Jupiter's famous Great Red Spot is steadily shrinking
The Great Red Spot is a giant, high-pressure storm on Jupiter that has raged for at least 150 years, and likely much longer. It is wider than Earth, but historical observations show it is shrinking. In the late 19th century, the storm was four times the diameter of Earth; today, it is only about 1.3 times as wide.
An Anticyclone Between Opposing Jet Streams
The Great Red Spot is a persistent high-pressure storm, classified meteorologically as an anticyclone, situated roughly 22 degrees south of Jupiter's equator. Unlike low-pressure cyclonic systems on Earth, which rotate around a central low, this Jovian storm rotates counterclockwise around a high-pressure center, completing a full rotation roughly every six Earth days. Its sustained existence is made possible by the planet's atmospheric dynamics: Jupiter lacks a solid planetary surface, meaning there is no landmass or terrain to generate the surface friction that typically slows down and breaks apart storms in Earth's atmosphere.
The storm is pinned in latitude between two powerful, opposing atmospheric jet streams. To its south, an eastward-flowing jet stream pushes against the storm's lower boundary, while a westward-flowing jet stream shears along its northern edge. These opposing planetary currents continuously feed rotational energy into the vortex, acting as atmospheric guardrails that keep the storm locked within its latitudinal band even as it drifts westward relative to the surrounding Jovian atmosphere.
Historical Observations and the Continuity Debate
Astronomers have tracked giant spots on Jupiter for centuries, though the continuous record of the modern Great Red Spot is more nuanced than is often assumed. In the mid-17th century, observers including Robert Hooke and Giovanni Cassini reported a prominent dark feature on Jupiter, often referred to in historical records as the Permanent Spot. Cassini documented this feature repeatedly between 1665 and 1713, but subsequent telescopic observations lost track of it, resulting in an observation gap of more than a century during which no definitive sightings were recorded.
The storm recognized today as the Great Red Spot was clearly documented beginning in the 1830s and observed with rigorous detail starting in the late 1870s. Because of the long gap in the 18th century, planetary scientists continue to debate whether the feature observed by Cassini was the exact same physical vortex that exists today or an earlier storm that dissipated before a new one formed in the same latitudinal band. Continuous photographic and telescopic tracking has remained uninterrupted only since the late 19th century.
Tracking the Dramatic Reduction in Scale
When detailed measurements were recorded in the late 19th century, the Great Red Spot was estimated to span roughly 40,000 kilometers in longitudinal length—large enough to comfortably fit three to four Earths side by side across its span. By the time NASA's Voyager 1 and Voyager 2 spacecraft flew past Jupiter in 1979, the storm had contracted to an east-west diameter of approximately 24,000 kilometers, showing a clear long-term trend of longitudinal shrinkage.
Modern measurements gathered by ground-based observatories, the Hubble Space Telescope, and orbital missions show that this reduction has continued at an accelerated pace. Today, the storm measures approximately 16,000 kilometers across, making it roughly 1.3 times the diameter of Earth. As its east-west length has contracted while its north-south width has remained comparatively stable, the storm's overall geometry has transformed from a distinctly elongated oval into a noticeably more circular shape.
Probing the Depths Beneath the Cloud Tops
For decades, scientists could only observe the visible cloud deck of the Great Red Spot, leaving its vertical structure and rooting depth an open question. Observations by NASA's Juno spacecraft, which entered orbit around Jupiter in 2016, provided the first three-dimensional measurements of the storm's interior using microwave radiometry and precise tracking of Jupiter's gravitational field during close flybys.
Juno's instruments revealed that the Great Red Spot extends downward far deeper than the water-condensation cloud layer, reaching roughly 300 to 500 kilometers into Jupiter's interior. While this depth is shallow compared to Jupiter's overall planetary radius of roughly 70,000 kilometers, it is extraordinarily deep compared to terrestrial weather systems, which are confined to the thin troposphere. The storm's deep roots connect the upper atmospheric dynamics directly to the internal heat-driven convection of the planet's deeper layers.
The Chemistry of the Reddish Hue
Despite its name, the exact chemical mechanism responsible for the Great Red Spot's characteristic reddish-orange color remains one of the unresolved questions in planetary meteorology. Jupiter's upper clouds are composed primarily of ammonia, ammonium hydrosulfide, and water, none of which are naturally red. The prevailing scientific hypothesis is that the color results from complex chemical products called chromophores, created when solar ultraviolet radiation interacts with chemicals dredged up from deeper in the atmosphere.
Laboratory simulations and spectroscopic observations suggest that sunlight breaks down compounds such as ammonium hydrosulfide and acetylene in the upper atmosphere, producing sulfur compounds, red phosphorus, or complex organic polymers known as tholins. Because the storm's high-altitude clouds rise higher and colder than surrounding atmospheric zones, these photochemical reactions are concentrated over the vortex. The color itself is not static, varying over time from pale salmon and yellowish-white to deep brick red depending on atmospheric activity and storm dynamics.
Vortices, Flaking, and Future Stability
The longevity of the Great Red Spot depends partly on its interactions with smaller atmospheric disturbances. The giant vortex regularly consumes and absorbs smaller anticyclonic eddies that drift into its path, a process that injects fresh kinetic energy and angular momentum into the system. In recent years, however, observers have documented prominent 'flaking' events, where reddish ribbons of atmospheric material detach from the main body of the storm and dissipate into surrounding bands.
These shedding events sparked discussions over whether the Great Red Spot might be entering a terminal phase of rapid disruption. Detailed fluid dynamics modeling and atmospheric monitoring indicate that while the visible upper-cloud periphery is changing shape and shedding material during interactions with neighboring vortices, the core vortex below the clouds remains dynamically robust. Whether the storm will eventually fade entirely, stabilize at a smaller circular diameter, or enter a new phase of growth remains an active subject of ongoing planetary observation.
Key takeaways
•The Great Red Spot is a high-pressure anticyclonic storm trapped between two opposing Jovian jet streams, allowing it to persist without a solid surface to slow it down.
•Since the late 19th century, the storm has shrunk from roughly 40,000 kilometers across (four times Earth's diameter) to about 16,000 kilometers (1.3 times Earth's diameter), becoming more circular.
•Data from NASA's Juno mission revealed that the storm extends approximately 300 to 500 kilometers deep into Jupiter's atmosphere.
•Its reddish color is thought to be caused by solar ultraviolet radiation breaking down atmospheric chemicals like ammonium hydrosulfide into colored compounds called chromophores.