Great Red Spot

• 1. Introduction

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  The Great Red Spot is a storm on Jupiter, and it may be the oldest continuous weather event in the known Solar System. Locked in place 22 degrees south of Jupiter's equator, it spins counterclockwise and generates winds of up to 496 kilometers per hour. What makes it stranger still is that no one knows what gives it its color. That mystery has persisted since astronomers first started watching it in earnest in September 1831. The questions that follow from that first serious look are still open: how deep does the storm actually go, what is it made of, and how much longer can it last?

• 2. A Disputed Beginning

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  Robert Hooke described a spot on Jupiter in May 1664, and he is often credited as the first person to see the Great Red Spot. But there are good reasons to doubt that. Hooke's spot appears to have been in the North Equatorial Belt, not the South Equatorial Belt where the modern storm lives. It may also have been caught in the shadow of a passing moon, most likely Callisto.

  The year after Hooke's sighting, Giovanni Cassini recorded a spot that he described as permanent, appearing in the same place with the same size and shape again and again. He calculated its rotation period at 9 hours 56 minutes. Cassini's spot was observed from 1665 to 1713. Then it vanished from the records entirely for over a century.

  A 1711 painting by Donato Creti introduces another thread. Exhibited in the Vatican, the canvas depicts Jupiter with a large spot rendered in red, making it the earliest known artistic depiction of a red feature on Jupiter. The astronomer Eustachio Manfredi oversaw the entire series of paintings for accuracy. But because of the optical inversion inherent to the telescopes of the time, Creti placed the spot in Jupiter's northern hemisphere rather than its south.

  A study published in 2024 examined the historical record carefully and concluded that the spot Cassini watched is probably not the same storm that exists today. On that reading, the original spot disappeared entirely, and a new one formed later. The modern storm, continuously watched since the 5th of September 1831, may simply be a younger phenomenon entirely.

• 3. The Shrinking Giant

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  Around a century ago, the Great Red Spot stretched roughly 40,000 kilometers across, about three times the diameter of Earth. By 2004, it had shrunk to roughly half that length. As of the time of writing, it measures 16,350 kilometers in width, just 1.3 times the diameter of Earth, based on measurements taken on the 3rd of April 2017.

  At its current rate of reduction, astronomers estimate it will become circular by 2040. In 2019, the situation grew more dramatic. Fragments of the storm began spinning off the edges, a behavior observers described as "flaking." Some astronomers read this as a sign the spot could dissipate within decades.

  Others are less alarmed. Those researchers argue that the apparent shrinkage reflects a reduction in cloud coverage rather than a reduction in the vortex itself. The flaking events, on this view, are the result of encounters with nearby cyclones and anticyclones, some of which the Great Red Spot partially absorbs without finishing the job. The underlying storm, they suggest, may be more stable than it looks from above.

  What is agreed upon is the reason the storm has persisted as long as it has: Jupiter has no solid surface beneath its atmosphere. Without ground friction to slow it down, the rotating mass of gas has nothing to push against. Circulating eddies keep spinning simply because nothing acts against their angular momentum.

• 4. Neighbors and Encounters

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  In March 2000, three white oval storms in Jupiter's southern hemisphere merged into a single larger system. By 2006, that merged oval had turned reddish, earning the nickname Red Jr., also catalogued as Oval BA. Astronomers at the Goddard Space Flight Center and at UC Berkeley, including Amy Simon-Miller, Imke de Pater, and Phil Marcus, began studying the two storms together using the Hubble Space Telescope in April 2006.

  Simon-Miller predicted the Great Red Spot and Oval BA would pass at their closest on the 4th of July 2006. On the 20th of July 2006, the Gemini Observatory photographed them doing exactly that, passing each other without merging. The two storms cross paths roughly every two years, but the 2002 and 2004 passes had produced little notable activity. In May 2008, a third storm independently turned red.

  On the 25th of February 1979, the Voyager 1 spacecraft transmitted the first detailed images of the Great Red Spot, taken when the probe was 9,200,000 kilometers from Jupiter. At that distance it could resolve cloud details as small as 160 kilometers across. Those images, along with later Voyager time-lapse footage, confirmed what careful ground-based tracking had hinted at since 1966: the storm's counterclockwise rotation was unmistakable. The Juno spacecraft, which entered a polar orbit around Jupiter in 2016, flew directly over the Great Red Spot on the 11th of July 2017, photographing the storm from roughly 5,000 miles above the clouds.

• 5. Depth and Structure

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  From above, the Great Red Spot looks like a single flat oval. Juno's Microwave Radiometer scans, collected during a pass in July 2017, showed something more complex. The storm extends roughly 240 kilometers below the cloud layer, with atmospheric pressure at that depth estimated to reach 100 bar. Two different analytical methods pushed that estimate even further: the mascon approach found a depth of roughly 290 kilometers, and the Slepian approach suggested the storm's winds extend to about 310 kilometers.

  Those same scans indicated that even at depth, the zonal winds retain about 50 percent of the velocity measured at cloud level, before decay sets in at lower depths. The full range suggested by combining gravity, microwave, and thermal data is a storm depth somewhere between 200 and 500 kilometers. Why the storm extends that far down remains unexplained.

  Thermal infrared data collected by Galileo and Cassini spacecraft between 1995 and 2008 painted a portrait of a storm with a cold core sitting inside a warmer surrounding ring. The core is warmer than the regions immediately east and west by roughly 1.0 to 1.5 kelvins, and warmer than the northern and southern edges by 3.0 to 3.5 kelvins. Ground-based imaging using the VISIR instrument on the ESO Very Large Telescope, gathered in 2006, confirmed this thermal structure and showed that the storm physically occupies a wide range of altitudes, spanning atmospheric pressures from 80 to 600 millibars.

  There is a further oddity above the storm. While the Great Red Spot itself is colder than the surrounding Jovian cloud deck, the upper atmosphere directly above it runs substantially hotter than the rest of the planet. Acoustic waves rising from the turbulence below have been proposed as the mechanism. These waves travel vertically upward to roughly 800 kilometers above the storm and break in the upper atmosphere, releasing heat in a process compared to ocean waves crashing on a beach. The result is a region of upper atmosphere sitting at 1,600 kelvins, several hundred kelvins above the planetary average at that altitude.

• 6. The Unanswered Color

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  The Great Red Spot has no agreed-upon explanation for its color, and it has not had one since scientists began tracking it closely. The hue shifts considerably over time, ranging from almost brick-red to pale salmon and occasionally fading to near-white. When it pales that far, the storm becomes visible only through an absence: the Red Spot Hollow, an indentation in the South Equatorial Belt where the storm sits.

  Between 1947 and 1997, the spot ran darkest during four distinct periods: 1961-1966-1968-1975-1989-1990, and 1992-1993. The color appears linked to the state of the South Equatorial Belt itself. When the belt runs bright white, the spot tends to appear dark; when the belt darkens, the spot fades.

  Laboratory experiments have produced one working hypothesis. Ultraviolet radiation from the Sun acts on ammonium hydrosulfide and the organic compound acetylene in Jupiter's upper atmosphere. The resulting chemical products include complex organic compounds called tholins, which produce a reddish material. The altitude at which these compounds form may further concentrate the color. No Jovian feature was described in writing as red before the late 19th century, which makes the 1711 Creti painting, with its red-tinted oval, an unexplained early outlier in the observational record.