1665

Great Red Spot

Robert Hooke (1635–1703), Giovanni Domenico Cassini (1625–1712)

When the seventeenth-century scientists Robert Hooke and Giovanni Domenico Cassini trained their early astronomical telescopes on the planet Jupiter, they were the first to notice and track a circular, reddish blotch in the planet’s southern hemisphere. Little did they imagine that they were tracking an enormous, hurricane-like storm system, more than twice the size of the Earth, that would continue churning for nearly 350 more years, and perhaps longer.

Astronomers and planetary scientists have studied Jupiter’s Great Red Spot in much more detail from modern telescopes and space missions. To atmospheric scientists, the Great Red Spot is now known to be a persistent, counterclockwise rotating atmospheric vortex. Time-lapse photography of the storm’s rotation shows that it takes about 6 Earth days (about 14 Jupiter days) for the storm to spin once. Wind speeds along the edge of the storm zone, where it interacts with other belts and zones in Jupiter’s atmosphere, peak around 270 miles per hour (430 kilometers per hour).

The Great Red Spot is cooler than the surrounding parts of Jupiter’s atmosphere because its storm clouds are about 6 miles (10 kilometers) higher than the surrounding clouds. If we could somehow fly through that part of Jupiter’s atmosphere, we would see something like a giant, slowly rotating thunderhead cloud rising up above the haze. Strong jet-stream winds to the north and the south of the Great Red Spot appear to provide the energy to keep the storm confined to the same latitude. While the spot’s size has decreased somewhat in the past few decades, no one knows how much longer it will continue to rage.

Why the Great Red Spot is red is somewhat of a mystery. Various hypotheses are that the color is caused by atmospheric gases or aerosols containing sulfur, phosphorus, or organic molecules. Actually, the spot’s color has been observed by astronomers to change over the past few decades, from reddish to brownish, yellowish, and even whitish. The effort to understand the origin of the color of the Great Red Spot, as well as the colors of Jupiter’s other lovely atmospheric patterns, is an active area of planetary science research. The mystery of the spot’s origin and future only enhances the Van Gogh–like beauty of its colorful, swirling clouds.

False color rendering of Jupiter’s Great Red Spot, photographed on July 10, 2017, by NASA’s Juno Jupiter orbiter mission. The spot is an enormous high-pressure storm system, with winds exceeding 250 miles per hour (400 kilometers per hour).