mission is currently on the way to Jupiter, where it will take an unprecedented close-up of the Great Red Spot.
“We’re going to skim within 3,100 miles of Jupiter’s cloud tops—in between the cloud tops and just inside the most intense portion of the radiation belt,” said Rick Nybakken, Juno’s project manager. “Nobody’s ever ventured there before. No spacecraft has ever operated this close to Jupiter.”
People have been captivated by the planet’s spot for centuries. Scientists today know that the Great Red Spot is an anti-cyclonic storm, a hurricane-like high pressure system that’s three times the size of Earth and has been raging for 400 years—maybe longer. (Its nickname seems to date back only to the 1870s, however.) The chaotic storm has churned for so long, astronomers surmise, because it has never made landfall—there is, after all, no land to fall upon.
The fact that the spot is actually a storm swirling in the Jovian atmosphere helps explain one of the characteristics that astronomers a century ago found most troubling. The Great Red Spot didn’t exactly stay put; it seemed detached from the planet’s surface. “No observer understands the cause of this huge rift,” The New York Times reported in 1880. “It may be an opening in the cloud-atmosphere disclosing the more solid matter beneath, and it may be something beyond human ken.”
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